C++ Tricks: Part I

Impress your interviewer

"Comma Expressions Hack"
Clearly, anyone who has done C++ knows the trick where you can initialize multiple variables of the same type at one time.

For example,
int a = 0, b= 1, c = 2;

This declares three variables with the same type, namely int. However, the comma is strictly an expression separator. This is quite useful when you hate the whole ordeal of using braces ("{}") for if-statements, loop type structures, and the like. You can use comma-separated expressions for "regular" statements. What do I mean by regular? Regular, in this context, means that an expression is not a "break" or a "return". These two (2) special statements are not "regular" expressions, not to be confused with a regular expression studied in theoretical computer science.

Therefore, you can do things like this:

if( a == 0 )
     b = 3, s = "hello", d = 4;

Multiple statements within an if-statement that doesn't exercise the utilization of braces. In any event, things like this should be properly avoided if adhering to coding standards within your academic or professional setting. But it's rather useful to know.

The "swap" without an extra variable.
Swapping two variables usually requires the use of another variable.

For example,

     int a = 3, b = 4;
     int tmp = a;
     // comma trick:
     a = b, b = tmp;

Therefore, we can do this without an extra variable, namely tmp.
It uses the XOR property of variables in C++.
A little clarification might help in this situation.
Say we have a and b as ints. Let's store the XOR of a and b into a variable named c:

         c = a ^ b;

Therefore, we know that:

        c ^ a == b
        c ^ b == a

We can use this feature, without having another variable, c.
If we do it correctly, and chain the XOR operation, there's no need for the extra variable.

       a ^= b;    // think of a holding the value that c would hold
       b ^= a;   // if a holds the value of c, then c ^b == a, so now b holds the value of a.
       a ^= b;  // if a held the value that c holds, then a ^ b, is actually (c ^ a) since b holds "a"
More compactly, you can do it as such:
      a ^= b ^= a ^= b;

Therefore, without using another variable, we have just swapped two integral types.

Note: These examples are for use with the C++ programming language. Though some of the tricks might work in the language, it is not wise to assume they all do.