c++ - 模板类中的静态模板函数

Question

这里我们有两个模板类。

第一个名为“Factory”，它不是一个单例工厂模型类，并且有一个模板成员函数负责在这个工厂中注册类生产者。

第二个名为“FactoryRegister”，它唯一的职责是在工厂 T1 中注册 T2。

最后，我们有一个声明 FactoryRegister 变量的宏，通过在全局范围内使用这个宏，我们实现了在我们的程序运行之前注册工厂中的所有类。

#include <memory>
#include <mutex>
#include <map>
#include <string>

template <typename Base>
class Factory {
public:

    static std::shared_ptr<Factory<Base> > get_instance() {
        static std::shared_ptr<Factory> factory;
        if (!factory) {
            static std::mutex _new_mutex;
            std::lock_guard<std::mutex> guard(_new_mutex);
            if (!factory) {
                factory.reset(new Factory);
            }
        }

        return factory;
    }

    template <typename Derived,
              typename = typename std::enable_if<std::is_base_of<Base, Derived>::value>::type>
    void add(std::string pname) {
        std::lock_guard<std::mutex> guard(_mutex);
        _producers.emplace(pname, [](std::string name) {return new Derived(name);});
    }

    void del(std::string pname) {
        std::lock_guard<std::mutex> guard(_mutex);
        _producers.erase(pname);
    }

    bool is_exist(std::string pname) {
        return _producers.find(pname) != _producers.end();
    }

    std::shared_ptr<Base> create(std::string pname, std::string name) {
        std::shared_ptr<Base> ret;
        if (is_exist(pname)) {
            ret.reset(_producers[pname](name));
        }
        return ret;
    }

private:
    Factory() = default;

    Factory(const Factory&) = delete;

    Factory& operator=(Factory&) = delete;

private:
    std::mutex _mutex;
    std::map<std::string, std::function<Base*(std::string)> > _producers;
};

template<typename T1, typename T2>
struct FactoryRegister {
    FactoryRegister(std::string name) {
        Factory<T1>::get_instance()->add<T2>(name);
    }
};

#define REGISTER_TO_FACTORY(BaseClass, DerivedClass) \
    const FactoryRegister<BaseClass, DerivedClass> \
    REGISTER_WITH_##BaseClass_##DerivedClass(#DerivedClass)

代码，编译器(gcc4.8.2)显示如下:

factory.h: In constructor 'FactoryRegister<T1, T2>::FactoryRegister(std::string)':
factory.h:62:44: error: expected primary-expression before '>' token
         Factory<T1>::get_instance()->add<T2>(name);

谁能说出原因？

Answer 1

引自 C++'03 标准 14.2/4:

When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter (14.6.2), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template.

Factory<T1>::get_instance()-> template add<T2>(name);

顺便说一句，一些编译器有特殊模式，可以无错地编译原始代码