Error: Jump to case label in switch statement

Question

I wrote a program which involves use of switch statements, however on compilation it shows:

Error: Jump to case label.

Why does it do that?

#include <iostream> int main()  {     int choice;     std::cin >> choice;     switch(choice)     {       case 1:         int i=0;         break;       case 2: // error here      } } 

Answer 1

The problem is that variables declared in one case are still visible in the subsequent cases unless an explicit { } block is used, but they will not be initialized because the initialization code belongs to another case.

In the following code, if foo equals 1, everything is ok, but if it equals 2, we'll accidentally use the i variable which does exist but probably contains garbage.

switch(foo) {   case 1:     int i = 42; // i exists all the way to the end of the switch     dostuff(i);     break;   case 2:     dostuff(i*2); // i is *also* in scope here, but is not initialized! } 

Wrapping the case in an explicit block solves the problem:

switch(foo) {   case 1:     {         int i = 42; // i only exists within the { }         dostuff(i);         break;     }   case 2:     dostuff(123); // Now you cannot use i accidentally } 

Edit

To further elaborate, switch statements are just a particularly fancy kind of a goto. Here's an analoguous piece of code exhibiting the same issue but using a goto instead of a switch:

int main() {     if(rand() % 2) // Toss a coin         goto end;      int i = 42;    end:     // We either skipped the declaration of i or not,     // but either way the variable i exists here, because     // variable scopes are resolved at compile time.     // Whether the *initialization* code was run, though,     // depends on whether rand returned 0 or 1.     std::cout << i; } 

Answer 2

Declaration of new variables in case statements is what causing problems. Enclosing all case statements in {} will limit the scope of newly declared variables to the currently executing case which solves the problem.

switch(choice) {     case 1: {        // .......     }break;     case 2: {        // .......     }break;     case 3: {        // .......     }break; }