Re: Why does my template need a type?

From:
=?windows-1252?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 12 Mar 2013 17:47:31 CST
Message-ID:
<khoa18$5u5$1@dont-email.me>
Am 12.03.2013 22:33, schrieb DeMarcus:

Hi,

Usually I want to separate the declaration from the definition also
for templates, but I bumped into an interesting error when doing that
recently. Consider this separation.

template<typename T>
class A
{
public:
     A& getMe( A& arg ); // Separated implementation below.
};

template<typename T>
A& A<T>::getMe( A& arg )
{
     return *this;
}

int main()
{
     A<int> a;
     A<int> b;
     a.getMe( b );
     return 0;
}

This code gives me on gcc 4.7.2.
error: invalid use of template-name ?A? without an argument list

But if I change the return type to look like the following, it
compiles.

template<typename T>
A<T>& A<T>::getMe( A& arg )
{
     return *this;
}

My question is; why do I get a compilation error for the return type
but not for the argument 'arg' that is still just an A& ?


I think the reason is that in your out-of-class definition, the injected
class name has not been inserted at that point (Take this with a grain
of salt). IMO 9 p2 says:

"A class-name is inserted into the scope in which it is declared
immediately after the class-name is seen. The class-name is also
inserted into the scope of the class itself; this is known as the
injected-class-name."

This happens not before the "A<T>::" in your example code. A different
way of fixing your problem would be to take advantage of the
"trailing-return-type" syntax, such as:

template<typename T>
auto A<T>::getMe( A& arg )-> A&
{
    return *this;
}

Now, the return type "A&" is *after* the start of the scope and thus
should be accepted as well.

HTH & Greetings from Bremen,

Daniel Kr?gler

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