decltype as a return type in class member function

Question

I got error compiling below code.

struct B{
    double operator()(){
        return 1.0;
    }
};

struct A {
    auto func() -> decltype(b())
    {
        return b();
    }

    B b;
};

However, if I reorganize the A , it compiles.

gcc 4.8 said that 'b' was not declared in this scope.

struct A {
    B b;
    auto func() -> decltype(b())
    {
        return b();
    }
};

So, what is wrong with the first??

Answer 1

Is it valid?

Your last example is well-formed, while the first one is not (so GCC is correct).

Paragraph 3.4.1/7 on unqualified name lookup specifies:

A name used in the definition of a class X outside of a member function body , default argument, brace-or- equal-initializer of a non-static data member, or nested class definition shall be declared in one of the following ways :

— before its use in class X or be a member of a base class of X (10.2), or

— [...]

And what follows are other conditions that do not apply in your case.

Answer 2

The definition of the class is processed it two passes: first the member declarations are collected, including function signatures , and then the bodies of definitions are parsed.

The function body therefore has access to all member declarations, including subsequent ones, but the function prototype only sees preceding declarations.

Answer 3

You can also get it to work like this:

struct B
{
    double operator()()
    {
        return 1.0;
    }
};

// my implementation does not have std::declval
template < typename T > T&& declval();

struct A
{
    B b;
    auto func() -> decltype(declval<B>().operator()())
    {
        return b();
    }
};

edit: or since B is in scope already anyway no need for auto, -> decltype and declval

struct A
{
    B b;
    decltype(Q()()) func()
    {
        return b();
    }
};