Question

reading combase.cpp code, I find following:

/* 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;       
    } 

I don’t understand why “max macro leads to one of the parameters being evaluated twice”?

Answer 1

Consider an usage like in this 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;

}

The intention probably was to print 1 and 2, but macro expands to:

int c = a++ > b++ ? a++ : b++;

b gets incremented twice, and the program prints 1 and 3.

Hence,
In some cases, expressions passed as arguments to macros can be evaluated more than once.