写入仅标头的模板库时出现“多重定义”错误

当我需要使用 my_template_library.h 在里面 main_class.h .

主.cpp

#include "main_class.h"

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

main_类.h

#ifndef MAIN_CLASS_H
#define MAIN_CLASS_H

#include "my_template_library.h"

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

#endif // MAIN_CLASS_H

主类.cpp

#include <iostream>

#include "main_class.h"

MainClass::MainClass(){}

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

我的模板库.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

在我编写的模板头库中,我首先声明类中的所有内容,然后定义类之外的所有内容 .h 和 .cpp 代码,但带有 .cpp文件 在末尾添加的文件 .h时 ,内部包括防护装置。只要我的模板库只包含一次,这就很好了,但当它开始包含的次数超过一次时,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。我添加了 #pragma message 对mytemplatelibrary.h进行一些解释,您可以在代码中看到注释。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

因此,通过以下方式包含头文件:

1. main.cpp->main_类.h
2. main_类.h
3. main_class.cpp->main_类.h
4. my_template_library.h(自己?)

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?