使用依赖于模板参数的参数编写模板类的构造函数 [英] Writing constructor of template class with template argument-depending parameters

问题描述

请考虑以下两个带有构造函数的类，这些构造函数接受完全不同的参数集。

Consider the following two classes with constructors that accept wildly different sets of parameters.

class A {
public:
  A(int x, double y) {
    //do something
  }
};

class B {
public:
  B(const std::vector<bool>& x) {
    //do something
  }
};

现在，我想编写一个具有对象的类模板TpC（可能是A或B或其他）。换句话说，我希望能够使用以下内容：

Now, I want to write a class template TpC that will have an object (which might be A or B or something else). In other words, I want to be able to use the following:

int x;
double y;
std::vector<bool> z;
TpC<A> obj_a (x, y);
TpC<B> obj_b (z);

如何为TpC编写一个依赖于A和B的构造函数的构造函数？编写类似特征/策略的内容来告诉TpC每个特定专业化应使用的参数是可以接受的。

How can I write a constructor for TpC that is dependent on the parameters of the constructors of A and B? It would be acceptable to write something like a trait/policy to tell TpC what the parameters should be for each particular specialization.

推荐答案

您将需要使用模板构造函数。但是，为了防止调用此构造函数而不是复制构造函数，您需要添加一些歧义。例如：

You would need to use a template constructor. However, in order to prevent this constructor being called instead of the copy constructor, you need to add a bit of disambiguation. For example:

template<class T>
class TpC {
     T t;
public:
     struct in_place_t { };
     static constexpr in_place_t in_place;
     template<class... ARGS>
     TpC(in_place_t, ARGS... args) : t(std::forward<ARGS>(args)...) {}
};

在C ++ 17中 in_place 可用在 std 中。

In C++17 in_place is available in std.

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