I came across this weird C++ program.
#include <iostream>
using namespace std;
int main()
{
int a = ({int x; cin >> x; x;});
cout << a;
}
Can anyone explain what is going on? What is this construct called?
I came across this weird C++ program.
#include <iostream>
using namespace std;
int main()
{
int a = ({int x; cin >> x; x;});
cout << a;
}
Can anyone explain what is going on? What is this construct called?
It creates an inline scope, declares x
within it, reads it from the standard input and the whole statement finally evaluates to x
, which is assigned to a
.
The comma operator works similarly, although it doesn't require a separate scope. For example:
int x;
int a = (cin >> x, x);
would do the same. All the statements connected with commas will be executed sequentially, and the result of the whole expression will be set to the value of the rightmost operand.
It assigns user input value to
a
and prints it out. it is done by using aStatement Expression
.Statement Expressions are a gnu gcc compiler extension and are not supported by the C/C++ standards. Hence, any code which uses statement expression is not standard conforming and non-portable.
The IBM XL C/C++ v7.0 also support Statement Expressions & its documentation explains them aptly:
Statement Expressions:
Always compile your code by selecting a standard in GCC: use one of the options
-ansi
,-std=c90
or-std=iso9899:1990
,-std=c++03
,-std=c++0x
; to obtain all the diagnostics required by the standard, you should also specify-pedantic
(or-pedantic-errors
if you want them to be errors rather than warnings).