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What is double evaluation and why should it be avoided?

(4个答案)


去年关闭。




阅读combase.cpp代码,我发现以下内容:
/* We have to ensure that we DON'T use a max macro, since these will typically   */
/* lead to one of the parameters being evaluated twice.  Since we are worried    */
/* about concurrency, we can't afford to access the m_cRef twice since we can't  */
/* afford to run the risk that its value having changed between accesses.        */

    template<class T> inline static T ourmax( const T & a, const T & b )
    {
        return a > b ? a : b;
    }

我不明白为什么“最大宏导致对其中一个参数进行两次评估”?

最佳答案

考虑这样的用法 code sample :

#define max(a,b) (a>b?a:b)

int main()
{

  int a = 0;
  int b = 1;

  int c = max(a++, b++);

  cout << a << endl << b << endl;
  return 0;

}

目的可能是打印12,但是宏扩展为:
int c = a++ > b++ ? a++ : b++;
b递增两次,程序将打印13

因此,
在某些情况下,可以多次评估作为参数传递给宏的表达式。

关于c++ - 获取最大var时,使用函数和宏有什么区别,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/10695945/

10-11 23:20
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