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)
- break;
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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 .
Example 4-7. Using break to exit a loop
// One way to implement the find_if standard algorithm. template<typename InIter, typename Predicate> InIter find_if(InIter first, InIter last, Predicate pred) { for ( ; first != last; ++first) if (pred(*first)) break; return first; } - continue;
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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.
Example 4-8. Using continue in a loop
#include <cmath> #include <iostream> #include <istream> #include <limits> #include <ostream> int main( ) { using std::cin; using std::cout; while(true) { cout << "Enter a number: "; double x; cin >> x; if (cin.eof( ) cin.bad( )) // Input error: exit break; else if (cin.fail( )) { // Invalid input: skip the rest of the line cin.clear( ); cin.ignore(std::numeric_limits<int>::max( ), '\n'); continue; } cout << "sqrt(" << x << ")=" << std::sqrt(x) << std::endl; } } - goto identifier ;
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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.
Example 4-9. goto statements
#include <iostream> #include <ostream> int main(int argc, char* argv[]) { int matrix[4][5]; for (int i = 0; i < 4; ++i) for (int j = 0; j < 5; ++j) if (! (std::cin >> matrix[i][j])) goto error; goto end; error: std::cerr << "Need 20 values for the matrix\n"; end: return 0; } - return ;
- return expr ;
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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.
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
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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.
