c++中头文件的相互依赖引发的问题

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先上代码：摘自essential c++

Triangular.h

#include "Triangular_iterator.h"

class Triangular
{
    friend int Triangular_iterator::operator * () const;

    ......

    private:
        .......
        static std::vector<int> elems;
};

Triangular_iterator.h

​
#include "Triangular.h"

class Triangular_iterator
{
    public:        
        ......
        int  operator * () const;

};

inline int Triangular_iterator::operator * () const
{
    check_integrity();
    return Triangular::elems[index];
}

​

error

error: 'Triangular' has not been declared|

原因：头文件的相互依赖

去掉Triangular_iterator.h中的#include “Triangular.h”产生同样的错误。

想一下先把Triangular.h文件中的#include "Triangular_iterator.h"展开

​#ifndef TRIANGULAR_H
#define TRIANGULAR_H

#ifndef TRIANGULAR_ITERATOR_H
#define TRIANGULAR_ITERATOR_H

#include "Triangular.h"

class Triangular_iterator
{
    public:
        ......
        int  operator * () const;

};

inline int Triangular_iterator::operator * () const
{
    check_integrity();
    return Triangular::elems[index];
}

#endif


class Triangular
{
    friend int Triangular_iterator::operator * () const;

    ......

    private:
        .......
        static std::vector<int> elems;
};

#endif

​

因为使用了条件编译，所以此时代码中的#include "Triangular.h"文件不会再被编译。

那么在解引用函数中

    return Triangular::elems[index];

自然会报错找不到Triangular

解决方案：

1.在.cpp文件中定义解引用操作符，包含#include "Triangular.h"文件

Triangular_iterator.cpp

#include "Triangular_iterator.h"
#include "Triangular.h"

int Triangular_iterator::operator * () const
{
    check_integrity();
    return Triangular::elems[index];
}

2.在Triangular类的定义后面定义解引用符，并声明为inline

Triangular.h

​
#include "Triangular_iterator.h"

class Triangular
{
    friend int Triangular_iterator::operator * () const;

    ......

    private:
        .......
        static std::vector<int> elems;
};

inline int Triangular_iterator::operator * () const
{
    check_integrity();
    return Triangular::elems[index];
}
​

在头文件里面定义一个函数要声明为inline，否则会产生重定义的错误，所以在类内部定义的成员函数都是inline的。