In which case if(a=b) is a good idea? [duplicate]

Question

Possible Duplicate:
Inadvertent use of = instead of ==

C++ Compilers let your know via warnings that you wrote

if( a = b ) { //...

and that it might be a mistake, that you certainly wanted to write

if( a == b ) { //...

But is there a case where the warning should be ignored because it's a good way to use this "feature"? I don't see any code clarity reason possible so is there a case where its useful?

Answer 1

Two possible reasons:

1. Assign & Check

   The = operator (when not overriden) normally returns the value that it assigned. This is to allow statements such as a=b=c=3. In the context of your question, it also allows you to do something like this:

   bool global;//a global variable
   
   //a function
   int foo(bool x){
   
      //assign the value of x to global
      //if x is equal to true, return 4
      if (global=x)
          return 4;
   
      //otherwise return 3
      return 3;
   }
   

   ...which is equivalent to but shorter than:

   bool global;//a global variable
   
   //a function
   int foo(bool x){
   
      //assign the value of x to global
      global=x;
   
      //if x is equal to true, return 4
      if (global==true)
          return 4;
   
      //otherwise return 3
      return 3;
   }
   

   Also, it should be noted (as stated by Billy ONeal in a comment below) that this can also work when the left-hand argument of the = operator is actually a class with a conversion operator specified for a type which can be coerced (implicitly converted) to a bool. In other words, (a=b) will evaulate to true or false if a is of a type which can be coerced to a boolean value.

   So the following is a similar situation to the above, except the left-hand argument to = is an object and not a bool:

   #include <iostream>
   using namespace std;
   
   class Foo {
   public:
       operator bool (){ return true; }
       Foo(){}
   };
   
   int main(){
       Foo a;
       Foo b;
   
       if (a=b)
           cout<<"true";
       else
           cout<<"false";
   }
   
   //output: true 
   

   Note: At the time of this writing, the code formatting above is bugged. My code (check the source) actually features proper indenting, shift operators and line spacing. The <'s are supposed to be <'s, and there aren't supposed to be enourmous gaps between each line.

2. Overridden = operator

   Since C++ allows the overriding of operators, sometimes = will be overriden to do something other than what it does with primitive types. In these cases, the performing the = operation on an object could return a boolean (if that's how the = operator was overridden for that object type).

   So the following code would perform the = operation on a with b as an argument. Then it would conditionally execute some code depending on the return value of that operation:

   if (a=b){
      //execute some code
   }
   

   Here, a would have to be an object and b would be of the correct type as defined by the overriding of the = operator for objects of a's type. To learn more about operator overriding, see this wikipedia article which includes C++ examples: Wikipedia article on operator overriding