CRTP 避免动态多态

问题描述

如何在 C++ 中使用 CRTP 来避免虚成员函数的开销?

How can I use CRTP in C++ to avoid the overhead of virtual member functions?

推荐答案

有两种方法.

第一个是通过静态指定类型结构的接口:

The first one is by specifying the interface statically for the structure of types:

template <class Derived>
struct base {
  void foo() {
    static_cast<Derived *>(this)->foo();
  };
};

struct my_type : base<my_type> {
  void foo(); // required to compile.
};

struct your_type : base<your_type> {
  void foo(); // required to compile.
};

第二个是避免使用引用到基或指针到基的习惯用法，并在编译时进行接线.使用上面的定义，您可以拥有如下所示的模板函数:

The second one is by avoiding the use of the reference-to-base or pointer-to-base idiom and do the wiring at compile-time. Using the above definition, you can have template functions that look like these:

template <class T> // T is deduced at compile-time
void bar(base<T> & obj) {
  obj.foo(); // will do static dispatch
}

struct not_derived_from_base { }; // notice, not derived from base

// ...
my_type my_instance;
your_type your_instance;
not_derived_from_base invalid_instance;
bar(my_instance); // will call my_instance.foo()
bar(your_instance); // will call your_instance.foo()
bar(invalid_instance); // compile error, cannot deduce correct overload

因此在函数中结合结构/接口定义和编译时类型推导可以让您进行静态分派而不是动态分派.这就是静态多态的本质.

So combining the structure/interface definition and the compile-time type deduction in your functions allows you to do static dispatch instead of dynamic dispatch. This is the essence of static polymorphism.

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