To define behaviour of new-delete on empty structs

[mlbrooks]

struct thing{};
void ?{}( thing & ) {
    printf("ctor\n");
}
void ^?{}( thing & ) {
    printf("dtor\n");
}

int main() {
    thing * one = new();
    delete(one);
}

Current: Outputs "ctor," but not "dtor."

An explanation of current behaviour:

• program calls new with return type thing
• new calls malloc with sizeof(thing), which is zero
• malloc returns 0p when asked for zero bytes
• new calls the thing constructor with argument *0p
• new returns 0p as the requested object, which populates main variable one
• program calls delete with argument one, which is 0p
• delete of 0p is a no-op

We learned that sizeof( struct{} ) is 0 in C and 1 in C++. It is currently 0 in CFA.

One option discussed:

• sizeof(struct{}) is 0
• new always calls malloc, requesting min(1, sizeof(T)) bytes
• new always calls constructor with a nonzero malloc result, except when malloc is unable to allocate
• new skips the constructor call when malloc is unable to allocate
• delete always calls free
• delete does not call a destructor when the argument is 0p
• delete calls a destructor when the argument is not 0p
• all constructor and destructor calls use a non *0p argument

Another option discussed:

• sizeof(struct{}) is 0
• new always calls malloc, requesting exactly sizeof(T) bytes
• new does not call a constructor when the type's size is nonzero and malloc returns 0p
• new calls a constructor with argument *0p when the type's size is zero
• delete always calls free
• delete does not call a destructor when the type's size is nonzero and the argument is 0p
• delete calls a destructor with argument 0p when the type's size is zero
• constructor and destructor calls use a 0p argument for {}-structs

[Peter A. Buhr <pabuhr@…>]

In 0f7a0ea:

[fixes #228] delete of 0-sized object always calls destructor; always call free