CODEWITHSUNDEEP
c++namespacestypedefforward-declaration
ilev4ik
ilev4ik

Reputation: 23

Substitute the forward declared struct with lately defined struct

I'm trying to substitute the forward declared struct B with lately defined struct A. Here is a code sample:

#include <iostream>
using namespace std;

namespace n1 {
struct B;

namespace n3 {
    void func(B& b) {
        cout << "b\n";
    }
}

}

namespace n2 {
struct A{};
}

namespace n1 {
using B = n2::A;
}

int main() {
    n1::B b;
    n1::n3::func(b);
    return 0;
}

and got the following error:

conflicting declaration ‘using B = struct n2::A’
 using B = n2::A;
                ^
prog.cpp:5:8: note: previous declaration as ‘struct n1::B’
 struct B;

Is this trick possible somehow if it's even legal? Am I missing something?

Upvotes: 0

Views: 43

Answers (2)

Jarod42
Jarod42

Reputation: 217235

You cannot forward declared aliases.

a workaround, if appropriate, is to use inheritance instead of alias:

namespace n1 {
struct B;

namespace n3 {
    void func(B& b) {
        cout << "b\n";
    }
}

}

namespace n2 {
struct A{};
}

namespace n1 {
struct B : n2::A{};
}

Demo

but

n2::A a;
n1::n3::func(a); // Fail, a is not n1::B

Upvotes: 1

pptaszni
pptaszni

Reputation: 8217

Your using directive is creating alias to n2::A with name n1::B that is already taken by structure declaration struct B; inside namespace n1 (it is not another declaration of struct B, it is name aliasing). What you probably try to achieve is to provide a definition of your n1::B structure that might be done like this:

namespace n1
{
struct B {
    // definition here, you might use your struct A if you need
};
}

namespace docs

Upvotes: 1

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