Labeled Statements
The latest version of this topic can be found at Labeled Statements.
Labels are used to transfer program control directly to the specified statement.
identifier : statement
case constant-expression : statement
default : statement
The scope of a label is the entire function in which it is declared.
Remarks
There are three types of labeled statements. All use a colon to separate some type of label from the statement. The case and default labels are specific to case statements.
#include <iostream>
using namespace std;
void test_label(int x) {
if (x == 1){
goto label1;
}
goto label2;
label1:
cout << "in label1" << endl;
return;
label2:
cout << "in label2" << endl;
return;
}
int main() {
test_label(1); // in label1
test_label(2); // in label2
}
The goto statement
The appearance of an identifier label in the source program declares a label. Only a goto statement can transfer control to an identifier label. The following code fragment illustrates use of the goto statement and an identifier label:
A label cannot appear by itself but must always be attached to a statement. If a label is needed by itself, place a null statement after the label.
The label has function scope and cannot be redeclared within the function. However, the same name can be used as a label in different functions.
// labels_with_goto.cpp
// compile with: /EHsc
#include <iostream>
int main() {
using namespace std;
goto Test2;
cout << "testing" << endl;
Test2:
cerr << "At Test2 label." << endl;
}
//Output: At Test2 label.
The case statement
Labels that appear after the case keyword cannot also appear outside a switch statement. (This restriction also applies to the default keyword.) The following code fragment shows the correct use of case labels:
// Sample Microsoft Windows message processing loop.
switch( msg )
{
case WM_TIMER: // Process timer event.
SetClassWord( hWnd, GCW_HICON, ahIcon[nIcon++] );
ShowWindow( hWnd, SW_SHOWNA );
nIcon %= 14;
Yield();
break;
case WM_PAINT:
memset( &ps, 0x00, sizeof(PAINTSTRUCT) );
hDC = BeginPaint( hWnd, &ps );
EndPaint( hWnd, &ps );
break;
default:
// This choice is taken for all messages not specifically
// covered by a case statement.
return DefWindowProc( hWnd, Message, wParam, lParam );
break;
}
Labels in the case statement
Labels that appear after the case keyword cannot also appear outside a switch statement. (This restriction also applies to the default keyword.) The following code fragment shows the correct use of case labels:
// Sample Microsoft Windows message processing loop.
switch( msg )
{
case WM_TIMER: // Process timer event.
SetClassWord( hWnd, GCW_HICON, ahIcon[nIcon++] );
ShowWindow( hWnd, SW_SHOWNA );
nIcon %= 14;
Yield();
break;
case WM_PAINT:
// Obtain a handle to the device context.
// BeginPaint will send WM_ERASEBKGND if appropriate.
memset( &ps, 0x00, sizeof(PAINTSTRUCT) );
hDC = BeginPaint( hWnd, &ps );
// Inform Windows that painting is complete.
EndPaint( hWnd, &ps );
break;
case WM_CLOSE:
// Close this window and all child windows.
KillTimer( hWnd, TIMER1 );
DestroyWindow( hWnd );
if ( hWnd == hWndMain )
PostQuitMessage( 0 ); // Quit the application.
break;
default:
// This choice is taken for all messages not specifically
// covered by a case statement.
return DefWindowProc( hWnd, Message, wParam, lParam );
break;
}
Labels in the goto statement
The appearance of an identifier label in the source program declares a label. Only a goto statement can transfer control to an identifier label. The following code fragment illustrates use of the goto statement and an identifier label:
A label cannot appear by itself but must always be attached to a statement. If a label is needed by itself, place a null statement after the label.
The label has function scope and cannot be redeclared within the function. However, the same name can be used as a label in different functions.
// labels_with_goto.cpp
// compile with: /EHsc
#include <iostream>
int main() {
using namespace std;
goto Test2;
cout << "testing" << endl;
Test2:
cerr << "At Test2 label." << endl;
// At Test2 label.
}