使用可变参数模板的工厂模式? [英] Factory pattern using variadic template?
问题描述
我有一个抽象类
template <class T> struct A { /* virtual methods */ };
以及具有各种构造函数的几个具体派生类
and several concrete derived classes with various constructors
// The constructor of B takes 1 input
template <class T>
struct B
: public A<T>
{
B() { /* default ctor */ }
B( T *input ) { /* initializer */ }
// .. implement virtual methods
}
// The constructor of C takes 2 inputs
template <class T>
struct C
: public A<T>
{
double some_member;
C() { /* default ctor */ }
C( T *input, double& value ) { /* initializer */ }
// .. implement virtual methods
}
我创建了一个工厂,该工厂返回指向<$ c的指针$ c> A ,我正在尝试使用可变参数模板将输入转发到所选派生类的构造函数。一切正常,但是我不得不为带有/不带有构造函数输入的情况复制代码,并且我正在寻找一种防止代码重复的方法(请参见下文)。
I created a Factory that returns pointers to A
, and I am trying to use variadic templates to forward inputs to the constructor of the selected derived class. It is working fine, but I had to duplicate the code for the cases with/without constructor inputs, and I am looking for a way to prevent code duplication (see below).
template <class T>
struct A_Factory
{
typedef std::shared_ptr<A> out_type;
// Version without constructor inputs
static out_type create( id_type id )
{
out_type out;
switch (id)
{
// .. select the derived class
case Type_B:
out.reset( new B() );
break;
}
return out;
}
// Version with constructor inputs
template <class... Args>
static out_type create( id_type id, Args&&... args )
{
out_type out;
switch (id)
{
// .. select the derived class
case Type_B:
out.reset( new B( std::forward<Args>(args)... ) );
break;
}
return out;
}
};
很抱歉,这个冗长的问题。
Very sorry for the long question. Any suggestion to make this shorter appreciated.
推荐答案
我们可以使用SFINAE和 std :: is_constructible
类型特征(h / t Y牛)。
We can solve this using SFINAE and the std::is_constructible
type trait (h/t Yakk).
我们只需要一个单独的create函数,该函数将分派给其他函数:
We just need one single create function, that will dispatch out to other functions:
template <class... Args>
static std::shared_ptr<A> create( id_type id, Args&&... args )
{
switch (id) {
case Type_B:
return create<B>(std::forward<Args>(args)...);
case Type_C:
return create<C>(std::forward<Args>(args)...);
// ...
}
每个基于标记的 create
函数将调用正确的构造函数,或者如果不存在该函数,则返回nullptr:
Each tag-based create
function will either call the correct constructor OR return nullptr if such a one does not exist:
// possible to construct
template <typename T, typename... Args>
std::enable_if_t<
std::is_constructible<T, Args...>::value,
std::shared_ptr<T>
>
create(Args&&... args) {
return std::make_shared<T>(std::forward<Args>(args)...);
}
// impossible to construct
template <typename T, typename... Args>
std::enable_if_t<
!std::is_constructible<T, Args...>::value,
std::shared_ptr<T>
>
create(Args&&... ) {
return nullptr;
}
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