Re: regarding Auto and decltype

From:
Nick Hounsome <nick.hounsome@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 25 May 2010 12:54:49 CST
Message-ID:
<5500c0b6-4db6-4a92-9e3b-008b3bbe3cb9@y21g2000vba.googlegroups.com>
On 24 May, 16:45, ManicQin <manic...@gmail.com> wrote:

Hello everybody.
In Scott Meyers lecture notes he states that one of the differences
between Auto and decltype is that the decltype does not evaluate the
expression.

I have a question regarding the evaluation of the expression, in the
next scenario what should I expect:

class B
{
public:
        B()
        {

        }
        virtual B* Clone()
        {
                cout << "B" << endl;
                return new B();
        }

};

class D : public B
{
public:
        D(){}
        virtual D* Clone()
        {
                cout << "D" << endl;
                return new D();
        }

};

int main()
{
        B* tmp = new D();
        auto test1 = tmp->Clone(); //returns D*!!!
        decltype(tmp->Clone()) test2 = tmp->Clone();
return 0;

}

Please note That D::Clone overloads with a different return type.

In my understanding if the "auto" is evaluating so it means that the
type of test1 should be D*, but VS10 understands different :) what am
I missing?


It's evaluated at runtime - The type is still the compile time return
type of B::Clone() i.e. B*.

All it means is that if you need an uninitialized variable you have to
use decltype.

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