有没有办法在非模板类中定义模板成员？ [英] Is there a way to define a template member in a non-template class?

问题描述

说我有一个名为 Compute 的类模板，另一个名为 Function_A 的类具有成员函数模板：

Say I have a class template named Compute, and another class named Function_A with a member function template:

template <typename T> void Evaluate(T parameter)

我必须使用 Function_A 原样。我已经知道 T 只能是 type_1 和 type_2

I am constrained to use the class Function_A as it is. I already know that T can only be one of two types type_1 and type_2.

是否有一种类似于 Compute< T>的方法？ C 作为 Function_A 的成员变量，而不是定义本地 Compute< T> 对象内 Evaluate（...）？我知道这与使用模板的原则背道而驰，因此这不可能，但在那种情况下可以做到吗？

Is there a way to have something similar to Compute<T> C as a member variable of Function_A instead of defining a local Compute<T> object inside Evaluate(...)? I know that this is against the philosophy of using templates, hence it is likely not possible, but can ideally be done in that case instead?

我尝试过拥有两个成员 Compute< type_1> C1 和 Compute< type_2> Function_A 中的C2 ，然后在下使用它们，如果（typeid（T）== typeid（type_1）），但这很荒谬，并且也反对使用模板的哲学。

I tried to have two members Compute<type_1> C1 and Compute<type_2> C2 in Function_A, and then use them under an if (typeid(T) == typeid(type_1)) but it's a pretty hideous, and against the philosophy of using templates as well.

仅说明我的意思：

template <class T>
class Compute
{
public:
  T Function_B(T parameter)
    {
      return f.eval(parameter);
    }

private:
  SomeClass<T> f;
}

和一个班级：

class Function_A
{
  public:
    template <typename T> T Evaluate(T parameter)
    {
      Compute<T> C; //this is very expensive!
      T value = C.Function_B(parameter);
      return value;
    }

  private:
    double SomeParameter;
    //Compute<T> C; //conceptually what I want
}

推荐答案

怎么样（未经测试）：

class Function_A
{
  public:
    template <typename T> void Evaluate(T parameter)
    {
      T value = std::get<Compute<T>>(computers).Function_B(parameter);
      return T(SomeParameter) * value;
    }

  private:
    double SomeParameter;
    std::tuple<Compute<type_1>, Compute<type_2>> computers;
};

注意： std :: pair 可以工作如果您喜欢它添加的第一个/第二个语义，则与 std :: tuple 完全相同。

Note: std::pair would work exactly the same as std::tuple here, if you fancy the first/second semantics it adds.

另外，请注意， T（SomeParameter）是C样式的强制转换，如果 T 不是类，则可能会出现问题类型。考虑 T {} 或 static_cast< T>（）。

Additionally, note that T(SomeParameter) is a C-style cast, which could be problematic if T is not a class type. Consider T{} or static_cast<T>().

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