Struct without definition used as pointer

Question

I have come across some code that looks like it is forward declaring a struct but I can not find any definition for the struct in the code base. It seems to be used as though the struct was defined. Could someone explain why the below code is valid c++?

What type is Frame? What is the size? I cannot use sizeof() as it will complain it is undefined.

I am trying to convert a similar piece of code to Visual Studio 2015 from 2010. The reinterpret_cast cast is complaining that it cannot be converted due to the fact that

'reinterpret_cast': conversion from 'unsigned long' to 'Frame *' of greater size

#include <stdio.h>

struct Frame;

int main()
{
    unsigned long currentFrame = 5;
    Frame* frame = reinterpret_cast<Frame*>(currentFrame);
    printf("%p", frame);
}

GCC 4.9.2 was used to compile this example.

I understand the error, but do not understand how the struct is being used. Is it defaulting to int?

struct Frame; is an incomplete struct definiton. That is you can pass around pointers to Frame but you cannot dereference pointers to Frame. ll that can be said is : struct Frame* is a pointer to some unknown struct. conversion from 'unsigned long' to 'Frame *' of greater size means that 'unsigned long is smaller than struct Frame* as you compile for 64 bits. So the size of unsigned long is 32 bits bujt the size of a pointer is 64 bits. — Jabberwocky

Bathsheba Bathsheba · Accepted Answer · 2017-09-08T13:35:30

The program behaviour is undefined, as a conversion from an unsigned long to Frame* where the former is set to a value not associated with a pointer value that you can set is not in accordance with one of the possibilities mentioned in http://en.cppreference.com/w/cpp/language/reinterpret_cast.

The fact that printf appears to output the address of a pointer is a manifestation of that undefined behaviour.

The fact that Frame is an incomplete type does not matter here. With the exception of nullptr, one past the address of a scalar (i.e. single object or a plain-old-data object), and one past the end of an array, the behaviour on setting a pointer type to memory you don't own is also undefined.

Struct without definition used as pointer

2 Answers