Which are the most common pitfalls with conditional (if) statements in C++? [closed]

Answer 1

Formatting and scoping errors

• if(mycondition);
  {
      // Why this code is always executed ???
  }
  

  There's a superfluous ; after the if() statement.

• if(mycondition)
       statement1();
       statement2(); // Why this code is always executed ???
  

  The code is equivalent to

  if(mycondition) {
       statement1();
  }
  statement2();
  

  statement2(); is outside the scope of the conditional block. Add {} to group statements.

• if (mycondition)
       if (mycondition2)
           statement1();
   else
       statement2();
  

  The code is equivalent to

  if(mycondition) {
   if (mycondition2)
       statement1();
   else
       statement2();
  }
  

  else apply on previous if. Add {}:

  if (mycondition) {
      if (mycondition2)
           statement1();
  }
  else
      statement2();
  

---

The same applies for any wrongly placed ; in loop statements like

•   for(int x = 0;x < 5;++x);
                         // ^
    {
         // statements executed only once
    }
  

or

• while(x < 5);
           // ^
  {
      // statements executed only once
  }