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

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 2

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 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();
    }
};