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Thinking in C++
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The ternary operator

The ternary if-else is unusual because it has three operands. It is truly an operator because it produces a value, unlike the ordinary if-else statement. It consists of three expressions: if the first expression (followed by a ?) evaluates to true, the expression following the ? is evaluated and its result becomes the value produced by the operator. If the first expression is false, the third expression (following a :) is executed and its result becomes the value produced by the operator.

The conditional operator can be used for its side effects or for the value it produces. Here’s a code fragment that demonstrates both:

a = --b ? b : (b = -99);

Here, the conditional produces the rvalue. a is assigned to the value of b if the result of decrementing b is nonzero. If b became zero, a and b are both assigned to -99. b is always decremented, but it is assigned to -99 only if the decrement causes b to become 0. A similar statement can be used without the “a =” just for its side effects:

--b ? b : (b = -99);

Here the second B is superfluous, since the value produced by the operator is unused. An expression is required between the ? and :. In this case, the expression could simply be a constant that might make the code run a bit faster.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire