c++在多个源文件中包含具有相同类实现的不同头文件

Question

例如

啊.h

class Dummy {
public:
  Dummy() { std::cout << "a.h" << std::endl; }
};

b.h

class Dummy {
public:
  Dummy() { std::cout << "b.h" << std::endl; }
};

抄送

#include "a.h"

void test() {
  Dummy a;
}

d.cc

#include "b.h"

int main() {
  Dummy a;
  return 0;
}

然后用命令编译源文件

g++ d.cc c.cc

输出是

b.h

但是有命令

g++ c.cc d.cc

输出是

a.h

我的问题是为什么没有multiple definition错误，为什么输出取决于编译的顺序？

Answer 1

您的程序有未定义的行为。总结 C++ 标准的做法，这里是 [basic.def.odr/6]我强调:

There can be more than one definition of a class type, [...] in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then

• each definition of D shall consist of the same sequence of tokens; and

• [...]

[...] If the definitions of D satisfy all these requirements, then the behavior is as if there were a single definition of D. If the definitions of D do not satisfy these requirements, then the behavior is undefined.

因此您观察到两种不同的行为。完全可以接受，因为语言对您应该看到的行为没有任何限制。你违反了契约(Contract)，所以没有任何保证。

现在，从实际的角度来看，您所看到的只是 GCC 在上述契约(Contract)下运行。它假设您不会违反它(即使您违反了)，并且只是忽略 Dummy 和/或其成员的任何后续重新定义。第一个“胜出”。