Thread: Why I can't declare a std::string type in a switch/case box?

Here is the code.

Code:

int main(int argc, char* argv[]){                                                                                                                                            
    int i=2;                                                                                                                                                                 
    switch (i)                                                                                                                                                               
    {                                                                                                                                                                        
        case 0: 
            cout<<0<<endl;                                                                                                                                                   
            break;                                                                                                                                                           
        case 1:                                                                                                                                                              
            string test="";                                                                                                                                                  
            cout<<1<<endl;                                                                                                                                                   
            break;                                                                                                                                                           
        case 2:                                                                                                                                                              
            cout<<2<<endl;                                                                                                                                                   
            break;                                                                                                                                                           
        default:                                                                                                                                                             
            break;                                                                                                                                                           
    }                                                                                                                                                                        
                                                                                                                                                                             
    return 0;                                                                                                                                                                
}

as you can see, when declare a string test it reports errors

main.cc: In function `int main(int, char**)':
main.cc:24: error: jump to case label
main.cc:21: error: crosses initialization of `std::string test'
main.cc:27: error: jump to case label
main.cc:21: error: crosses initialization of `std::string test'
main.cc:24: warning: destructor needed for `std::string test'
main.cc:24: warning: where case label appears here
main.cc:24: warning: (enclose actions of previous case statements requiring
destructors in their own scope.)
main.cc:27: warning: destructor needed for `std::string test'
main.cc:27: warning: where case label appears here
make: *** [main.o] Error 1

I don't understand the errors, what is "jump to case label" ?
why I "crosses initialization of `std::string test'" ?

Interestingly, if I put the declaration in a box, it's OK

Code:

int main(int argc, char* argv[]){                                                                                                                                            
    int i=2;                                                                                                                                                                 
    switch (i)                                                                                                                                                               
    {                                                                                                                                                                        
        case 0:                                                                                                                                                              
            cout<<0<<endl;                                                                                                                                                   
            break;                                                                                                                                                           
        case 1:                                                                                                                                                              
        {                                                                                                                                                                    
            string test="";                                                                                                                                                  
        }                                                                                                                                                                    
            cout<<1<<endl;                                                                                                                                                   
            break;                                                                                                                                                           
        case 2:                                                                                                                                                              
            cout<<2<<endl;                                                                                                                                                   
            break;                                                                                                                                                           
        default:                                                                                                                                                             
            break;                                                                                                                                                           
    }                                                                                                                                                                        
                                                                                                                                                                             
    return 0;                                                                                                                                                                
}

Interesting little corner of the language.

A switch case is the same as a labeled "goto" statement. You are not allowed to "goto" a location in a block following the construction point of a local variable, and you can't do it with a switch either.

The problem is, the string is still in scope at case 2. Each labeled section of a switch statement is NOT its own scope. So going to case 2 would bring the variable into scope even though it has not been constructed.

Placing it in a block creates a new scope, so that the variable's scope is limited to case 1.

EDIT: Some code to help clarify:

Code:

    goto foo;
    std::string x;
foo:
    std::cout << x << std::endl;

The declaration brings a variable 'x' into scope -- however, the line itself, which constructs the variable, is skipped by the goto statement. So x is used without being initialized. The same thing is happening here.