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10 February, 2015 - 15:37

The conditional operator is unique in that it has three operands separated by two unconnected operator symbols. All other C++ operators are either unary (one operator and one operand) or binary (one operator and two operands). On the "Abbreviated Precedence Chart for C++ Operators" the conditional operator has the word "trinary" in the comments column. This prefix "tri" means three, thus three operands.

C++ Operator



? :


trinary - three operands with two operators

As an operator it produces a value for the expression. An easy way to explain the conditional operator is to convert an "if then else" control structure to an expression using the conditional operator.

Example 17.1: if then else

if (age  > 17)  {  cout  << "You can vote.";  }else  {  cout  << "You can't vote.";  } 

Example 17.2: conditional = option 1

age > 17 ? cout << "You can vote." : cout << "You can't vote.";

Example 17.3: conditional = option 2

cout <<  (age  > l7 ? "You can vote." : "You can't vote."); 
Note: The use of parenthesizes is needed because of the precedence of operators. The conditional expression is of lower precedence than the insertion (writing) operator.

The first operand is a test expression similar to those that control program fow in control structures. This type of expression is also known as a Boolean expression because they create a Boolean answer of true or false. If the test is true the second operand becomes the value of the expression. If false, the third operand becomes the value of the expression. The operators of the question mark and colon separate the three operands.

Example 17.4: general format

test expression ? expression true : expression false