Various errors with CRTP (C++)

Question

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)

You should put the code directly in the question if it's not extremely long. — kennytm, Dec 20 '11 at 14:36

Luchian Grigore · Accepted Answer · 2011-12-20T14:42:06.283

4

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();
}

edited Dec 20 '11 at 14:42

answered Dec 20 '11 at 14:34

Luchian Grigore

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Thank you very much, compiles now. – Asmodiel Dec 20 '11 at 14:36
This only creates specialisations for `Client`; you probably want to define the generic functions instead, assuming you want more than one class to inherit from `Entity`. – Mike Seymour Dec 20 '11 at 14:39
@MikeSeymour I added the generic implementation. – Luchian Grigore Dec 20 '11 at 14:42

score 2 · Answer 2 · answered Dec 20 '11 at 14:37

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
};

Various errors with CRTP (C++)

2 Answers2