C ++结构的多重定义 [英] C++ Multiple Definition of Struct

问题描述

我有一个包含简单结构的标头 util.hpp :

I have header util.hpp containing a simple struct:

// util.hpp

struct Point {
  float x;
  float y;
};

两个cpp文件，我们称它们为 a.cpp 和 b.cpp ，它们都包含 util.hpp :

Two cpp files, let's call them a.cpp and b.cpp, both include util.hpp:

// a.cpp

#include "util.hpp"

void funcA(float _x, float _y) {
  Point p;
  p.x = _x;
  p.y = _y;
  // ...
}

// b.cpp

#include "util.hpp"

void funcB(float _x, float _y) {
  Point p;
  p.x = _x;
  p.y = _y;
  // ...
}

int main() {
  // ...
}

当我分别编译 a.cpp 和 b.cpp 并将它们链接在一起时，我没有任何错误.

When I compile a.cpp and b.cpp individually and then link them together I get no errors.

那是为什么?因为我在两个文件中都包含了 util.hpp ，所以我们不是对 struct Point 进行双重定义吗?例如，当我像这样将变量定义添加到 util.hpp 时:

Why is that? Since I include util.hpp in both files, wouldn't we have a double definition of struct Point? When I for example add a variable definition to util.hpp like this:

// util.hpp

struct Point {
  float x;
  float y;
};

// New variable
int foo;

链接时出现以下错误:

g++ a.o b.o -o test -O0
b.o:(.bss+0x0): multiple definition of `foo'
a.o:(.bss+0x0): first defined here

这对我来说很有意义，但是为什么结构不会出现相同的错误?

which makes sense to me, but why doesn't the same error occur for the struct?

推荐答案

b.o :(.bss + 0x0):'foo'的多个定义

b.o:(.bss+0x0): multiple definition of `foo'

拥有

// util.hpp
...
// New variable
int foo;

每次包含 util.hpp 时，您再次定义全局变量foo，从而产生错误

each time you include util.hpp you define again the global variable foo, producing your error

只需在头文件中声明( extern int foo; )并将其定义在一个源文件上

in the header file just declare it (extern int foo;) and define it on one source file

结构点没有相同的问题，因为这是类型定义，而不是全局变量定义

There is not the same problem for the struct Point because this is a type definition, not a global variable definition

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