Control Statements

Control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. (see Chapter 2 for a discussion of automatic and other object lifetimes)

C++ supports the following control statements:

break;

A break statement can be used only in the body of a loop or switch statement. It terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.

In a nested loop or switch, the break applies only to the innermost statement. To break out of multiple loops and switches, you must use a goto statement or redesign the block to avoid nested loops and switches (by factoring the inner statement into a separate function, for example). Example 4-7 shows a simple use of break.

continue;

A continue statement can be used only in the body of a loop. It causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating (if the condition is true). In a for loop, the iterate-expr is evaluated before testing the condition. Example 4-8 shows how continue is used in a loop.

goto identifier ;

The goto statement transfers control to the statement that has identifier as a label. The goto statement and the labeled statement must be in the same function. Jumping into a block is usually a bad idea. In particular, if the jump bypasses the declaration of an object, the results are undefined unless the object has POD type and no initializer. (See Chapter 2 for information about POD types and initializers.) Example 4-9 shows some uses of goto statements.

return ;, return expr ;

The return statement transfers execution out of a function to the caller. The first form does not return a value, so it should be used only in functions of type void, in constructors, and in destructors. The latter form cannot be used in constructors and destructors. In a function of type void, you can use the second form, but only if expr has type void. See Chapter 5 for more information about returning from functions.

goto statements

The value of expr is converted to the function's return type and returned to the caller. The compiler is free to construct a temporary object and copy expr when returning. Some compilers optimize away the extra copy.

If execution reaches the last statement of a function without executing a return statement, an implicit return; is assumed. If the function has a non-void return type, the behavior is undefined.

The main function is special. If it ends without a return statement, return 0; is assumed.

identifier : statement

Any statement can have a label. A label is used only as a target of a goto statement. Label identifiers must be unique within a function; the scope of a label is the function in which it is declared. Label identifiers do not conflict with any other identifiers.

A statement can have multiple labels, including case and default labels.