How do I forward declare a typedef in C++?

Question

I have two namespaces (F and M) where I used typedef to define something. I use the typedef in one namespace to declare a variable in the other namespace.

For example I have these files:

File M.hpp

#ifndef M_HPP
#define M_HPP

#include "F.hpp"

namespace M{
    typedef std::vector<F::myD> VectorDouble;

class M{
    private:
        VectorDouble Diction;
};
}

#endif  // M_HPP

File F.hpp

#ifndef F_HPP
#define F_HPP

#include "M.hpp"

namespace F{
    typedef double myD;

class MyF{
    private:
        M::VectorDouble myVar;
};
}

#endif  // F_HPP

It is immediately clear that these two header files create a circular dependance so forward declaration may be necessary, but how to do that with namespaces and typedefs?

File namespace.cpp to drive the code:

#include <iostream>
#include <vector>

#include "M.hpp"
#include "F.hpp"

int main(){
    std::cout << "Learning how to access stuff in a namespace." << std::endl;

    F::MyF myFInstance;
    M::M myMInstance;

    return 0;
}

When I try to compile, I get an error that my M is an undeclared identifier (see exact error message below). I don't understand why M isn't seen as a namespace.

$ clang++ -std=c++11 -stdlib=libc++ namespace.cpp -o namespace
In file included from namespace.cpp:5:
In file included from ./M.hpp:5:
./F.hpp:12:9: error: use of undeclared identifier 'M'
        M::VectorDouble myVar;
        ^
1 error generated.

How can I access a typedef from another namespace? Is this a forward declaration issue?

Pull the `typedef` out into it's own header... then `M.hpp` no longer has a dependency on `F.hpp` — Michael Fredrickson, Jun 04 '14 at 18:00
@MichaelFredrickson Is it good practice to put typedefs like that in a single header? — jlconlin, Jun 04 '14 at 18:03
I'd prefer it over the alternative of duplicating the `typedef` between the two existing headers, since you [can't forward declare a typedef, only duplicate the identical definitions](http://stackoverflow.com/questions/804894/forward-declaration-of-a-typedef-in-c). — Michael Fredrickson, Jun 04 '14 at 18:06

score 1 · Accepted Answer · edited May 23 '17 at 12:11

1

Your issue is that you have created circular includes.

By your own coding, your file F.hpp can't be compiled without first including M.hpp.

And M.hpp can't be compiled without first including F.hpp.

Therefore, neither header can be compiled. See this SO post for solutions to circular dependencies.

Edit:

You can forward declare your typedef like this.

File F_fwd.hpp

#ifndef F_FWD_HPP
#define F_FWD_HPP

namespace F{
    typedef double myD;
}

#endif // F_FWD_HPP

edited May 23 '17 at 12:11

Community

1
1

answered Jun 04 '14 at 17:43

Drew Dormann

59,987
13
123
180

I recognize that my code has circular dependencies. I know how to forward declare classes, but my problem is not with the classes, but with the namespace and typedefs. How do I forward declare them? – jlconlin Jun 04 '14 at 17:50

score 0 · Answer 2 · answered Jun 04 '14 at 17:47

0

your two headers are included in each other, that would lead to circular reference and with the header gaurds, one file can be completely excluded in one of the other headers.

answered Jun 04 '14 at 17:47

Richard Peters

98
8

How do I forward declare a typedef in C++?

2 Answers2