Is there a way to force the "most vexing parse" to be an error, even on a class by class basis?

Question

Is it possible (with any modification of class A) to have the following work? i.e., make the most vexing parse an error?

class A {
};

int main() {
    A a(); // can this be forced to be an error??
    A b;   // this should work
}

Unlikely. Remember that this is parsed as a function declaration, so you would need a modification to the type `A` that prevents `A` from being a return type of a function (even if no such function is ever even defined) while still being instantiable. If this is a real problem for you, I'd try to find a static analysis tool that will catch possible instances of this. — us2012, Sep 25 '13 at 15:42
Why? It's not an error. It's your expectations that are erred. — Luchian Grigore, Sep 25 '13 at 15:44
This is **not** the [most vexing parse](http://en.wikipedia.org/wiki). It is simply a function declaration. — Pete Becker, Sep 25 '13 at 15:52
If `A a()` was an error, how would you declare a function that takes no arguments and returns an object of type `A`? — Pete Becker, Sep 25 '13 at 15:55
I could envision a checker in clang to disable local function declarations and complains there. Really, how many people do use local function declarations in real code? — David Rodríguez - dribeas, Sep 25 '13 at 15:57
Oops, the link that I intended was [most vexing parse](http://en.wikipedia.org/wiki/Most_vexing_parse). — Pete Becker, Sep 25 '13 at 21:00

bames53 · Accepted Answer · 2013-09-25T16:03:13.390

No modification of the class A will have any effect on how a declaration A a(); is parsed. The parser determines that this is a function declaration before it even bothers to look at the definition of A. In fact the definition of A doesn't even need to be visible to parse this statement; A forward declaration is sufficient.

However compilers generally have a warning for this and you can probably turn that into an error. For example with clang you can use the flag -Werror=vexing-parse.

struct A;

A a(); // no error

int main() {
    A a(); // error
}

clang++ -std=c++11 -Weverything -Werror=vexing-parse main.cpp

main.cpp:6:8: error: empty parentheses interpreted as a function declaration [-Werror,-Wvexing-parse]
    A a();
       ^~
main.cpp:6:8: note: replace parentheses with an initializer to declare a variable
    A a();
       ^~
       {}
1 error generated.

Although technically speaking A a(); isn't the syntax known as the most vexing parse. That would be:

A a(B());

score 1 · Answer 2 · answered Sep 25 '13 at 15:57

There is no way in the current language specification which could make this code an error. Normally, you just get a funny error message when you try to use the "object". However, some compilers do warn about the situation (e.g. clang):

clang++ -W -Wall  -Werror -c -o vexing.o vexing.cpp
vexing.cpp:5:8: error: empty parentheses interpreted as a function declaration [-Werror,-Wvexing-parse]
   A a(); // can this be forced to be an error??
      ^~

Is there a way to force the "most vexing parse" to be an error, even on a class by class basis?

2 Answers2

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