c++: why can we declare a variable in case of switch
int i;
i = 2;
switch(i)
{
case 1:
int k;
break;
case 2:
k = 1;
cout<<k<<endl;
break;
}
I don't know why the code above works.
Here, we can never go into case 1 but why we can use k in case 2?
There are actually 2 questions:
1. Why can I declare a variable after case label?
It's because in C++ label has to be in form:
N3337 6.1/1
labeled-statement:
...
case constant-expression : statement ...
And in C++ declaration statement is also considered as statement (as opposed to C ):
N3337 6/1:
statement:
...
...
2. Why can I jump over variable declaration and then use it?
Because: N3337 6.7/3
It is possible to transfer into a block, but not in a way that bypasses declarations with initialization . A program that jumps (The transfer from the condition of a switch statement to a case label is considered a jump in this respect.)
from a point where a variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has scalar type , class type with a trivial default constructor and a trivial destructor, a cv-qualified version of one of these types, or an array of one of the preceding types and is declared without an initializer (8.5).
Since k is of scalar type, and is not initialized at point of declaration jumping over it's declaration is possible. This is semantically equivalent:
goto label;
int x;
label:
cout << x << endl;
However this wouldn't work if x was initialized at point of declaration:
goto label;
int x = 58; //error, jumping over declaration with initialization
label:
cout << x << endl;
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