Why does "if (char a = f())" compile whereas "if ((char a = f()))" does not?

Question

#include <iostream>

char f()
{
    return 0;
}

int main()
{
    // Compiles
    if (char a = f())
        std::cout << a;
        
    // Does not compile (causes a compilation error)
    // if ((char a = f()))
    //     std::cout << a;

    return 0;
}

One can declare a local variable and assign a value to it inside an if statement as such:

if (char a = f())

However, adding an additional pair of parentheses, leading to if ((char a = f())), causes the following compilation error:

error: expected primary-expression before ‘char’

Why is that? What is the difference between both? Why is the additional pair of parentheses not just considered redundant?

To have the same scope, and a parenthesized expression, try either `if (char a = (f()))` or `if (char a; (a = f()))` — Ben Voigt, Jun 23 '22 at 19:26
@BenVoigt My intent was to be able to do something like `if ( !(char a = f()) )`, which your second suggestion allows (`if ( char a; !(a = f()) )`), but it is not available in C++14. — Ramanewbie, Jun 23 '22 at 20:30
@Ramanewbie: That particular code can be handled by swapping the controlled-block of `if` with the controlled-block of `else`, instead of using `!`. In particular, accessing `a` within the controlled-block of `if (!a)` is pretty useless.... `a` must be zero or you wouldn't enter the `if` at all. — Ben Voigt, Jun 23 '22 at 21:05
@BenVoigt "accessing `a` within the controlled-block of `if (!a)` is pretty useless" > Yes. I should have said instead that my intent was to do something like `if ( (char a = f()) > 0 )`. Then it could have been useful to access `a` within the controlled-block. "That particular code can be handled by swapping the controlled-block of `if` with the controlled-block of `else`" > That only stands if there is an `else`. — Ramanewbie, Jun 23 '22 at 21:42

score 3 · Accepted Answer · answered Jun 23 '22 at 18:49

To put it simply, C++ syntax allows the condition inside an if statement to be either an expression or a declaration.

So, char a = f() is a declaration (of the variable named a).

But (char a = f()) is not a declaration (and is also not an expression convertible to bool).

Why does "if (char a = f())" compile whereas "if ((char a = f()))" does not?

1 Answers1