当我需要在my_template_library.h
中使用main_class.h
时,我的项目类似于此设置。
的main.cpp
#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
在我编写的模板标题库中,我首先在类中声明所有内容然后定义类外的所有内容,就像将类分成.h
和.cpp
时通常所做的那样代码,但在.cpp
末尾添加了.h
文件,内部包含警卫。只要我的模板库只包含一次,这种方法就可以正常工作,但是当它开始被包含在内时,我会遇到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。我已经将#pragma message
添加到my_template_library.h中以便从中理解这一点,您可以在此处看到代码中注释掉的内容。当我取消注释并运行代码时,我得到了
$ 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:
所以,我的问题是:
答案 0 :(得分:6)
为什么包括警卫不帮忙?
包含防护措施可防止相同编译单元内的冗余代码。
您收到关于不同编辑单元声称拥有相同功能定义的链接器错误。
如何防止这种情况发生?
您需要制作这些非模板成员函数inline
,以避免违反One Definition Rule。
一种方法是明确地将它们内联声明。
inline MyTemplateLibrary::MyTemplateLibrary(){}
或者,类定义中定义的函数是隐式内联的。
class MyTemplateLibrary
{
public:
MyTemplateLibrary() {}
void function()
{
std::cout << "function called!" << std::endl;
}
};