Re: Template subclass trouble

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 13 May 2009 21:20:15 -0400

Message-ID:

<gufrkf$kd7$1@news.datemas.de>

ekstrand wrote:

I am having problems with subclassing a templated class with a
templated subclass. Consider the following program (I have simplified
it greatly from its origional form, the actual header file can be
provided upon request):

class A
{
public:
    static void f() { }
};

template<typename T, typename Q>
class B
{
public:
    B(T* t = 0) { }
    B(B& b) { }
    ~B() { Q::f(); }
};

template<typename T>
class C:
    public B<T, A>
{
public:
    C(T* t = 0): B<T, A>(t) { }
    C(C& c): B<T, A>(c) { }
};

C<int>
function() {
    C<int> var;
    return var;
}

int main()
{
    C<int> var = function();
    return 0;
}

I have a class that takes two template parameters. In order to specify
one of these template parameters without specifying the other, I
create a second templated class that is a subclass of the other. This
method seems to be the generally accepted method for getting around
the no templated typedefs problem. However, when I try to build it
with g++, I get the following error message:

template_test.cpp: In function ?int main()?:
template_test.cpp:33: error: no matching function for call to
?C<int>::C(C<int>)?
template_test.cpp:22: note: candidates are: C<T>::C(C<T>&) [with T =
int]
template_test.cpp:21: note: C<T>::C(T*) [with T = int]

g++ cannot find the copy constructor even though the first candidate
is exactly what it needs. If I make the copy constructor const, the
problem goes away. Also, this code correctly builds on VisualC++ 2005.
Is there something I'm missing, or have I found a bug in g++?

My g++ version is as follows:
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian
4.3.2-1.1' --with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs --
enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-
shared --with-system-zlib --libexecdir=/usr/lib --without-included-
gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/
usr/include/c++/4.3 --program-suffix=-4.3 --enable-clocale=gnu --
enable-libstdcxx-debug --enable-objc-gc --enable-mpfr --enable-
targets=all --enable-cld --enable-checking=release --build=i486-linux-
gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.3.2 (Debian 4.3.2-1.1)

Thanks in advance for your response.

This has nothing to do with templates. Try this

    class A {
    public:
        A(A&) {}
    };

    A foo() {
        A a;
        return a;
    }

    int main() {
        A aa = foo();
    }

The problem is that you're trying to initialise 'aa' using a temporary
returned from 'foo'. To do that a copy constructor would be used. You
defined a copy constructor that takes a reference to non-const 'A'. A
temporary cannot be bound to a reference to non-const. Period.

V
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