CODEWITHSUNDEEP
c++templates
user4670511
user4670511

Reputation:

Can parameter type and count be specified by a template

I want to have a function template inside a regular (non-template) class, which allocates a derived class with the needed parameters and returns a pointer to the base class:

class Factory {
public:
    BaseClass_T * createObject(parameters) { return new Derived_T(parameters); }
};

So far I've only been using templates where parameters vary in type, but are always the same count. Since my different derived classes have different constructor signatures, I wonder if there is a way to template that and specify the generated function's parameter type and count via a template, something like this:

Factory f;
f.createObject<Derived1, void>(); // create createObject() for Derivev1() constructor
f.createObject<Derived2, int, double>(); // create createObject() for Derived2(int, double) constructor

Upvotes: 1

Views: 81

Answers (1)

Kerrek SB
Kerrek SB

Reputation: 476970

You can use a variadic template:

#include <utility>

struct BaseClass_T { ~virtual BaseClass_T() = default; };

struct Factory
{
    template <typename Der, typename ...Args>
    BaseClass_T * create(Args &&... args)
    { 
        return new Der(std::forward<Args>(args)...);
    }
};

An even better design would be for your factory to return std::unique_ptr<BaseClass_T>:

#include <memory>

struct SaneFactory
{
    template <typename Der, typename ...Args>
    std::unique_ptr<BaseClass_T> create(Args &&... args)
    { 
        return std::make_unique<Der>(std::forward<Args>(args)...);
    }
};

Upvotes: 1

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