Question

So, say I have the following C++ header, testheader.h:

struct mystruct
{
  struct myinnerstruct
  {
        int x;
  } astruct;
};

struct myinnerstruct
{
    int x;
};

and the following C++ source, test.cpp:

#include "testheader.h"
using namespace std;
int main()
{
    return 0;
}

g++ gives no issues during compile/link.

Now, if I have the same header, but instead of the C++ source, a C source file test.c:

#include "testheader.h"
int main()
{
  return 0;
}

And I compile with gcc, I get the following error:

error: redefinition of struct myinnerstruct

So, I gather that the scope of the C version is the translation unit, and the C++ version is block scoped? Can someone confirm this is the case, and maybe give me a reason of why it makes sense? I’m doing some mixing of C and C++ code, and this is giving me quite a bit of trouble.

Any insight is greatly appreciated. Thanks!

Answer 1

In C nested structs do not actually live in their parents scope. However in C++ they do- teststruct::innerstruct is a different type to innerstruct. This is to improve encapsulation in C++ code. Without this rule, no two types in the same namespace could define a nested iterator class, for example, which would be extremely bad.

C treats structs in plenty of other very silly ways, so it’s no surprise that they did the wrong thing here. However, C does not otherwise allow type scoping and having a sane rule here would have introduced many additional concepts to the language- ultimately, it would have required the introduction of namespaces, which for some reason was never done.