g++ 和 clang++ 中的宏扩展

Question

#include <iostream>
#define DOUBLE1(x) x + x
#define DOUBLE2(x) 2 * x
int main() {
    int a = 2;
    std::cout << DOUBLE2(a++) << " " << DOUBLE1(++a) << std::endl;
    std::cout << a;
}

该程序的执行会在

4 9 5

中打印

clang++

，并在

4 10 5

中打印

g++

。 我对操作优先级如何发生感到困惑。

注意： 我没有编写此代码，这是课程中作为

predict output question

提出的要求。

Answer 1

你的课程应该教你这样的东西：

#include <iostream>

// Macros are a last resort 
// and usually only used when
// you need to concatenate some strings to 
// make new lines of code
//#define DOUBLE1(x) x + x
//#define DOUBLE2(x) 2 * x

int double1(int a)
{
    return a+a;
}

int double2(int a)
{
    return 2*a;
}

int main() {
    int a = 2;
    
    // epxlicitly model evaluation order
    // the compiler will not care you write
    // extra lines. It will still create
    // good (optimized) assembly code.
    // and at least now you can set breakpoints
    // to check what your code is doing in a debugger
    int a2 = double2(a);
    a++;
    ++a;
    int a3 = double1(a);
        
    std::cout << a2 << " " << a3 << "\n"; //std::endl; only use endl when you need to flush
    std::cout << a;
}