在main.cpp以外的文件中包含标头时，链接器错误 [英] Linker error when including header in files other than main.cpp

问题描述

我正在尝试以某种方式在代码中添加一些实用程序函数和全局变量，以便可以在项目中所需的每个类中使用它们.我想在定义时使用.hpp文件，在实现时使用.cpp文件结束.

I'm trying to add some utility functions and global variables to my code in such a way that I can be able to use them in every class I want in my project. I would like to use a .hpp file for the definitions end a .cpp file for the implementation.

这是这两个文件的摘要:

This is a summary of these two files:

// This is Utilities.hpp
#ifndef utilities_hpp
#define utilities_hpp


namespace utils {

    int global_variable1;

    int global_variable2;

    void utility_function1(...);

    void utility_function2(...);

    void utility_function3(...);
}

#endif /* utilities_hpp */

和实现:

// This is Utilities.cpp
#include "Utilities.hpp"


namespace utils {

    void utility_function1(...) {
        // Some code
    }


    void utility_function2(...) {
        // Some code
    }


    void utility_function3(...) {
        // Some code
    }

}

除了我的 main.cpp 文件外，我还有另外两个类.我的 main.cpp 文件包含 Class1.hpp 标头，其中包含 Class2.hpp 标头.

Other than my main.cpp file I have two other classes. My main.cpp file includes Class1.hpp header that includes Class2.hpp header.

现在我认为我可以在 Class1.hpp 或 Class2.hpp 中放置另一个 #include"Utilities.hpp" 因为我已经在标题中使用了警卫.问题是，当我尝试执行此操作时，链接器将向我抛出此错误: Apple Mach-O Linker(ld)错误组-铛:错误:链接器命令失败，退出代码为1(使用-v查看调用)，我不明白为什么或怎么解决它.

Now I thought that I could put another #include "Utilities.hpp" in Class1.hpp or Class2.hpp without any problems since I've used the guards in that header. The thing is that when I try to do that the linker throws me this error: Apple Mach-O Linker (ld) Error Group - clang: error: linker command failed with exit code 1 (use -v to see invocation) and I can't understand why or what to do to solve it.

我正在macOS Sierra 10.12.4上使用Xcode 8.3.

I'm using Xcode 8.3 on a macOS Sierra 10.12.4.

希望我能解释我的问题，非常感谢.

I hope I was able to explain my problem, thank you all very much in advance.

推荐答案

您违反了单一定义规则.应该在标头中将 global_variable1 和 global_variable2 声明为 extern ，并在一个翻译单元(可能是Utilities.cpp)中进行定义.

You've violated the One Definition Rule. global_variable1 and global_variable2 should be declared extern in your header and defined in exactly one translation unit (probably Utilities.cpp).

您已经在一个标头中定义了全局变量，该标头包含在多个转换单元中，因此在main.cpp中定义了一个 utils :: global_variable1 ，在Utilities.cpp中定义了一个.关于链接时间，链接器无法知道要使用哪个 global_variable1 ，因此会引发错误.

You've defined global variables in a header that gets included in multiple translation units, so there's a utils::global_variable1 defined in main.cpp, and one in Utilities.cpp. When it comes to link time, the linker has no way to know which global_variable1 to use, so it throws an error.

要解决此问题，请将 extern 关键字添加到声明中，然后在"Utilities.cpp"中添加定义:

To fix it, add the extern keyword to your declarations and add a definition in "Utilities.cpp":

Utilities.hpp:

Utilities.hpp:

// This is Utilities.hpp
#ifndef utilities_hpp
#define utilities_hpp


namespace utils {

    extern int global_variable1;
  //^^^^^^ <-----HERE
    extern int global_variable2;
  //^^^^^^ <-----HERE
    void utility_function1(...);

    void utility_function2(...);

    void utility_function3(...);
}

#endif /* utilities_hpp */

Utilities.cpp:

Utilities.cpp:

// This is Utilities.cpp
#include "Utilities.hpp"

namespace utils {

    int global_variable1;  //<---- Definitions added
    int global_variable2;

    void utility_function1(...) {
        // Some code
    }

    void utility_function2(...) {
        // Some code
    }

    void utility_function3(...) {
        // Some code
    }
}

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