CODEWITHSUNDEEP
c++crtp
Asmodiel
Asmodiel

Reputation: 1062

Various errors with CRTP (C++)

I know I've just asked a question about this, but I cannot figure out what I'm doing wrong. I've rewritten just the small part and cannot find any errors (used C++ function in parent return child as reference)

My code:

#include <iostream>
#include <stdlib.h>
#include <cstring>

using namespace std;

template<class Derived>
class Entity {
    private:
        string _name;
    public:
        const string& name() const;
        Derived& name( const string& );
        Derived* This() { return static_cast<Derived*>(this); }
};

class Client : Entity<Client> {
    private:
        long int _range;
    public:
        const long int& range() const;
        Client& range( const long int& );
};

const string& Entity::name() const {
    return _name;
}

Derived& Entity::name(const string& name) {
    _name = name;
    return *This();
}

const long int& Client::range() const {
    return _range;
}

Client& Client::range( const long int& range ) {
    _range = range;
    return *this;
}

int main() {
    Client ().name("Buck").range(50);
    return 0;
}

The result: 

untitled:25: error: ‘template<class Derived> class Entity’ used without template parameters
untitled:25: error: non-member function ‘const std::string& name()’ cannot have cv-qualifier
untitled: In function ‘const std::string& name()’:
untitled:26: error: ‘_name’ was not declared in this scope
untitled: At global scope:
untitled:29: error: expected constructor, destructor, or type conversion before ‘&’ token
untitled: In function ‘int main()’:
untitled:13: error: ‘Derived& Entity<Derived>::name(const std::string&) [with Derived = Client]’ is inaccessible
untitled:44: error: within this context
untitled:44: error: ‘Entity<Client>’ is not an accessible base of ‘Client’

I'd be very grateful for answers (my incompetence might be due to sleep deprivation though :D)

Upvotes: 1

Views: 314

Answers (2)

Luchian Grigore
Luchian Grigore

Reputation: 258548

You need to implement your specialized functions as this:

template<>
const string& Entity<Client>::name() const {
    return _name;
}

template<>
Client& Entity<Client>::name(const string& name) {
    _name = name;
    return *This();
}

and also add public inheritance:

class Client : public Entity<Client>

so you can access name().

If you want generic implementations:

template<class x>
const string& Entity<x>::name() const {
    return _name;
}

template<class x>
x& Entity<x>::name(const string& name) {
    _name = name;
    return *This();
}

Upvotes: 4

Mike Seymour
Mike Seymour

Reputation: 254431

If you define members of a class template outside the template definition, then you need to include the template specification:

template <class Derived>
const string& Entity<Derived>::name() const {
    return _name;
}

template <class Derived>
Derived& Entity<Derived>::name(const string& name) {
    _name = name;
    return *This();
}

You also need to inherit publicly from Entity:

class Client : public Entity<Client> {
    // stuff
};

Upvotes: 2

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