Case statement with braces

Question

Why does a case statement allow declarations within braces but not without them?

For example, the following is not OK

switch (op->name) {
    case 0:
        int a = 2;
    case 1:  
        int b = 3;
}

But the following is OK:

switch (op->name) {
    case 0: 
       {int a = 2;}
    case 1:  
       {int b = 3;}
}

What does the braces resolve that, without them, a declaration would be ambiguous to the compiler? To me (a beginner in C) it seems like each case statement should have an implied braces until the next case/default/end-of-switch, but that is obviously wrong!

Answer 1

Why does a case statement allow declarations within braces but not without them?

My compiler emits this error message:

error: a label can only be part of a statement and a declaration is not a statement

That's pretty clear I think. Labels, including case labels, are not considered independent statements or declarations, but rather components of statements. When control branches to a label, whether via a switch or a goto, it is the statement of which the label is a part to which the program jumps. Not to the label itself, which typically has no runtime representation.

Declarations such as int a; are not statements, and labels cannot be part of declarations. On the other hand, blocks such as { int a; } are statements, and as statements, they can have labels.

There's no fundamental underlying constraint here. It would have been possible for C syntax to have been defined differently, so that both of your examples were valid. It just wasn't.

Answer 2

The reason this is invalid:

switch (op->name) {
    case 0:
        int a = 2;
    case 1:  
        int b = 3;
}

Is because you have a label (in this case a case label) in front of a declaration. A label may only appear before a statement.

Using braces works because they are used to denote a compound statement, which is a particular type of statement, and a statement can immediately precede a label.

Formally, a label is part of a labeled statement. This is spelled out in section 6.8.1 of the C standard:

labeled-statement:
  identifier : statement
  case constant-expression : statement
  default : statement