模板类工厂混乱

Question

我实现了一个小类层次结构，需要从工厂类实例化子类。我的课程ConcreteProduct来自AbstractProduct，我的课程ConcreteFactory来自AbstractFactory。我的代码如下：

factory.h

#ifndef FACTORY_H
#define FACTORY_H

#include <cstddef>
#include <vector>
#include <memory>

template <typename DATATYPE, typename RETURNTYPE>
class AbstractProduct {
    protected:
        DATATYPE data;
        std::size_t count;
    public:
        explicit AbstractProduct(DATATYPE _data, std::size_t _count):
            data(_data),
            count(_count) {}
        virtual void doSomething() = 0;
};
template <typename DATATYPE, typename RETURNTYPE>
class ConcreteProduct: public AbstractProduct<DATATYPE, RETURNTYPE> {
    public:
        ConcreteProduct(DATATYPE _data, std::size_t _count):
            AbstractProduct<DATATYPE, RETURNTYPE>(_data, _count) {}
        virtual void doSomething() override;
};

template <typename DATATYPE, typename RETURNTYPE>
class AbstractFactory {
    public:
        virtual 
            std::vector<std::unique_ptr<AbstractProduct<DATATYPE, RETURNTYPE>>>
            createProducts(DATATYPE _data) = 0;
};
template <typename DATATYPE, typename RETURNTYPE>
class ConcreteFactory: public AbstractFactory<DATATYPE, RETURNTYPE> {
    public:
        virtual 
            std::vector<std::unique_ptr<ConcreteProduct<DATATYPE, RETURNTYPE>>>
            createProducts(DATATYPE _data) override;
};

#endif

factory.cpp

#include "factory.h"

template class ConcreteProduct<int*, int*>;
template class ConcreteFactory<int*, int*>;

template <typename DATATYPE, typename RETURNTYPE>
void ConcreteProduct<DATATYPE, RETURNTYPE>::doSomething() {
    int a = 5+3;
}

template <typename DATATYPE, typename RETURNTYPE>
std::vector<std::unique_ptr<ConcreteProduct<DATATYPE, RETURNTYPE>>> ConcreteFactory<DATATYPE, RETURNTYPE>::createProducts(DATATYPE _data) {
    std::vector<std::unique_ptr<ConcreteProduct<DATATYPE, RETURNTYPE>>> result;
    for(std::size_t i = 0; i < 5; ++i) {
        result.push_back(std::make_unique<ConcreteProduct<DATATYPE, RETURNTYPE>>(_data, 1));
    }
    return result;
}

的main.cpp

#include "factory.h"

int main() {

    int* data = new int[20];

    ConcreteFactory<int*, int*> factory;
    std::vector<std::unique_ptr<ConcreteProduct<int*, int*>>> products = factory.createProducts(data);

    return 0;
}

当我尝试使用g++ -o factory -std=c++14 main.cpp factory.cpp（g ++ 4.9.2）编译时，我收到此错误：

In file included from main.cpp:1:0:
factory.h: In instantiation of ‘class ConcreteFactory<int*, int*>’:
main.cpp:7:33:   required from here
factory.h:39:13: error: invalid covariant return type for ‘std::vector<std::unique_ptr<ConcreteProduct<DATATYPE, RETURNTYPE> > > ConcreteFactory<DATATYPE, RETURNTYPE>::createProducts(DATATYPE) [with DATATYPE = int*; RETURNTYPE = int*]’
             createProducts(DATATYPE _data) override;
             ^
factory.h:32:13: error:   overriding ‘std::vector<std::unique_ptr<AbstractProduct<DATATYPE, RETURNTYPE> > > AbstractFactory<DATATYPE, RETURNTYPE>::createProducts(DATATYPE) [with DATATYPE = int*; RETURNTYPE = int*]’
             createProducts(DATATYPE _data) = 0;
             ^
In file included from factory.cpp:1:0:
factory.h: In instantiation of ‘class ConcreteFactory<int*, int*>’:
factory.cpp:4:16:   required from here
factory.h:39:13: error: invalid covariant return type for ‘std::vector<std::unique_ptr<ConcreteProduct<DATATYPE, RETURNTYPE> > > ConcreteFactory<DATATYPE, RETURNTYPE>::createProducts(DATATYPE) [with DATATYPE = int*; RETURNTYPE = int*]’
             createProducts(DATATYPE _data) override;
             ^
factory.h:32:13: error:   overriding ‘std::vector<std::unique_ptr<AbstractProduct<DATATYPE, RETURNTYPE> > > AbstractFactory<DATATYPE, RETURNTYPE>::createProducts(DATATYPE) [with DATATYPE = int*; RETURNTYPE = int*]’
             createProducts(DATATYPE _data) = 0;
             ^

由于ConcreteProduct是AbstractProduct的子类，这应该有效，不应该吗？我做错了什么？