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

c++ switch-statement
Rob picture Rob · Sep 18, 2008 · Viewed 294k times · Source

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?

Answer

TJ Seabrooks picture TJ Seabrooks · Sep 18, 2008

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;
}

Related questions

Why the switch statement cannot be applied on strings?

Compiling the following code and got the error of type illegal. int main() { // Compilation error - switch expression of type illegal switch(std::string("raj")) { case"sda": } } You cannot use string in either switch or case. Why? Is there any …

Read More
How to break out of a loop from inside a switch?

I'm writing some code that looks like this: while(true) { switch(msg->state) { case MSGTYPE: // ... break; // ... more stuff ... case DONE: break; // **HERE, I want to break out of the loop itself** } } Is there any direct way to do that? …

Read More
Advantage of switch over if-else statement

What's the best practice for using a switch statement vs using an if statement for 30 unsigned enumerations where about 10 have an expected action (that presently is the same action). Performance and space need to be considered but are not critical. …

Read More