在C ++中的多重分派 [英] Multiple dispatch in C++

问题描述

我想了解多重派遣是什么。我读了很多不同的文本，但我仍然不知道多个调度是什么，它是什么好。也许我缺少的东西是一块代码使用多个分派。请，你可以在C ++中使用多个分派写一小段代码，以便我可以看到它不能被编译/运行正常，因为C ++只有单个分派？我需要看到的区别。非常感谢。

I am trying to understand what multiple dispatch is. I read a lot of various texts but I still have no idea what multiple dispatch is and what it is good for. Maybe the thing I am missing is piece of code using multiple dispatch. Please, can you write a little piece of code in C++ using multiple dispatch so that I can see it cannot be compiled/runned properly because C++ has only single dispatch? I need to see the difference. Thanks.

推荐答案

多分派是基于参数的运行时类型选择调用哪个版本的函数的能力传递给函数调用。

Multi-dispatch is the ability to choose which version of a function to call based on the runtime type of the arguments passed to the function call.

下面是一个在C ++中未正确工作的示例：

Here's an example that won't work right in C++ (untested):

class A { };
class B : public A { };
class C : public A { }


class Foo
{
  virtual void MyFn(A* arg1, A* arg2) { printf("A,A\n"); }
  virtual void MyFn(B* arg1, B* arg2) { printf("B,B\n"); }
  virtual void MyFn(C* arg1, B* arg2) { printf("C,B\n"); }
  virtual void MyFn(B* arg1, C* arg2) { printf("B,C\n"); }
  virtual void MyFn(C* arg1, C* arg2) { printf("C,C\n"); }
};

void CallMyFn(A* arg1, A* arg2)
{
  // ideally, with multi-dispatch, at this point the correct MyFn() 
  // would be called, based on the RUNTIME type of arg1 and arg2
  pFoo->MyFn(arg1, arg2);
}

...

A* arg1 = new B();
A* arg2 = new C();
// Using multi-dispatch this would print "B,C"... but because C++ only
// uses single-dispatch it will print out "A,A"
CallMyFn(arg1, arg2);

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