C++11 Uniform Initialization and function overloading

Question

Simple program:

void f(const std::string& s);
void f(const char* p);
f({});

Why clang calls f((const char*)nullptr)? I've expected compiler warning about ambiguous call.

Answer 1

This is covered in the draft C++11 standard section 13.3.3.1.5 [over.ics.list] which says:

Otherwise, if the parameter type is not a class:

[...]

• if the initializer list has no elements, the implicit conversion sequence is the identity conversion. [ Example:

  void f(int);
  f( { } ); // OK: identity conversion
  

—end example ]

and so an identity conversion would be better than a constructor call.

The reason why we get a nullptr is because it is value initializing the pointer. From section 8.5.4 [dcl.init.list]:

List-initialization of an object or reference of type T is defined as follows:

[...]

• Otherwise, if the initializer list has no elements, the object is value-initialized. [ Example:

  int** pp {}; // initialized to null pointer
  

—end example ]