Why can't variables be declared in a switch statement?


Question

I've always wondered this - why can't you declare variables after a case label in a switch statement? In C++ you can declare variables pretty much anywhere (and declaring them close to first use is obviously a good thing) but the following still won't work:

switch (val)  
{  
case VAL:  
  // This won't work
  int newVal = 42;  
  break;
case ANOTHER_VAL:  
  ...
  break;
}  

The above gives me the following error (MSC):

initialization of 'newVal' is skipped by 'case' label

This seems to be a limitation in other languages too. Why is this such a problem?

1
890
1/15/2018 5:58:36 AM

Accepted Answer

Case statements are only labels. This means the compiler will interpret this as a jump directly to the label. In C++, the problem here is one of scope. Your curly brackets define the scope as everything inside the switch statement. This means that you are left with a scope where a jump will be performed further into the code skipping the initialization. The correct way to handle this is to define a scope specific to that case statement and define your variable within it.

switch (val)
{   
case VAL:  
{
  // This will work
  int newVal = 42;  
  break;
}
case ANOTHER_VAL:  
...
break;
}
1081
10/27/2016 6:12:39 PM

This question is was originally tagged as [C] and [C++] at the same time. The original code is indeed invalid in both C and C++, but for completely different unrelated reasons.

  • In C++ this code is invalid because the case ANOTHER_VAL: label jumps into the scope of variable newVal bypassing its initialization. Jumps that bypass initialization of automatic objects are illegal in C++. This side of the issue is correctly addressed by most answers.

  • However, in C language bypassing variable initialization is not an error. Jumping into the scope of a variable over its initialization is legal in C. It simply means that the variable is left uninitialized. The original code does not compile in C for a completely different reason. Label case VAL: in the original code is attached to the declaration of variable newVal. In C language declarations are not statements. They cannot be labeled. And this is what causes the error when this code is interpreted as C code.

    switch (val)  
    {  
    case VAL:             /* <- C error is here */
      int newVal = 42;  
      break;
    case ANOTHER_VAL:     /* <- C++ error is here */
      ...
      break;
    }
    

Adding an extra {} block fixes both C++ and C problems, even though these problems happen to be very different. On the C++ side it restricts the scope of newVal, making sure that case ANOTHER_VAL: no longer jumps into that scope, which eliminates the C++ issue. On the C side that extra {} introduces a compound statement, thus making the case VAL: label to apply to a statement, which eliminates the C issue.

  • In C case the problem can be easily solved without the {}. Just add an empty statement after the case VAL: label and the code will become valid

    switch (val)  
    {  
    case VAL:;            /* Now it works in C! */
      int newVal = 42;  
      break;
    case ANOTHER_VAL:  
      ...
      break;
    }
    

    Note that even though it is now valid from C point of view, it remains invalid from C++ point of view.

  • Symmetrically, in C++ case the the problem can be easily solved without the {}. Just remove the initializer from variable declaration and the code will become valid

    switch (val)  
    {  
    case VAL: 
      int newVal;
      newVal = 42;  
      break;
    case ANOTHER_VAL:     /* Now it works in C++! */
      ...
      break;
    }
    

    Note that even though it is now valid from C++ point of view, it remains invalid from C point of view.


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