Issue
Consider the following snippet:
auto f() { return void({}); }
int main() { f(); }
What's exactly the {} in void({})?
How is it interpreted?
Just out of curiosity, of course. Let's go a bit further anyway.
Note that both GCC 6.1 and clang 3.8 compile it with no errors (-std=c++14 -pedantic).
The latter doesn't complain, the former shows a warning:
warning: list-initializer for non-class type must not be parenthesized
Using -pedantic-errors instead, GCC ends with an error while clang compiles it.
Is this discrepancy an error of one of the two compilers? I mean, is it valid code that should be accepted or not?
Solution
Conversions to void type as well as the possibility to returning a void value have been present in the C++ language since the very beginning. The only part that raises questions is the role of {} in this context.
A quick experiment with clang
int a({});
generates an error message saying
error: cannot initialize a variable of type 'int' with an rvalue of type 'void'
which indicates that Clang interprets {} as a void value. This appears to be a non-standard behavior. I don't see any place in language specification that would say that {} should produce a void value in this context.
But since this happens to be the case in clang, there isn't anything unusual in void({}) compiling in clang. Any value in C++ can be converted to void type, meaning that as long as the compiler accepts {} in this context, the rest just follows naturally.
In GCC, it is actually an error in -pedantic-errors mode:
error: list-initializer for non-class type must not be parenthesized
So formally it is an "error", not a "warning" in GCC.
The combination of opening ({ and closing }) makes these compilers to interpret it as a GNU C language extension known as Statement Expression (which is incidentally supported by clang as well). This is, for example, what makes the following code compile
int a = ({ 3; });
Under that extension, the expression ({}) is seen as a statement expression of void type. However, this conflicts with the uniform initialization syntax in C++.
Answered By - AnT stands with Russia Answer Checked By - Mary Flores (WPSolving Volunteer)