switch statement (C++)
Allows selection among multiple sections of code, depending on the value of an integral expression.
Syntax
selection-statement:
switch(init-statementoptC++17condition)statement
init-statement:
expression-statement
simple-declaration
condition:
expression
attribute-specifier-seqoptdecl-specifier-seqdeclaratorbrace-or-equal-initializer
labeled-statement:
caseconstant-expression:statement
default:statement
Remarks
A switch statement causes control to transfer to one labeled-statement in its statement body, depending on the value of condition.
The condition must have an integral type, or be a class type that has an unambiguous conversion to integral type. Integral promotion takes place as described in Standard conversions.
The switch statement body consists of a series of case labels and an optional default label. A labeled-statement is one of these labels and the statements that follow. The labeled statements aren't syntactic requirements, but the switch statement is meaningless without them. No two constant-expression values in case statements may evaluate to the same value. The default label may appear only once. The default statement is often placed at the end, but it can appear anywhere in the switch statement body. A case or default label can only appear inside a switch statement.
The constant-expression in each case label is converted to a constant value that's the same type as condition. Then, it's compared with condition for equality. Control passes to the first statement after the case constant-expression value that matches the value of condition. 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; no default label is present. |
Control is transferred to the statement after the switch statement. |
If a matching expression is found, execution can continue through later 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() {
const char *buffer = "Any character stream";
int uppercase_A, lowercase_a, other;
char c;
uppercase_A = lowercase_a = other = 0;
while ( c = *buffer++ ) // Walks buffer until NULL
{
switch ( c )
{
case 'A':
uppercase_A++;
break;
case 'a':
lowercase_a++;
break;
default:
other++;
}
}
printf_s( "\nUppercase A: %d\nLowercase a: %d\nTotal: %d\n",
uppercase_A, lowercase_a, (uppercase_A + lowercase_a + other) );
}
In the above example, uppercase_A is incremented if c is an uppercase 'A'. The break statement after uppercase_A++ terminates execution of the switch statement body and control passes to the while loop. Without the break statement, execution would "fall through" to the next labeled statement, so that lowercase_a and other would also be incremented. A similar purpose is served by the break statement for case 'a'. If c is a lowercase 'a', lowercase_a is incremented and the break statement terminates the switch statement body. If c isn't an 'a' or 'A', the default statement is executed.
Visual Studio 2017 and later (available in /std:c++17 mode and later): The [[fallthrough]] attribute is specified in the C++17 standard. You can use it in a switch statement. It's a hint to the compiler, or anyone who reads the code, that fall-through behavior is intentional. The Microsoft C++ compiler currently doesn't warn on fallthrough behavior, so this attribute has no effect on compiler behavior. In the example, the attribute gets applied to an empty statement within the unterminated labeled statement. In other words, the semicolon is necessary.
int main()
{
int n = 5;
switch (n)
{
case 1:
a();
break;
case 2:
b();
d();
[[fallthrough]]; // I meant to do this!
case 3:
c();
break;
default:
d();
break;
}
return 0;
}
Visual Studio 2017 version 15.3 and later (available in /std:c++17 mode and later): A switch statement may have an init-statement clause, which ends with a semicolon. It introduces and initializes a variable whose scope is limited to the block of the switch statement:
switch (Gadget gadget(args); auto s = gadget.get_status())
{
case status::good:
gadget.zip();
break;
case status::bad:
throw BadGadget();
};
An inner block of a switch statement can contain definitions with initializers as long as they're 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. When nested, the case or default labels associate with the closest switch statement that encloses them.
Microsoft-specific behavior
Microsoft C++ doesn't limit the number of case values in a switch statement. The number is limited only by the available memory.