一个非常奇怪的C ++行为

Question

While I was trying to make my DX11 engine, I had a very strange problem. I couln't find what was wrong. So, I decided to make some simpler code and try to get the same error. This:

Source.cpp

#include <iostream>
#include "Header.h"

using namespace std;

struct1 g_struct1(12);
struct2 g_struct2(&g_struct1);

int main()
{
    cout << "Value of g_struct1.num: " << g_struct1.num << endl;
    cout << "Value of g_struct2.copynum: " << g_struct2.copynum << endl;
    getchar();
    return 0;
}

Header.h

#ifndef _HEADER_H_
#define _HEADER_H_

#include "Header1.h"
#include "Header2.h"

#endif

Source1.cpp

#include "Header1.h"

struct1::struct1(int number)
{
    num = number;
};

Header1.h

#ifndef _HEADER1_H_
#define _HEADER1_H_

#include "Header1.h"
#include "Header2.h"

struct struct1
{
    struct1(int number);
    int num;
};

#endif

Source2.cpp

#include "Header2.h"

struct2::struct2(struct1 *par1)
{
    copynum = par1->num;
};

Header2.h

#ifndef _HEADER2_H_
#define _HEADER2_H_

#include "Header1.h"
#include "Header2.h"

struct struct2
{
    struct2(struct1 *par1);
    int copynum;
};

#endif

Error

1>------ Build started: Project: ConsoleApplication1, Configuration: Debug Win32 ------
1>  Source.cpp
1>c:\users\<My Username>\desktop\consoleapplication1\consoleapplication1\header2.h(10): error C2061: syntax error : identifier 'struct1'
1>c:\users\<My Username>\desktop\consoleapplication1\consoleapplication1\source.cpp(8): error C2664: 'struct2::struct2(const struct2 &)' : cannot convert argument 1 from 'struct1 *' to 'const struct2 &'
1>          Reason: cannot convert from 'struct1 *' to 'const struct2'
1>          No constructor could take the source type, or constructor overload resolution was ambiguous
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Of course, the second error doesn't matter because the compiler thinks it's a copy constructor.

If I change struct2::struct2(struct1 *par1) to struct2::struct2(void *par1) and then cast void *par1 to struct1* , it works fine.

Well, if you made it this far, thanks. And sorry for bad English.

Answer 1

Your headers have circular dependencies. They even include themself which is silly. The end result is that neither header gets processed properly.

In Header1 remove both #include lines. In Header2 remove #include "Header2.h" . This should fix the problem.

Answer 2

It does not work because during the definition of struct2 the struct1 is not defined and you are using the non-defined struct poniter in your construction.

Simply fix would be

struct struct2
{
    struct2(struct struct1 *par1); // note the struct struct1 *
    int copynum;
};

Also you don't have to depend on Header1.h in Header2.h (but you do in Source2.cpp) if you do use this.