switch Statement (C++)

 

The latest version of this topic can be found at switch Statement (C++).

Allows selection among multiple sections of code, depending on the value of an integral expression.

Syntax

  
      switch ( expression )  
   case constant-expression : statement  
   [default  : statement]  

Remarks

The expression must be of an integral type or of a class type for which there is an unambiguous conversion to integral type. Integral promotion is performed as described in Integral Promotions.

The switch statement body consists of a series of case labels and an optional default label. No two constant expressions in case statements can evaluate to the same value. The default label can appear only once. The labeled statements are not syntactic requirements, but the switch statement is meaningless without them. The default statement need not come at the end; it can appear anywhere in the body of the switch statement. A case or default label can only appear inside a switch statement.

The constant-expression in each case label is converted to the type of expression and compared with expression for equality. Control passes to the statement whose case constant-expression matches the value of expression. The resulting behavior is shown in the following table.

Switch Statement Behavior

Condition	Action
Converted value matches that of the promoted controlling expression.	Control is transferred to the statement following that label.
None of the constants match the constants in the case labels; a default label is present.	Control is transferred to the default label.
None of the constants match the constants in the case labels; default label is not present.	Control is transferred to the statement after the switch statement.

If a matching expression is found, control is not impeded by subsequent case or default labels. The break statement is used to stop execution and transfer control to the statement after the switch statement. Without a break statement, every statement from the matched case label to the end of the switch, including the default, is executed. For example:

// switch_statement1.cpp  
#include <stdio.h>  
  
int main() {  
   char *buffer = "Any character stream";  
   int capa, lettera, nota;  
   char c;  
   capa = lettera = nota = 0;  
  
   while ( c = *buffer++ )   // Walks buffer until NULL  
   {  
      switch ( c )  
      {  
         case 'A':  
            capa++;  
            break;  
         case 'a':  
            lettera++;  
            break;  
         default:  
            nota++;  
      }  
   }  
   printf_s( "\nUppercase a: %d\nLowercase a: %d\nTotal: %d\n",  
      capa, lettera, (capa + lettera + nota) );  
}  

In the above example, capa is incremented if c is an uppercase A. The break statement after capa++ terminates execution of the switch statement body and control passes to the while loop. Without the break statement, lettera and nota would also be incremented. A similar purpose is served by the break statement for case 'a'. If c is a lowercase a, lettera is incremented and the break statement terminates the switch statement body. If c is not an a or A, the default statement is executed.

An inner block of a switch statement can contain definitions with initializations as long as they are reachable — that is, not bypassed by all possible execution paths. Names introduced using these declarations have local scope. For example:

// switch_statement2.cpp  
// C2360 expected  
#include <iostream>  
using namespace std;  
int main(int argc, char *argv[])  
{  
   switch( tolower( *argv[1] ) )  
   {  
       // Error. Unreachable declaration.  
       char szChEntered[] = "Character entered was: ";  
  
   case 'a' :  
       {  
       // Declaration of szChEntered OK. Local scope.  
       char szChEntered[] = "Character entered was: ";  
       cout << szChEntered << "a\n";  
       }  
       break;  
  
   case 'b' :  
       // Value of szChEntered undefined.  
       cout << szChEntered << "b\n";  
       break;  
  
   default:  
       // Value of szChEntered undefined.  
       cout << szChEntered << "neither a nor b\n";  
       break;  
   }  
}  

A switch statement can be nested. In such cases, case or default labels associate with the closest switch statement that encloses them.

Microsoft Specific

Microsoft C does not limit the number of case values in a switch statement. The number is limited only by the available memory. ANSI C requires at least 257 case labels be allowed in a switch statement.

The default for Microsoft C is that the Microsoft extensions are enabled. Use the /Za compiler option to disable these extensions.

END Microsoft Specific

See Also

Selection Statements
Keywords
(NOTINBUILD) Using Labels in the case Statement