Question

Trying to compile the following code on different compilers gives me two different results:

struct S{};
struct T{S S;};
int main(){}

As you can see, inside T, I have an object named the same as the previously defined class S.

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On GCC 4.7.2, I get the following error pertaining to the S S; declaration inside T:

error: declaration of ‘S T::S’ [-fpermissive]
error: changes meaning of ‘S’ from ‘struct S’ [-fpermissive]

However, moving it outside of the class (or into main) works fine:

struct S{};
S S;
int main(){}

What exactly does it mean by the error it’s giving me?

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In Visual Studio 2012, the whole thing compiles and runs without any errors. Pasting it into this Clang 3.0 compiler gives me no errors as well.

Which is right? Can I actually do this or not?

Answer 1

gcc is correct, from [3.3.7 Class Scope]

A name N used in a class S shall refer to the same declaration in its
context and when re-evaluated in the completed scope of S. No
diagnostic is required for a violation of this rule.

However, note that no diagnostic is required, so all compilers are conforming.

The reason is because of how class scope works. When you write S S; S is visible within the entire class and changes the meaning when you use S.

struct S{};
struct T{
    void foo()
    { 
        S myS; // Not possible anymore because S refers to local S
    }
    S S;
};