#ifdef 在同一个头文件中返回不同的值

Question

我有两个 C++ 文件 main.cpp 和 client.cpp，以及一个头文件 action.h。我正在尝试使用预处理器指令模拟一个简单的客户端-服务器场景。代码如下：

//main.cpp
#include "action.h" 
extern void connect(); 
static Action server; 
int main()
{
    server.init(); 
    connect(); 
    server.execute(); 
    return 0; 
}

//client.cpp
#define CLIENT 
#include "action.h" 
void connect()
{
    Action client; 
    client.init(); 
    client.execute(); 
}

和头文件。

#include <iostream>
struct Action
{
    #ifdef CLIENT
    int data = 10;
    #else
    int data = 13; 
    #endif
    inline void init()
    {
        #ifdef CLIENT
        std::cout << "Client " << data << std::endl;
        data = -1;
        #else
        std::cout << "Server " << data << std::endl;
        data = 0;
        #endif
    }
    inline void execute()
    {
        #ifdef CLIENT
        std::cout << "Server is " << data << std::endl;
        #else
        std::cout << "Number of clients is " << data << std::endl;
        #endif
    } 
};

当我运行这个时，我得到以下结果：

Server 13

Server 10

Number of clients is 0

Number of clients is 0

现在我的问题是为什么 #ifdef CLIENT 块在 action.h (int data = 10) 的声明部分中执行，而 #else 块在 [=20= 中执行] 和 execute() 功能，当我从 client.cpp 文件中调用它们时？为什么不是所有三个调用都转到 #ifdef 部分？

Answer 1

此程序表现出未定义的行为。它违反了 One Definition Rule (ODR)。具体这部分：

[basic.def.odr]/6 There can be more than one definition of a class type, ... inline function with external linkage ... 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

(6.1) — each definition of D shall consist of the same sequence of tokens...
...

If the definitions of D do not satisfy these requirements, then the behavior is undefined.

在你的例子中，你有两个 class Action 的定义以及它的内联成员函数 Action::init 和 Action::execute，它们由不同的标记序列组成，多亏了宏技巧。

#ifdef 在同一个头文件中返回不同的值

#ifdef is returning different values inside the same header file

c++

preprocessor-directive