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

Answer 1

Misconceptions of conditional expressions

• if(x = 1) // ...
  

  Equality comparisons are expressed using the ==. = is an assignment, and the result is evaluated as a cast to bool. I.e. any value evaluated to != 0 results in true. As a prevention mechanism consider the below expressions:

•    if(1 = x)  // invalid assignment compilation error!
     if(1 == x) // valid equality comparison
  

  A wrong use of an assignment operator can be avoided by always placing the constant on the left hand side of the expression. The compiler will flag any mistake triggering an invalid assignment error.

• if(answer == 'y' || 'Y')
  

  _{Variations: if(answer == 'y','Y')}
  Conditions must be tested with separate comparisons. The || operator binding doesn't do what's expected here. Use if(answer == 'y' || answer == 'Y')instead.

• if (0 < x < 42)  
  

  Valid syntax in Python, with expected behaviour, that syntax is valid in C++, but parsed as if ((0 < x) < 42) so false/true converted to 0/1 and then tested against < 42 -> always true. Condition must be tested with separate comparisons: if (0 < x && x < 42)

Answer 2

if (north) {

} else if (south) {

} else if (west) {

} else if (east) {

}

potential missing else-clause for wrong direction.

Funny placement of {} because they add them, misplace them and then delete the wrong ones.

if (a) {
}
if (b) {
}
if (c) {
}

missing else as only one of them should be done even if more is true.

Answer 3

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
  }