C++模拟C#中的委托

我们知道，C#中有一个关键字delegate，相对应于C++中的函数指针。当然，由于语言本身的原因，二者不可能完全相同。相较于函数指针，delegate用起来要便捷许多。由于个人兴趣的关系，前些日子写了个C++程序用来扩展一个自己的关键字Delegate，以便达到比较容易定义函数指针的目的。

先来看看C#中的委托与C++的函数指针是如何定义与使用的:

C#

private delegate string GetAString();static void Main(){int x = 40;GetAString firstStringMethod = new GetAString(x.ToString);Console.WriteLine("String is {0}", firstStringMethod());}

C++

typedef char* (*GetAString)();char* strHelloWorld = "Hello World";char* HelloWorld(){ return strHelloWorld;}void main(){ GetAString p1 = HelloWorld; printf("String is %s/n", (*p1)());}

 

二者相比，真的是没有什么大的不同。接着试着加一个宏Delegate,

#define DELEGATE(RtType,FuncType) typedef RtType(*FuncType)()DELEGATE(char*, GetAString);char* strHelloWorld = "Hello World";char* HelloWorld(){ return strHelloWorld;}void main(){ GetAString p1 = HelloWorld; printf("String is %s/n", (*p1)());}

看起来一切都很好，然而，我忽然意识到了一个问题，如果函数原型是带参数的怎么办？因为宏是不能重复定义的，我总不能重新定义一个DELEGATEWITHARGUMENTS的宏吧？其实这个问题还是可以处理的，那就是使用预定义的可变参数宏__VA_ARGS__。最后，略加修饰，就写了以下的代码：

 

Macro.h

#ifndef _DELEGATE_TEST_H_#define _DELEGATE_TEST_H_/// <description>/// Simulates keyword delegate of C#/// </description>/// <params>/// RtType: return value type./// FuncType: function type./// ...: variable arguments list/// </params>/// <remarks>/// ## makes preprocessor remove the previous comma if no argument is passed in. /// In VC++, it's allowed if the empty param is passed to "...", but it's NOT allowed/// in ANSI C++. Of course, in this case, it's fine if there is no "##". Add it to /// emphasize it./// </remarks>#define DELEGATE(RtType,FuncType,...) typedef RtType(*FuncType)(##__VA_ARGS__)#endif

 

MainEntry.cpp

#include <iostream>#include "Macro.h"using namespace std;DELEGATE(void,FuncPtr0);DELEGATE(void,FuncPtr1,int);DELEGATE(void,FuncPtr2,int,int);void PrintHelloWorld(){ cout << "Hello World! 0" << endl;}void PrintHelloWorld(int){ cout << "Hello World! 1" << endl;}void PrintHelloWorld(int,int){ cout << "Hello World! 2" << endl;}void main(){ FuncPtr0 ptr0 = NULL; ptr0 = PrintHelloWorld; (*ptr0)(); FuncPtr1 ptr1 = NULL; ptr1 = PrintHelloWorld; (*ptr1)(0); FuncPtr2 ptr2 = NULL; ptr2 = PrintHelloWorld; (*ptr2)(0,0);}

 

Works well!