Consider the following code:
#include <iostream>
using namespace std;
int main()
{
int x, y, i;
cin >> x >> y >> i;
switch(i) {
case 1:
// int r = x + y; -- OK
int r = 1; // Failed to Compile
cout << r;
break;
case 2:
r = x - y;
cout << r;
break;
};
}
G++ complains crosses initialization of 'int r'
. My questions are:
crosses initialization
?x + y
pass the compilation, but the latter failed?crosses initialization
?I know I should use brackets to specify the scope of r
, but I want to know why, for example why non-POD could not be defined in a multi-case switch statement.
The version with int r = x + y;
won't compile either.
The problem is that it is possible for r
to come to scope without its initializer being executed. The code would compile fine if you removed the initializer completely (i.e. the line would read int r;
).
The best thing you can do is to limit the scope of the variable. That way you'll satisfy both the compiler and the reader.
switch(i)
{
case 1:
{
int r = 1;
cout << r;
}
break;
case 2:
{
int r = x - y;
cout << r;
}
break;
};
The Standard says (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 from a point where a local variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has POD type (3.9) and is declared without an initializer (8.5).