CODEWITHSUNDEEP
c++c++11initializer-listargument-dependent-lookup
Joel
Joel

Reputation: 1315

initializer_list and argument-dependent lookup

I'm trying to use an std::initializer_list as an argument in a function that uses argument-dependent lookup (ADL). But I don't get it to work and I don't understand why. The following is a minimal failing example:

    #include <initializer_list>
    #include <iostream>

    class Foo {
    public:
      inline friend void bar(std::initializer_list<Foo> v) {
        std::cout << "size = " << v.size() << std::endl;
      }
    };

    void baz(std::initializer_list<Foo> v) {
      std::cout << "size = " << v.size() << std::endl;
    }

    int main(){
      Foo a;
      //bar({a,a});   // error: use of undeclared identifier 'bar'
      baz({a,a});   // works

      return 0;
    }

As seen above, an equivalent global function works just fine. Why does the above not work?

I'm using clang on OS X 10.10.

Upvotes: 4

Views: 213

Answers (2)

David Rodr&#237;guez - dribeas
David Rodr&#237;guez - dribeas

Reputation: 208353

I believe that the problem is that the subexpression1 { a, a } does not really have a type, and as such it does not have associated types or namespaces which in turn means that ADL does not kick in. If you have a function in the global namespace, normal lookup will find it, and it will find that { a, a } can match the function call as the initializer for std::initializer_list<Foo>.

1 The syntax { a, a } is called braced-init-list and it is not really an expression (or subexpression) in the language.

Upvotes: 8

Mooing Duck
Mooing Duck

Reputation: 66922

When the compiler sees bar({a,a}), it doesn't know the type of the arguments, so it searches for bar in the global namespace (::), and nowhere else. If you changed that line to Foo{a,a}, then it knows the argument is a Foo, and so also searches the Foo class for functions.

Upvotes: 2

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