How to solve this undeclared identifier

Question

I'm getting an undeclared identifier error (C2065 on vs2013) on my project, I managed to replicate the problem in the example code below:

foo.h:

#pragma once

#include "bar.h"

class Foo
{

public:
    inline void doStuff() { someFunction(); }

};

bar.h:

#pragma once

#include <map>

#include "foo.h"

extern std::map<const char*, Foo> myMap;

void someFunction();

bar.cpp:

#include "bar.h"

std::map<const char*, Foo> myMap;

void someFunction()
{

}

main.cpp:

#include "foo.h"

int main()
{
    Foo foo;
    foo.doStuff();
    return 0;
}

When building on Visual Studio Express 2013, it gives these errors:

error C2065: 'Foo' : undeclared identifier

error C2923: 'std::map' : 'Foo' is not a valid template type argument for parameter '_Ty'

error C3861: 'someFunction': identifier not found

What is the problem here and how can it be solved?

Answer 1

The problem is caused by circular dependency between foo.h and bar.h. It can be resolved using any of the following methods that I can think of.

Method 1

1. Use forward declaration of the class Foo in bar.h.
2. Don't use #include "foo.h" in bar.h.

foo.h:

#pragma once

#include "bar.h"

class Foo
{

public:
    inline void doStuff() { someFunction(); }

};

bar.h:

#pragma once

#include <map>

// #include "foo.h"
class Foo;

extern std::map<const char*, Foo> myMap;

void someFunction();

Method 2

1. Create foo.cpp and add it to the project.
2. Move the implementation of Foo::doStuff() to foo.cpp.
3. Don't use #include "bar.h" in foo.h.

foo.h:

#pragma once

class Foo
{

public:
    void doStuff();

};

bar.h:

#pragma once

#include <map>

#include "foo.h"

extern std::map<const char*, Foo> myMap;

void someFunction();

foo.cpp:

#include "foo.h"
#include "bar.h"

void Foo::doStuff(){ someFunction(); }

Method 3

Use both Method 1 and Method 2.

foo.h:

#pragma once

class Foo
{

public:
    void doStuff();

};

bar.h:

#pragma once

#include <map>

// #include "foo.h"
class Foo;

extern std::map<const char*, Foo> myMap;

void someFunction();

foo.cpp:

#include "foo.h"
#include "bar.h"

void Foo::doStuff(){ someFunction(); }

I strongly suggest using Method 3. As a general guide, it's better to use forward declarations in header files if you don't need the full definition of a class.