Get decltype to deduce member variable constness

Consider the following code:

Code:

---

template <typename T>
class really_long_complicated_typename
{
public:
    really_long_complicated_typename(T& x) : x(x) {}
private:
    T& x;
};

template <typename T>
really_long_complicated_typename<T> f(T& t)
{
    return really_long_complicated_typename<T>(t);
}


class A
{
    class C {};
    C c;
public:
    A() {}

    decltype(f(c)) get_c() const { return f(c); }
};

int main()
{
    const A a;
    a.get_c();
}

---

g++ gives the error:

Code:

---

test.cpp: In member function really_long_complicated_typename<A::C> A::get_c() const:
test.cpp:24: error: conversion from really_long_complicated_typename<const A::C> to non-scalar type really_long_complicated_typename<A::C> requested

---

The problem is that the expression f(c) in the return statement is really_long_complicated_typename<const A::C>, but decltype doesn't "know" that this is a const member function, so decltype(f(c)) is deduced to be really_long_complicated_typename<A::C>.

Is there a way around this?

have you tried "auto get_c() const -> decltype(f(c)) { return f(c); }" or the more explicit "auto get_c() const -> decltype(f( this->c )) { return f(c); }" instead ?

Quote:

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Originally Posted by superbonzo

have you tried "auto get_c() const -> decltype(f(c)) { return f(c); }" or the more explicit "auto get_c() const -> decltype(f( this->c )) { return f(c); }" instead ?

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The first makes no difference.

The second gives the error "invalid use of 'this' at top level".

Perhaps it's just my compiler that doesn't support this feature fully? I am using g++ 4.4.