What is the dynamic type of the object allocated by malloc?

Question

The C++ standard refers to the term "dynamic type" (and the C standard refers to "effective type" in the similar context), for example

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:

• the dynamic type of the object,

But how is the dynamic type of the object allocated with malloc determined?

For example:

void *p = malloc(sizeof(int));
int *pi = (int*)p;

Will the dynamic type of the object pointed to by pi be int?

Answer 1

According to the C++ specification:

Dynamic type:

<glvalue> type of the most derived object (1.8) to which the glvalue denoted by a glvalue expression refers

The return value of malloc is a block of uninitialized storage. No object has been constructed within that storage. And therefore it has no dynamic type.

The void* does not point to an object, and only objects have a dynamic type.

You can create an object within that storage by beginning its lifetime. But until you do so, it's just storage.

Answer 2

In C, the effective type is only relevant when you access an object. Then in is determined by

• the declaration type, if it has one
• the type of another object of which it is a copy (eg. memcpy)
• the type of the lvalue through which it is accessed, e.g if a void* is converted to another pointer type (e.g int*), which then is dereferenced.

The latter is usually what happens with malloced objects, if you assign the return value of malloc to a pointer type.