Order of Header files being included in a C++ app

Question

Is there software/tool to see the order in which header files are being included after compilation of a C++ application? I find myself often running into circular dependency issues and seeing the "undefined reference" error message :(

UPDATE #1

I am familiar with header include guards and I am using them, but for some reason I'm still having an issue with circular dependency.

In my case, I have many situations where a class A uses class B, and vice versa. In those cases, I use forward declaration for example "class B;" at the top of A.h and vice versa.

Should it ever be the case where I'll need to do "#include "B.h"" at the top of A.h? Or is "class B;" sufficient on its' own at the top of A.h?

UPDATE #2

Hello, below is a snippet of the code that I am having trouble compiling/linking. There are 3 classes below:

A.cpp

#include "A.h"

namespace sef
{
    A::A() {
        // TODO Auto-generated constructor stub
        b = 0;
    }

    A::~A() {
        // TODO Auto-generated destructor stub
    }

    bool A::execute()
    {
        C::connectAndSaveFile();
        b->start();
        return true;
    }
}

A.h

#ifndef A_H_
#define A_H_

//#include "B.h"
class B;
#include "C.h"

namespace sef {

    class A {
    public:
        B* b;
        bool execute();

        A();
        virtual ~A();
    };

}

#endif /* A_H_ */

B.cpp

#include "B.h"

B::B() {
    // TODO Auto-generated constructor stub
    engine = 0;
    bool result = engine->execute();
    cout << result << endl;
}

B::~B() {
    // TODO Auto-generated destructor stub
}

void B::start()
{

    cout << "B::start()" << endl;
}

B.h

#ifndef B_H_
#define B_H_

#include <iostream>
#include <string>
using namespace std;

//#include "A.h"

namespace sef {
    class A;
}

class B
{
public:
    sef::A* engine;

    B();
    virtual ~B();
    void start();
};

#endif /* B_H_ */

C.cpp

#include "C.h"

C::C() {
    // TODO Auto-generated constructor stub

}

C::~C() {
    // TODO Auto-generated destructor stub
}

void C::connectAndSaveFile()
{
    cout << "C::connectAndSaveFile()" << endl;
}

C.h

#ifndef C_H_
#define C_H_

#include <iostream>
#include <string>
using namespace std;

class C {
public:
    C();
    virtual ~C();

    static void connectAndSaveFile();
};

#endif /* C_H_ */

I seem to be getting the error:

***g++ -O0 -g3 -Wall -c -fmessage-length=0 -o "src\\A.o" "..\\src\\A.cpp" 
..\src\A.cpp: In member function 'bool sef::A::execute()':
..\src\A.cpp:23:4: error: invalid use of incomplete type 'class B'
In file included from ..\src\A.cpp:7:0:
..\src\A.h:12:7: error: forward declaration of 'class B'***

Answer 1

Your forward declarations are a good start, but in addition, A.cpp will need to include B.h and C.h since it dereferences a B* and calls a static function in C. Similarly, B.cpp needs A.h since it dereferences an A*. There may be other issues too, but that should get you on the right track.

Long story short, a forward declaration of class A only tells the compiler that A is the name of a class. This is enough for it to know what you mean when you say A* or A& (or a couple of other things), but if you want to actually call a function on an A, or access a member variable, or return/take an A by value, or have a variable of type A¹, you need the complete definition of A from its header.

¹ Basically, any operation that requires the compiler to know either sizeof(A) or information about its members/ancestors.

---

Also, this wasn't your question, but this code will give you undefined behavior:

B::B() {
    // TODO Auto-generated constructor stub
    engine = 0;
    bool result = engine->execute();
    cout << result << endl;
}

You're effectively setting engine to null and then calling a function on it.

And even further off the original topic, it's generally considered bad practice to using namespace std in a header file because everyone who includes it will be forced to use that namespace too, whether they want to or not.