Friday, November 19, 2010

Switch Statements

The following program compiles:

#include <iostream>
using namespace std;
int main() {
  int i = 1;
  int const j = 1;
  switch (-1)          cout<<"switch(-1)\n"       ;
  switch ( 0)          cout<<"switch( 0)\n"       ;
  switch ( 1)      i : cout<<"switch( 1) i\n"     ;
  switch ( 1) case 1 : cout<<"switch( 1) case 1\n";
  switch ( 1) case j : cout<<"switch( 1) case j\n";
  return 0;
I was reading the grammer for the switch statement in Stroustrup's C++ Programming Language, Third Edition, and I saw all of the above represented in the production rules. I thought the 'i:' fragment was very strange, until I realized this 'i' has nothing to do with the 'int i'. The 'i:' is just a statement label. There is no "case in front of it. The first 3 switch statements end up doing nothing. The last two switch statements print. For another example of a crazy switch statement, see Duff's Device: Reference: The C++ Programming Language by Bjarne Stroustrup, Addison-Wesley, 1997, p. 803.

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