18.1: Conditional Operator

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Overview

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	Meaning	Comments
? :	conditional	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.

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

conditional = option 1
age > 17 ? cout << "You can vote." : cout << "You can’t vote.";

conditional = option 2
cout << (age > 17 ? "You can vote." : "You can’t vote.");

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 flow 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.

general format
test expression ? expression true : expression false

Definitions

Conditional: A trinary C++ operator that acts like an if then else control structure.