c++ - 编写仅标头模板库时出现“多重定义”错误

Question

我的项目类似于这个设置，当我需要my_template_library.h在main_class.h.

主文件

#include "main_class.h"

int main()
{
    MainClass m;
    return m.exec();
}

main_class.h

#ifndef MAIN_CLASS_H
#define MAIN_CLASS_H

#include "my_template_library.h"

class MainClass {
public:
    MainClass();
    int exec();
};

#endif // MAIN_CLASS_H

main_class.cpp

#include <iostream>

#include "main_class.h"

MainClass::MainClass(){}

int MainClass::exec()
{
    std::cout << "exec!" << std::endl;
    return 0;
}

my_template_library.h

#ifndef MY_TEMPLATE_LIBRARY_H
#define MY_TEMPLATE_LIBRARY_H

#include <iostream>

//#pragma message ("I'm being included past the include guards!")

class MyTemplateLibrary
{
public:
    MyTemplateLibrary();

    void function();
};

MyTemplateLibrary::MyTemplateLibrary(){}

void MyTemplateLibrary::function()
{
    std::cout << "function called!" << std::endl;
}

#endif // MY_TEMPLATE_LIBRARY_H

在我编写的仅包含头文件的模板库中，我首先声明类中的所有内容，然后定义类外部的所有内容，就像您通常将类分成.h和.cpp代码时所做的那样，但在.cpp末尾添加文件.h，在 include守卫。只要我的模板库只包含一次，这工作就很好，但是当它开始包含更多时，我遇到了一些非常令人困惑的问题。

$ g++ -o test main.cpp main_class.h main_class.cpp my_template_library.h 
/tmp/ccuFlEDZ.o: In function `MyTemplateLibrary::MyTemplateLibrary()':
main_class.cpp:(.text+0x0): multiple definition of `MyTemplateLibrary::MyTemplateLibrary()'
/tmp/ccZikorv.o:main.cpp:(.text+0x0): first defined here
/tmp/ccuFlEDZ.o: In function `MyTemplateLibrary::MyTemplateLibrary()':
main_class.cpp:(.text+0x0): multiple definition of `MyTemplateLibrary::MyTemplateLibrary()'
/tmp/ccZikorv.o:main.cpp:(.text+0x0): first defined here
/tmp/ccuFlEDZ.o: In function `MyTemplateLibrary::function()':
main_class.cpp:(.text+0xa): multiple definition of `MyTemplateLibrary::function()'
/tmp/ccZikorv.o:main.cpp:(.text+0xa): first defined here
collect2: error: ld returned 1 exit status

我对发生了什么感到困惑。我已将其添加#pragma message到 my_template_library.h 以对此有所了解，您在此处看到代码中已将其注释掉。当我取消注释并运行我得到的代码时

$ g++ -o test main.cpp main_class.h main_class.cpp my_template_library.h 
In file included from main_class.h:4:0,
                 from main.cpp:1:
my_template_library.h:6:63: note: #pragma message: I'm being included past the include guards!
 #pragma message ("I'm being included past the include guards!")
                                                           ^
In file included from main_class.h:4:0:
my_template_library.h:6:63: note: #pragma message: I'm being included past the include guards!
 #pragma message ("I'm being included past the include guards!")
                                                           ^
In file included from main_class.h:4:0,
             from main_class.cpp:3:
my_template_library.h:6:63: note: #pragma message: I'm being included past the include guards!
 #pragma message ("I'm being included past the include guards!")
                                                           ^
my_template_library.h:6:63: note: #pragma message: I'm being included past the include guards!
 #pragma message ("I'm being included past the include guards!")
                                                           ^
/tmp/ccmawdhP.o: In function `MyTemplateLibrary::MyTemplateLibrary()':
main_class.cpp:(.text+0x0): multiple definition of `MyTemplateLibrary::MyTemplateLibrary()'
/tmp/cc4XSnui.o:main.cpp:(.text+0x0): first defined here
/tmp/ccmawdhP.o: In function `MyTemplateLibrary::MyTemplateLibrary()':
main_class.cpp:(.text+0x0): multiple definition of `MyTemplateLibrary::MyTemplateLibrary()'
/tmp/cc4XSnui.o:main.cpp:(.text+0x0): first defined here
/tmp/ccmawdhP.o: In function `MyTemplateLibrary::function()':
main_class.cpp:(.text+0xa): multiple definition of `MyTemplateLibrary::function()'
/tmp/cc4XSnui.o:main.cpp:(.text+0xa): first defined here
collect2: error: ld returned 1 exit status

所以头文件是通过以下方式包含的：

1. main.cpp -> main_class.h
2. main_class.h
3. main_class.cpp -> main_class.h
4. my_template_library.h （本身？）

所以，我的问题是：

1. 为什么包括警卫没有帮助？
2. 如何防止这种情况发生？

Answer 1

为什么包括警卫没有帮助？

包含警卫可防止同一编译单元中的冗余代码。

您收到关于声称拥有相同函数定义的不同编译单元的链接器错误。

如何防止这种情况发生？

您需要制作那些非模板成员函数inline以避免违反One Definition Rule。

One way is to explicitly declare them inline.

inline MyTemplateLibrary::MyTemplateLibrary(){}

Alternately, functions defined within the class definition are implicitly inline.

class MyTemplateLibrary
{
public:
    MyTemplateLibrary() {}

    void function()
    {
        std::cout << "function called!" << std::endl;
    }
};