c++ - 是否允许类在程序中的不同翻译单元中有不同的定义？

Question

如果类在每个翻译单元中最多定义一次，那么在不同的翻译单元中定义不同的类是否合式？

用例是在没有动态分配的情况下访问实现细节。 C++ 代码将对已由 C 库分配的指针进行操作。

为了举例，请忽略内存泄漏。

通用.hpp

#pragma once

namespace Test {

class Impl;

class A {
    void *ptr;
    A(void *ptr) : ptr(ptr) {}
    friend class Impl;

   public:
    int plus_one();
};

class B {
    void *ptr;
    B(void *ptr) : ptr(ptr) {}
    friend class Impl;

   public:
    int plus_two();
};

class Factory {
   public:
    A getA(int val);
    B getB(int val);
};

}  // namespace Test

A.cpp

#include "common.hpp"

namespace Test {

class Impl {
   public:
    static int as_int(A *a) { return *static_cast<int *>(a->ptr) + 1; }
};

int A::plus_one() { return Impl{}.as_int(this); }

}  // namespace Test

B.cpp

#include "common.hpp"

namespace Test {

class Impl {
   public:
    static int as_int(B *b) { return *static_cast<int *>(b->ptr) + 2; }
};

int B::plus_two() { return Impl{}.as_int(this); }

}  // namespace Test

工厂.cpp

#include "common.hpp"

namespace Test {

class Impl {
   public:
    static A getA(int val) { return A(new int{val}); }
    static B getB(int val) { return B(new int{val}); }
};

A Factory::getA(int val) { return Impl{}.getA(val); }
B Factory::getB(int val) { return Impl{}.getB(val); }

}  // namespace Test

主要.cpp

#include <iostream>

#include "common.hpp"

int main() {
    Test::Factory factory;
    std::cout << factory.getA(1).plus_one() << std::endl;
    std::cout << factory.getB(1).plus_two() << std::endl;
    return 0;
}

输出:

$ g++ A.cpp B.cpp Factory.cpp main.cpp -o test 
$ ./test
2
3

Answer 1

不行，同一个类类型不允许有不同的定义。您的程序直接违反了 ODR，并表现出未定义的行为。

[basic.def.odr]

6 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.

您的两个定义在它们的 token 序列上已经很明显不同，因此不支持 ODR 的条件。