如何使用在命名空间中的单独标题中定义的类 [英] How to use class defined in a separate header within a namespace

问题描述

我有一个命名空间，我想在其中定义一个类。这个类是相当复杂的，所以我宁愿在一个单独的头文件中定义它，但即使最简单的代码给我一个未定义的引用错误。

I have a namespace in which I'd like to define a class. The class is rather complex so I'd rather define it in a separate header file, but even the simplest code gives me an "undefined reference" error.

main.cpp

#include <iostream>
namespace A {
    #include "C.hpp"
}

int main()
{
    A::C foo;
    std::cout << foo.member << std::endl;
    return 0;
}

C.hpp

class C {
    public:
     C();
     int member;
}

C.cpp

C::C()
{
    this->member = 10;
}

当我运行 g ++ C.cpp main.cpp 我得到main.cpp :(。文本+ 0x10）：未定义的引用`A :: C :: C（）'错误。我想这是构造函数的C :: C（）定义是不对的，但我不知道如何解决它。

When I run g++ C.cpp main.cpp I get "main.cpp:(.text+0x10): undefined reference to `A::C::C()'" error. I suppose that it's the C::C() definition of the constructor that is somehow wrong, but I'm uncertain how to fix it.

推荐答案

namespace A {
    #include "C.hpp"
}

这是一件非常奇怪的事情。

This is a very odd thing to do.

这会将标题中声明的所有内容放在名为 A ;具体来说，它给你一个 A :: C 的声明，这是与 :: C 不同的类

This places everything declared in the header inside a namespace called A; specifically, it gives you a declaration of A::C, which is a different class to the ::C you get when you include the header without a surrounding namespace.

您为 :: C :: C（）提供了一个定义;但 main.cpp 需要定义 A :: C :: C（），因为它是类使用。

You have provided a definition for ::C::C(); but main.cpp requires a definition for A::C::C(), since that is the class it uses. That's not defined anywhere, hence the error.

将 C 正确地放入命名空间A 将命名空间移动到头文件（并修复 C.cpp 以使用该命名空间），或除去命名空间A 。

Either put C properly into namespace A by moving the namespace to the header file (and fix C.cpp to use that namespace), or get rid of namespace A.

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