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

 

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?

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;
}

This question is 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. I believe this important detail was missed (or obfuscated) by the existing answers.

  • 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 local 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.

    • Ok. Just to clarify this strictly has nothing to do with the declaration. It relates only to "jumping over the initialization" (ISO C++ '03 6.7/3)

    A lot of the posts here have mentioned that jumping over the declaration may result in the variable "not being declared". This is not true. An POD object can be declared without an initializer but it will have an indeterminate value. For example: 

    switch (i)
{
   case 0:
     int j; // 'j' has indeterminate value
     j = 0; // 'j' initialized to 0, but this statement
            // is jumped when 'i == 1'
     break;
   case 1:
     ++j;   // 'j' is in scope here - but it has an indeterminate value
     break;
}

    Where the object is a non-POD or aggregate the compiler implicitly adds an initializer, and so it is not possible to jump over such a declaration: 

    class A {
public:
  A ();
};

switch (i)  // Error - jumping over initialization of 'A'
{
   case 0:
     A j;   // Compiler implicitly calls default constructor
     break;
   case 1:
     break;
}

    This limitation is not limited to the switch statement. It is also an error to use 'goto' to jump over an initialization: 

    goto LABEL;    // Error jumping over initialization
int j = 0; 
LABEL:
  ;

    A bit of trivia is that this is a difference between C++ and C. In C, it is not an error to jump over the initialization.

    As others have mentioned, the solution is to add a nested block so that the lifetime of the variable is limited to the individual case label.

    链接地址: http://www.djcxy.com/p/274.html

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