Re: Compilation problem with templates

From:

Jerome Durand <aNOSPAMaj.durand@def2shoot.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 26 Apr 2007 15:43:28 +0200

Message-ID:

<f0q8vb$fil$1@reader1.imaginet.fr>

Victor Bazarov a ?crit :

Jerome Durand wrote:

Victor Bazarov a ?crit :

Jerome Durand wrote:

Hello,

I'm trying to write something along the following lines
but I cannot get this to compile.

template <typename derived> struct Base {
typedef typename derived::valueType valueType;
virtual valueType Value() = 0;
};

struct CharValue: Base<CharValue>{
typedef char valueType ;
valueType Value() {return 'a';}

};

struct IntValue: Base<IntValue> {
typedef int valueType ;
valueType Value() {return 1234;}
};

The compiler outputs (first error only):
Error 1: 'valueType' : is not a member of CharValue'

Could someone tell me what is wrong with this?

It can be that the compiler is trying to resolve 'derived::valueType'
too soon (at the time when it encounters the 'typedef' in 'Base', the
'derived' is not completely defined yet. The behaviour of compilers
in this matter can differ somewhat, but the most concervative will
likely choke because 'derived' is incomplete by the time 'Base'
instantiation is attemtped. You might consider replacing the
'typedef' dance with normal type processing:

    template<typename valueType> struct Base {
        virtual valueType Value() = 0;
    };

    struct CharValue : Base<char> {
        char Value() { return 'a'; }
    };

    ...

V

This is not really an option because I intend to have
several functions declared in base with different return
types which should depend on types defined in derived...

Then you're SOL since those types are members of the class which
is not going to be fully defined by the time Base is instantiated.
Catch 22.

V

Thank you Victor.

I did something a little differently to break the incomplete definition
dependency... Could have done it without an intermediate class, but I
felt it was better like this:

template <class aderived, class avaluetype> struct DerivedDescription {
    typedef aderived derived;
    typedef avaluetype valueType;

    };
template <typename derivedDescription> struct Base{
    typedef typename derivedDescription::valueType valueType;
    virtual valueType Value() = 0;
    };

struct CharValue: Base< DerivedDescription<CharValue, char> >{
    typedef char valueType ;
    valueType Value() {return 'a';}
};

struct IntValue: Base<DerivedDescription<IntValue, int>>{
    typedef int valueType ;
    valueType Value() {return 1234;}
};

Jerome