| 1 | \chapter{Background}
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| 2 |
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| 3 | Since this work builds on C, it is necessary to explain the C mechanisms and their shortcomings for array, linked list, and string.
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| 4 |
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| 5 |
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| 6 | \section{Array}
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| 7 |
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| 8 | At the start, the C programming language made a significant design mistake.
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| 9 | \begin{quote}
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| 10 | In C, there is a strong relationship between pointers and arrays, strong enough that pointers and arrays really should be treated simultaneously.
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| 11 | Any operation which can be achieved by array subscripting can also be done with pointers.~\cite[p.~93]{C:old}
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| 12 | \end{quote}
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| 13 | Accessing any storage requires pointer arithmetic, even if it is just base-displacement addressing in an instruction.
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| 14 | The conjoining of pointers and arrays could also be applied to structures, where a pointer references a structure field like an array element.
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| 15 | Finally, while subscripting involves pointer arithmetic (as does field references @x.y.z@), it is very complex for multi-dimensional arrays and requires array descriptors to know stride lengths along dimensions.
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| 16 | Many C errors result from performing pointer arithmetic instead of using subscripting.
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| 17 | Some C textbooks erroneously teach pointer arithmetic suggesting it is faster than subscripting.
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| 18 |
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| 19 | C semantics want a programmer to \emph{believe} an array variable is a ``pointer to its first element.''
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| 20 | This desire becomes apparent by a detailed inspection of an array declaration.
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| 21 | \lstinput{34-34}{bkgd-carray-arrty.c}
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| 22 | The inspection begins by using @sizeof@ to provide definite program semantics for the intuition of an expression's type.
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| 23 | \lstinput{35-36}{bkgd-carray-arrty.c}
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| 24 | Now consider the sizes of expressions derived from @ar@, modified by adding ``pointer to'' and ``first element'' (and including unnecessary parentheses to avoid confusion about precedence).
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| 25 | \lstinput{37-40}{bkgd-carray-arrty.c}
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| 26 | Given the size of @float@ is 4, the size of @ar@ with 10 floats being 40 bytes is common reasoning for C programmers.
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| 27 | Equally, C programmers know the size of a \emph{pointer} to the first array element is 8 (or 4 depending on the addressing architecture).
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| 28 | % Now, set aside for a moment the claim that this first assertion is giving information about a type.
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| 29 | Clearly, an array and a pointer to its first element are different things.
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| 30 |
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| 31 | In fact, the idea that there is such a thing as a pointer to an array may be surprising and it is not the same thing as a pointer to the first element.
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| 32 | \lstinput{42-45}{bkgd-carray-arrty.c}
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| 33 | The first assignment gets
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| 34 | \begin{cfa}
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| 35 | warning: assignment to `float (*)[10]' from incompatible pointer type `float *'
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| 36 | \end{cfa}
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| 37 | and the second assignment gets the opposite.
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| 38 |
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| 39 | The inspection now refutes any suggestion that @sizeof@ is informing about allocation rather than type information.
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| 40 | Note, @sizeof@ has two forms, one operating on an expression and the other on a type.
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| 41 | Using the type form yields the same results as the prior expression form.
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| 42 | \lstinput{46-49}{bkgd-carray-arrty.c}
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| 43 | The results are also the same when there is \emph{no allocation} using a pointer-to-array type.
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| 44 | \lstinput{51-57}{bkgd-carray-arrty.c}
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| 45 | Hence, in all cases, @sizeof@ is informing about type information.
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| 46 |
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| 47 | So, thinking of an array as a pointer to its first element is too simplistic an analogue and it is not backed up by the type system.
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| 48 | This misguided analogue works for a single-dimension array but there is no advantage other than possibly teaching beginning programmers about basic runtime array-access.
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| 49 |
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| 50 | Continuing, a short form for declaring array variables exists using length information provided implicitly by an initializer.
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| 51 | \lstinput{59-62}{bkgd-carray-arrty.c}
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| 52 | The compiler counts the number of initializer elements and uses this value as the first dimension.
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| 53 | Unfortunately, the implicit element counting does not extend to dimensions beyond the first.
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| 54 | \lstinput{64-67}{bkgd-carray-arrty.c}
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| 55 |
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| 56 | My contribution is recognizing:
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| 57 | \begin{itemize}
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| 58 | \item There is value in using a type that knows its size.
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| 59 | \item The type pointer to (first) element does not.
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| 60 | \item C \emph{has} a type that knows the whole picture: array, e.g. @T[10]@.
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| 61 | \item This type has all the usual derived forms, which also know the whole picture.
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| 62 | A usefully noteworthy example is pointer to array, e.g. @T (*)[10]@.\footnote{
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| 63 | The parenthesis are necessary because subscript has higher priority than pointer in C declarations.
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| 64 | (Subscript also has higher priority than dereference in C expressions.)}
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| 65 | \end{itemize}
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| 66 |
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| 67 |
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| 68 | \section{Reading Declarations}
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| 69 |
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| 70 | A significant area of confusion for reading C declarations results from embedding a declared variable in a declaration, mimicking the way the variable is used in executable statements.
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| 71 | \begin{cquote}
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| 72 | \begin{tabular}{@{}ll@{}}
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| 73 | \multicolumn{1}{@{}c}{\textbf{Array}} & \multicolumn{1}{c@{}}{\textbf{Function Pointer}} \\
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| 74 | \begin{cfa}
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| 75 | int @(*@ar@)[@5@]@; // definition
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| 76 | ... @(*@ar@)[@3@]@ += 1; // usage
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| 77 | \end{cfa}
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| 78 | &
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| 79 | \begin{cfa}
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| 80 | int @(*@f@())[@5@]@ { ... }; // definition
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| 81 | ... @(*@f@())[@3@]@ += 1; // usage
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| 82 | \end{cfa}
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| 83 | \end{tabular}
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| 84 | \end{cquote}
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| 85 | Essentially, the type is wrapped around the name in successive layers (like an \Index{onion}).
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| 86 | While attempting to make the two contexts consistent is a laudable goal, it has not worked out in practice, even though Dennis Richie believed otherwise:
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| 87 | \begin{quote}
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| 88 | In spite of its difficulties, I believe that the C's approach to declarations remains plausible, and am comfortable with it; it is a useful unifying principle.~\cite[p.~12]{Ritchie93}
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| 89 | \end{quote}
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| 90 | After all, reading a C array type is easy: just read it from the inside out, and know when to look left and when to look right!
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| 91 |
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| 92 | \CFA provides its own type, variable and routine declarations, using a simpler syntax.
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| 93 | The new declarations place qualifiers to the left of the base type, while C declarations place qualifiers to the right of the base type.
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| 94 | The qualifiers have the same meaning in \CFA as in C.
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| 95 | Then, a \CFA declaration is read left to right, where a function return type is enclosed in brackets @[@\,@]@.
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| 96 | \begin{cquote}
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| 97 | \begin{tabular}{@{}l@{\hspace{3em}}ll@{}}
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| 98 | \multicolumn{1}{c@{\hspace{3em}}}{\textbf{C}} & \multicolumn{1}{c}{\textbf{\CFA}} & \multicolumn{1}{c}{\textbf{read left to right}} \\
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| 99 | \begin{cfa}
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| 100 | int @*@ x1 @[5]@;
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| 101 | int @(*@x2@)[5]@;
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| 102 | int @(*@f( int p )@)[5]@;
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| 103 | \end{cfa}
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| 104 | &
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| 105 | \begin{cfa}
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| 106 | @[5] *@ int x1;
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| 107 | @* [5]@ int x2;
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| 108 | @[ * [5] int ]@ f( int p );
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| 109 | \end{cfa}
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| 110 | &
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| 111 | \begin{cfa}
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| 112 | // array of 5 pointers to int
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| 113 | // pointer to array of 5 int
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| 114 | // function returning pointer to array of 5 ints
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| 115 | \end{cfa}
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| 116 | \\
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| 117 | & &
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| 118 | \LstCommentStyle{//\ \ \ and taking an int argument}
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| 119 | \end{tabular}
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| 120 | \end{cquote}
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| 121 | As declaration complexity increases, it becomes corresponding difficult to read and understand the C declaration form.
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| 122 | Note, writing declarations left to right is common in other programming languages, where the function return-type is often placed after the parameter declarations.
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| 123 |
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| 124 | \VRef[Table]{bkgd:ar:usr:avp} introduces the many layers of the C and \CFA array story, where the \CFA story is discussion in \VRef{XXX}.
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| 125 | The \CFA-thesis column shows the new array declaration form, which is my contributed improvements for safety and ergonomics.
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| 126 | The table shows there are multiple yet equivalent forms for the array types under discussion, and subsequent discussion shows interactions with orthogonal (but easily confused) language features.
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| 127 | Each row of the table shows alternate syntactic forms.
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| 128 | The simplest occurrences of types distinguished in the preceding discussion are marked with $\triangleright$.
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| 129 | Removing the declared variable @x@, gives the type used for variable, structure field, cast or error messages \PAB{(though note Section TODO points out that some types cannot be casted to)}.
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| 130 | Unfortunately, parameter declarations \PAB{(section TODO)} have more syntactic forms and rules.
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| 131 |
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| 132 | \begin{table}
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| 133 | \centering
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| 134 | \caption{Syntactic Reference for Array vs Pointer. Includes interaction with \lstinline{const}ness.}
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| 135 | \label{bkgd:ar:usr:avp}
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| 136 | \begin{tabular}{ll|l|l|l}
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| 137 | & Description & \multicolumn{1}{c|}{C} & \multicolumn{1}{c|}{\CFA} & \multicolumn{1}{c}{\CFA-thesis} \\
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| 138 | \hline
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| 139 | $\triangleright$ & value & @T x;@ & @T x;@ & \\
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| 140 | \hline
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| 141 | & immutable value & @const T x;@ & @const T x;@ & \\
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| 142 | & & @T const x;@ & @T const x;@ & \\
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| 143 | \hline \hline
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| 144 | $\triangleright$ & pointer to value & @T * x;@ & @* T x;@ & \\
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| 145 | \hline
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| 146 | & immutable ptr. to val. & @T * const x;@ & @const * T x;@ & \\
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| 147 | \hline
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| 148 | & ptr. to immutable val. & @const T * x;@ & @* const T x;@ & \\
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| 149 | & & @T const * x;@ & @* T const x;@ & \\
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| 150 | \hline \hline
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| 151 | $\triangleright$ & array of value & @T x[10];@ & @[10] T x@ & @array(T, 10) x@ \\
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| 152 | \hline
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| 153 | & ar.\ of immutable val. & @const T x[10];@ & @[10] const T x@ & @const array(T, 10) x@ \\
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| 154 | & & @T const x[10];@ & @[10] T const x@ & @array(T, 10) const x@ \\
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| 155 | \hline
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| 156 | & ar.\ of ptr.\ to value & @T * x[10];@ & @[10] * T x@ & @array(T *, 10) x@ \\
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| 157 | & & & & @array(* T, 10) x@ \\
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| 158 | \hline
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| 159 | & ar.\ of imm. ptr.\ to val. & @T * const x[10];@ & @[10] const * T x@ & @array(* const T, 10) x@ \\
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| 160 | & & & & @array(const * T, 10) x@ \\
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| 161 | \hline
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| 162 | & ar.\ of ptr.\ to imm. val. & @const T * x[10];@ & @[10] * const T x@ & @array(const T *, 10) x@ \\
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| 163 | & & @T const * x[10];@ & @[10] * T const x@ & @array(* const T, 10) x@ \\
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| 164 | \hline \hline
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| 165 | $\triangleright$ & ptr.\ to ar.\ of value & @T (*x)[10];@ & @* [10] T x@ & @* array(T, 10) x@ \\
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| 166 | \hline
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| 167 | & imm. ptr.\ to ar.\ of val. & @T (* const x)[10];@ & @const * [10] T x@ & @const * array(T, 10) x@ \\
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| 168 | \hline
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| 169 | & ptr.\ to ar.\ of imm. val. & @const T (*x)[10];@ & @* [10] const T x@ & @* const array(T, 10) x@ \\
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| 170 | & & @T const (*x)[10];@ & @* [10] T const x@ & @* array(T, 10) const x@ \\
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| 171 | \hline
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| 172 | & ptr.\ to ar.\ of ptr.\ to val. & @T *(*x)[10];@ & @* [10] * T x@ & @* array(T *, 10) x@ \\
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| 173 | & & & & @* array(* T, 10) x@ \\
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| 174 | \hline
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| 175 | \end{tabular}
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| 176 | \end{table}
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| 177 |
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| 178 | TODO: Address these parked unfortunate syntaxes
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| 179 | \begin{itemize}
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| 180 | \item static
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| 181 | \item star as dimension
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| 182 | \item under pointer decay: @int p1[const 3]@ being @int const *p1@
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| 183 | \end{itemize}
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| 184 |
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| 185 |
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| 186 | \subsection{Arrays decay and pointers diffract}
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| 187 |
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| 188 | The last section established the difference between these four types:
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| 189 | \lstinput{3-6}{bkgd-carray-decay.c}
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| 190 | But the expression used for obtaining the pointer to the first element is pedantic.
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| 191 | The root of all C programmer experience with arrays is the shortcut
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| 192 | \lstinput{8-8}{bkgd-carray-decay.c}
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| 193 | which reproduces @pa0@, in type and value:
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| 194 | \lstinput{9-9}{bkgd-carray-decay.c}
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| 195 | The validity of this initialization is unsettling, in the context of the facts established in the last section.
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| 196 | Notably, it initializes name @pa0x@ from expression @ar@, when they are not of the same type:
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| 197 | \lstinput{10-10}{bkgd-carray-decay.c}
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| 198 |
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| 199 | So, C provides an implicit conversion from @float[10]@ to @float *@.
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| 200 | \begin{quote}
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| 201 | Except when it is the operand of the @sizeof@ operator, or the unary @&@ operator, or is a string literal used to
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| 202 | initialize an array an expression that has type ``array of \emph{type}'' is converted to an expression with type
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| 203 | ``pointer to \emph{type}'' that points to the initial element of the array object~\cite[\S~6.3.2.1.3]{C11}
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| 204 | \end{quote}
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| 205 | This phenomenon is the famous ``pointer decay,'' which is a decay of an array-typed expression into a pointer-typed one.
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| 206 | It is worthy to note that the list of exception cases does not feature the occurrence of @ar@ in @ar[i]@.
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| 207 | Thus, subscripting happens on pointers not arrays.
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| 208 |
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| 209 | Subscripting proceeds first with pointer decay, if needed. Next, \cite[\S~6.5.2.1.2]{C11} explains that @ar[i]@ is treated as if it were @(*((a)+(i)))@.
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| 210 | \cite[\S~6.5.6.8]{C11} explains that the addition, of a pointer with an integer type, is defined only when the pointer refers to an element that is in an array, with a meaning of ``@i@ elements away from,'' which is valid if @ar@ is big enough and @i@ is small enough.
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| 211 | Finally, \cite[\S~6.5.3.2.4]{C11} explains that the @*@ operator's result is the referenced element.
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| 212 | Taken together, these rules illustrate that @ar[i]@ and @i[a]@ mean the same thing!
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| 213 |
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| 214 | Subscripting a pointer when the target is standard-inappropriate is still practically well-defined.
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| 215 | While the standard affords a C compiler freedom about the meaning of an out-of-bound access,
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| 216 | or of subscripting a pointer that does not refer to an array element at all,
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| 217 | the fact that C is famously both generally high-performance, and specifically not bound-checked,
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| 218 | leads to an expectation that the runtime handling is uniform across legal and illegal accesses.
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| 219 | Moreover, consider the common pattern of subscripting on a @malloc@ result:
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| 220 | \begin{cfa}
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| 221 | float * fs = malloc( 10 * sizeof(float) );
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| 222 | fs[5] = 3.14;
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| 223 | \end{cfa}
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| 224 | The @malloc@ behaviour is specified as returning a pointer to ``space for an object whose size is'' as requested (\cite[\S~7.22.3.4.2]{C11}).
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| 225 | But \emph{nothing} more is said about this pointer value, specifically that its referent might \emph{be} an array allowing subscripting.
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| 226 |
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| 227 | Under this assumption, a pointer being subscripted (or added to, then dereferenced) by any value (positive, zero, or negative), gives a view of the program's entire address space, centred around the @p@ address, divided into adjacent @sizeof(*p)@ chunks, each potentially (re)interpreted as @typeof(*p)@.
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| 228 | I call this phenomenon ``array diffraction,'' which is a diffraction of a single-element pointer into the assumption that its target is in the middle of an array whose size is unlimited in both directions.
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| 229 | No pointer is exempt from array diffraction.
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| 230 | No array shows its elements without pointer decay.
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| 231 |
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| 232 | A further pointer--array confusion, closely related to decay, occurs in parameter declarations.
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| 233 | \cite[\S~6.7.6.3.7]{C11} explains that when an array type is written for a parameter,
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| 234 | the parameter's type becomes a type that can be summarized as the array-decayed type.
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| 235 | The respective handling of the following two parameter spellings shows that the array-spelled one is really, like the other, a pointer.
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| 236 | \lstinput{12-16}{bkgd-carray-decay.c}
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| 237 | As the @sizeof(x)@ meaning changed, compared with when run on a similarly-spelled local variable declaration,
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| 238 | @gcc@ also gives this code the warning for the first assertion:
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| 239 | \begin{cfa}
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| 240 | warning: 'sizeof' on array function parameter 'x' will return size of 'float *'
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| 241 | \end{cfa}
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| 242 | The caller of such a function is left with the reality that a pointer parameter is a pointer, no matter how it is spelled:
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| 243 | \lstinput{18-21}{bkgd-carray-decay.c}
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| 244 | This fragment gives no warnings.
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| 245 |
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| 246 | The shortened parameter syntax @T x[]@ is a further way to spell ``pointer.''
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| 247 | Note the opposite meaning of this spelling now, compared with its use in local variable declarations.
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| 248 | This point of confusion is illustrated in:
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| 249 | \lstinput{23-30}{bkgd-carray-decay.c}
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| 250 | The basic two meanings, with a syntactic difference helping to distinguish,
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| 251 | are illustrated in the declarations of @ca@ vs.\ @cp@,
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| 252 | whose subsequent @edit@ calls behave differently.
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| 253 | The syntax-caused confusion is in the comparison of the first and last lines,
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| 254 | both of which use a literal to initialize an object declared with spelling @T x[]@.
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| 255 | But these initialized declarations get opposite meanings,
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| 256 | depending on whether the object is a local variable or a parameter.
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| 257 |
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| 258 | In summary, when a function is written with an array-typed parameter,
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| 259 | \begin{itemize}
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| 260 | \item an appearance of passing an array by value is always an incorrect understanding
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| 261 | \item a dimension value, if any is present, is ignored
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| 262 | \item pointer decay is forced at the call site and the callee sees the parameter having the decayed type
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| 263 | \end{itemize}
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| 264 |
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| 265 | Pointer decay does not affect pointer-to-array types, because these are already pointers, not arrays.
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| 266 | As a result, a function with a pointer-to-array parameter sees the parameter exactly as the caller does:
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| 267 | \lstinput{32-42}{bkgd-carray-decay.c}
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| 268 | \VRef[Table]{bkgd:ar:usr:decay-parm} gives the reference for the decay phenomenon seen in parameter declarations.
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| 269 |
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| 270 | \begin{table}
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| 271 | \caption{Syntactic Reference for Decay during Parameter-Passing.
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| 272 | Includes interaction with \lstinline{const}ness, where ``immutable'' refers to a restriction on the callee's ability.}
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| 273 | \label{bkgd:ar:usr:decay-parm}
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| 274 | \centering
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| 275 | \begin{tabular}{llllll}
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| 276 | & Description & Type & Parameter Declaration & \CFA \\
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| 277 | \hline
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| 278 | & & & @T * x,@ & @* T x,@ \\
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| 279 | $\triangleright$ & pointer to value & @T *@ & @T x[10],@ & @[10] T x,@ \\
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| 280 | & & & @T x[],@ & @[] T x,@ \\
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| 281 | \hline
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| 282 | & & & @T * const x,@ & @const * T x@ \\
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| 283 | & immutable ptr.\ to val. & @T * const@ & @T x[const 10],@ & @[const 10] T x,@ \\
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| 284 | & & & @T x[const],@ & @[const] T x,@\\
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| 285 | \hline
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| 286 | & & & @const T * x,@ & @ * const T x,@ \\
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| 287 | & & & @T const * x,@ & @ * T const x,@ \\
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| 288 | & ptr.\ to immutable val. & @const T *@ & @const T x[10],@ & @[10] const T x,@ \\
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| 289 | & & @T const *@ & @T const x[10],@ & @[10] T const x,@ \\
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| 290 | & & & @const T x[],@ & @[] const T x,@ \\
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| 291 | & & & @T const x[],@ & @[] T const x,@ \\
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| 292 | \hline \hline
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| 293 | & & & @T (*x)[10],@ & @* [10] T x,@ \\
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| 294 | $\triangleright$ & ptr.\ to ar.\ of val. & @T(*)[10]@ & @T x[3][10],@ & @[3][10] T x,@ \\
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| 295 | & & & @T x[][10],@ & @[][10] T x,@ \\
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| 296 | \hline
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| 297 | & & & @T ** x,@ & @** T x,@ \\
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| 298 | & ptr.\ to ptr.\ to val. & @T **@ & @T * x[10],@ & @[10] * T x,@ \\
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| 299 | & & & @T * x[],@ & @[] * T x,@ \\
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| 300 | \hline
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| 301 | & ptr.\ to ptr.\ to imm.\ val. & @const char **@ & @const char * argv[],@ & @[] * const char argv,@ \\
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| 302 | & & & \emph{others elided} & \emph{others elided} \\
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| 303 | \hline
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| 304 | \end{tabular}
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| 305 | \end{table}
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| 306 |
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| 307 |
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| 308 | \subsection{Lengths may vary, checking does not}
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| 309 |
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| 310 | When the desired number of elements is unknown at compile time, a variable-length array is a solution:
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| 311 | \begin{cfa}
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| 312 | int main( int argc, const char * argv[] ) {
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| 313 | assert( argc == 2 );
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| 314 | size_t n = atol( argv[1] );
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| 315 | assert( 0 < n );
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| 316 | float ar[n];
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| 317 | float b[10];
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| 318 | // ... discussion continues here
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| 319 | }
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| 320 | \end{cfa}
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| 321 | This arrangement allocates @n@ elements on the @main@ stack frame for @ar@, called a \newterm{variable length array} (VLA), as well as 10 elements in the same stack frame for @b@.
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| 322 | The variable-sized allocation of @ar@ is provided by the @alloca@ routine, which bumps the stack pointer.
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| 323 | Note, the C standard supports VLAs~\cite[\S~6.7.6.2.4]{C11} as a conditional feature, but the \CC standard does not;
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| 324 | both @gcc@ and @g++@ support VLAs.
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| 325 | As well, there is misinformation about VLAs, \eg VLAs cause stack failures or are inefficient.
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| 326 | VLAs exist as far back as Algol W~\cite[\S~5.2]{AlgolW} and are a sound and efficient data type.
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| 327 |
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| 328 | For high-performance applications, the stack size can be fixed and small (coroutines or user-level threads).
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| 329 | Here, VLAs can overflow the stack, so a heap allocation is used.
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| 330 | \begin{cfa}
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| 331 | float * ax1 = malloc( sizeof( float[n] ) );
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| 332 | float * ax2 = malloc( n * sizeof( float ) );
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| 333 | float * bx1 = malloc( sizeof( float[1000000] ) );
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| 334 | float * bx2 = malloc( 1000000 * sizeof( float ) );
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| 335 | \end{cfa}
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| 336 |
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| 337 | Parameter dependency
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| 338 |
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| 339 | Checking is best-effort / unsound
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| 340 |
|
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| 341 | Limited special handling to get the dimension value checked (static)
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| 342 |
|
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| 343 |
|
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| 344 | \subsection{Dynamically sized, multidimensional arrays}
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| 345 |
|
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| 346 | In C and \CC, ``multidimensional array'' means ``array of arrays.'' Other meanings are discussed in TODO.
|
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| 347 |
|
|---|
| 348 | Just as an array's element type can be @float@, so can it be @float[10]@.
|
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| 349 |
|
|---|
| 350 | While any of @float*@, @float[10]@ and @float(*)[10]@ are easy to tell apart from @float@, telling them apart from each other may need occasional reference back to TODO intro section.
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| 351 | The sentence derived by wrapping each type in @-[3]@ follows.
|
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| 352 |
|
|---|
| 353 | While any of @float*[3]@, @float[3][10]@ and @float(*)[3][10]@ are easy to tell apart from @float[3]@,
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| 354 | telling them apart from each other is what it takes to know what ``array of arrays'' really means.
|
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| 355 |
|
|---|
| 356 | Pointer decay affects the outermost array only
|
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| 357 |
|
|---|
| 358 | TODO: unfortunate syntactic reference with these cases:
|
|---|
| 359 |
|
|---|
| 360 | \begin{itemize}
|
|---|
| 361 | \item ar. of ar. of val (be sure about ordering of dimensions when the declaration is dropped)
|
|---|
| 362 | \item ptr. to ar. of ar. of val
|
|---|
| 363 | \end{itemize}
|
|---|
| 364 |
|
|---|
| 365 |
|
|---|
| 366 | \subsection{Arrays are (but) almost values}
|
|---|
| 367 |
|
|---|
| 368 | Has size; can point to
|
|---|
| 369 |
|
|---|
| 370 | Can't cast to
|
|---|
| 371 |
|
|---|
| 372 | Can't pass as value
|
|---|
| 373 |
|
|---|
| 374 | Can initialize
|
|---|
| 375 |
|
|---|
| 376 | Can wrap in aggregate
|
|---|
| 377 |
|
|---|
| 378 | Can't assign
|
|---|
| 379 |
|
|---|
| 380 |
|
|---|
| 381 | \subsection{Returning an array is (but) almost possible}
|
|---|
| 382 |
|
|---|
| 383 |
|
|---|
| 384 | \subsection{The pointer-to-array type has been noticed before}
|
|---|
| 385 |
|
|---|
| 386 | \subsection{Multi-Dimensional}
|
|---|
| 387 |
|
|---|
| 388 | As in the last section, we inspect the declaration ...
|
|---|
| 389 | \lstinput{16-18}{bkgd-carray-mdim.c}
|
|---|
| 390 | The significant axis of deriving expressions from @ar@ is now ``itself,'' ``first element'' or ``first grand-element (meaning, first element of first element).''
|
|---|
| 391 | \lstinput{20-44}{bkgd-carray-mdim.c}
|
|---|
| 392 |
|
|---|
| 393 |
|
|---|
| 394 | \section{Linked List}
|
|---|
| 395 |
|
|---|
| 396 | Linked-lists are blocks of storage connected using one or more pointers.
|
|---|
| 397 | The storage block is logically divided into data and links (pointers), where the links are the only component used by the list structure.
|
|---|
| 398 | Since the data is opaque, list structures are often polymorphic over the data, which is normally homogeneous.
|
|---|
| 399 |
|
|---|
| 400 |
|
|---|
| 401 | \section{String}
|
|---|
| 402 |
|
|---|
| 403 | A string is a logical sequence of symbols, where the form of the symbols can vary significantly: 7/8-bit characters (ASCII/Latin-1), or 2/4/8-byte (UNICODE) characters/symbols or variable length (UTF-8/16/32) characters.
|
|---|
| 404 | A string can be read left-to-right, right-to-left, top-to-bottom, and have stacked elements (Arabic).
|
|---|
| 405 |
|
|---|
| 406 | An integer character constant is a sequence of one or more multibyte characters enclosed in single-quotes, as in @'x'@.
|
|---|
| 407 | A wide character constant is the same, except prefixed by the letter @L@, @u@, or @U@.
|
|---|
| 408 | With a few exceptions detailed later, the elements of the sequence are any members of the source character set;
|
|---|
| 409 | they are mapped in an implementation-defined manner to members of the execution character set.
|
|---|
| 410 |
|
|---|
| 411 | A C character-string literal is a sequence of zero or more multibyte characters enclosed in double-quotes, as in @"xyz"@.
|
|---|
| 412 | A UTF-8 string literal is the same, except prefixed by @u8@.
|
|---|
| 413 | A wide string literal is the same, except prefixed by the letter @L@, @u@, or @U@.
|
|---|
| 414 |
|
|---|
| 415 | For UTF-8 string literals, the array elements have type @char@, and are initialized with the characters of the multibyte character sequence, as encoded in UTF-8.
|
|---|
| 416 | For wide string literals prefixed by the letter @L@, the array elements have type @wchar_t@ and are initialized with the sequence of wide characters corresponding to the multibyte character sequence, as defined by the @mbstowcs@ function with an implementation-defined current locale.
|
|---|
| 417 | For wide string literals prefixed by the letter @u@ or @U@, the array elements have type @char16_t@ or @char32_t@, respectively, and are initialized with the sequence of wide characters corresponding to the multibyte character sequence, as defined by successive calls to the @mbrtoc16@, or @mbrtoc32@ function as appropriate for its type, with an implementation-defined current locale.
|
|---|
| 418 | The value of a string literal containing a multibyte character or escape sequence not represented in the executioncharacter set is implementation-defined.
|
|---|
| 419 |
|
|---|
| 420 |
|
|---|
| 421 | Another bad C design decision is to have null-terminated strings rather than maintaining a separate string length.
|
|---|
| 422 | \begin{quote}
|
|---|
| 423 | Technically, a string is an array whose elements are single characters.
|
|---|
| 424 | The compiler automatically places the null character @\0@ at the end of each such string, so programs can conveniently find the end.
|
|---|
| 425 | This representation means that there is no real limit to how long a string can be, but programs have to scan one completely to determine its length.
|
|---|
| 426 | \end{quote}
|
|---|
