source: doc/theses/mike_brooks_MMath/programs/bkgd-carray-arrty.c @ baa1d5d

#include <stdio.h>
#include <assert.h>
#include <stdlib.h>

#define SHOW(x, fmt) printf( #x ": " fmt "\n", x )

#ifdef ERRS
#define ERR(...) __VA_ARGS__
#else
#define ERR(...)
#endif

// int main( int argc, const char *argv[] ) {
//     assert(argc == 2);
//     const int n = atoi(argv[1]);
//     assert(0 < n && n < 1000);

//     float a1[42];
//     float a2[n];
//     SHOW(sizeof(a1), "%zd");
//     SHOW(sizeof(a2), "%zd");

// }


    // SHOW(sizeof( a   ), "%zd");
    // SHOW(sizeof(&a   ), "%zd");
    // SHOW(sizeof( a[0]), "%zd");
    // SHOW(sizeof(&a[0]), "%zd");



int main() {

/*
When a programmer works with an array, C semantics provide access to a type that is different in every way from ``pointer to its first element.''

Its qualities become apparent by inspecting the declaration
*/
    float a[10];
/*

The inspection begins by using @sizeof@ to provide definite program semantics for the intuition of an expression's type.

Assuming a target platform keeps things concrete:
*/
    static_assert(sizeof(float)==4);    // floats (array elements) are 4 bytes
    static_assert(sizeof(void*)==8);    // pointers are 8 bytes
/*

Consider the sizes of expressions derived from @a@, modified by adding ``pointer to'' and ``first element'' (and including unnecessary parentheses to avoid confusion about precedence).
*/
    static_assert(sizeof(  a    ) == 40); // array
    static_assert(sizeof(& a    ) == 8 ); // pointer to array
    static_assert(sizeof(  a[0] ) == 4 ); // first element
    static_assert(sizeof(&(a[0])) == 8 ); // pointer to first element
/*
That @a@ takes up 40 bytes is common reasoning for C programmers.
Set aside for a moment the claim that this first assertion is giving information about a type.
For now, note that an array and a pointer to its first element are, sometimes, different things.

The idea that there is such a thing as a pointer to an array may be surprising.
It is not the same thing as a pointer to the first element:
*/
    typeof(& a    ) x;   // x is pointer to array
    typeof(&(a[0])) y;   // y is pointer to first element
                       ERR(
    x = y;             // ill-typed
    y = x;             // ill-typed
                       )
/*
The first gets
    warning: warning: assignment to `float (*)[10]' from incompatible pointer type `float *'
and the second gets the opposite.
*/

/*
We now refute a concern that @sizeof(a)@ is reporting on special knowledge from @a@ being an local variable,
say that it is informing about an allocation, rather than simply a type.

First, recognizing that @sizeof@ has two forms, one operating on an expression, the other on a type, we observe that the original answers are unaffected by using the type-parameterized form:
*/
    static_assert(sizeof(typeof(  a    )) == 40);
    static_assert(sizeof(typeof(& a    )) == 8 );
    static_assert(sizeof(typeof(  a[0] )) == 4 );
    static_assert(sizeof(typeof(&(a[0]))) == 8 );

/*
Finally, the same sizing is reported when there is no allocation at all, and we launch the analysis instead from the pointer-to-array type.
*/
    void f( float (*pa)[10] ) {
        static_assert(sizeof(   *pa     ) == 40); // array
        static_assert(sizeof(    pa     ) == 8 ); // pointer to array
        static_assert(sizeof(  (*pa)[0] ) == 4 ); // first element
        static_assert(sizeof(&((*pa)[0])) == 8 ); // pointer to first element
    }
    f( & a );

/*
So, in spite of considerable programmer success enabled by an understanding that
an array just a pointer to its first element (revisited TODO pointer decay),
this understanding is simplistic.
*/

/*
A shortened form for declaring local variables exists, provided that length information is given in the initializer:
*/
    float fs[] = {3.14, 1.707};
    char cs[] = "hello";

    static_assert( sizeof(fs) == 2 * sizeof(float) );
    static_assert( sizeof(cs) == 6 * sizeof(char) );  // 5 letters + 1 null terminator

/*
In these declarations, the resulting types are both arrays, but their lengths are inferred.
*/

}


void syntaxReferenceCheck(void) {
    // $\rightarrow$ & (base element)
    //     & @float@ 
    //     & @float x;@ 
    //     & @[ float ]@
    //     & @[ float ]@
    float x0;

    // $\rightarrow$ & pointer
    //     & @float *@ 
    //     & @float * x;@ 
    //     & @[ * float ]@
    //     & @[ * float ]@
    float * x1;

    // $\rightarrow$ & array 
    //     & @float[10]@ 
    //     & @float x[10];@ 
    //     & @[ [10] float ]@
    //     & @[ array(float, 10) ]@
    float x2[10];

    typeof(float[10]) x2b;

    // & array of pointers
    //     & @(float*)[10]@
    //     & @float *x[10];@
    //     & @[ [10] * float ]@
    //     & @[ array(*float, 10) ]@
    float *x3[10];
//    (float *)x3a[10];  NO

    // $\rightarrow$ & pointer to array
    //     & @float(*)[10]@
    //     & @float (*x)[10];@
    //     & @[ * [10] float ]@
    //     & @[ * array(float, 10) ]@
    float (*x4)[10];

    // & pointer to array
    //     & @(float*)(*)[10]@
    //     & @float *(*x)[10];@
    //     & @[ * [10] * float ]@
    //     & @[ * array(*float, 10) ]@
    float *(*x5)[10];
    x5 =     (float*(*)[10]) x4;
//    x5 =     (float(*)[10]) x4;  // wrong target type; meta test suggesting above cast uses correct type

    // [here]
    // const

    // [later]
    // static
    // star as dimension
    // under pointer decay:                int p1[const 3]  being  int const *p1

    const float * y1;
    float const * y2;
    float * const y3;

    y1 = 0;
    y2 = 0;
    // y3 = 0; // bad

    // *y1 = 3.14; // bad
    // *y2 = 3.14; // bad
    *y3 = 3.14;

    const float z1 = 1.414;
    float const z2 = 1.414;

    // z1 = 3.14; // bad
    // z2 = 3.14; // bad


}

#define T float
void stx2() { const T x[10];
//            x[5] = 3.14; // bad
            }
void stx3() { T const x[10];
//            x[5] = 3.14; // bad
            }