什么是C ++中的代理类 [英] What is Proxy Class in C++

问题描述

我想知道什么是C ++中的代理类，为什么它被创建和它是有用的。

I wanted to know What is Proxy Class in C++, why it is created and where it is useful.

提前多谢。

推荐答案

代理是一个为另一个类提供修改的接口的类。这里是一个例子 - 假设我们有一个数组类，我们只想要能够包含二进制digist 1或0.这是第一次尝试：

A proxy is a class that provides a modified interface to another class. Here is an example - suppose we have an array class that we we only want to be able to contain the binary digist 1 or 0. Here is a first try:

struct array1 {
    int mArray[10];
    int & operator[]( int i) {
      /// what to put here
    }
}; `

我们希望operator []如果我们说像[1] = 42，这是不可能的，因为操作符只看到数组的索引，而不是存储的值。

We want operator[] to complain if we say something like a[1] = 42, but that isn't possible because the operator only sees the index of into the array, not the value being stored.

我们可以使用代理来解决这个问题：

We can solve this using a proxy:

#include <iostream>
using namespace std;;

struct aproxy {
    aproxy( int & r ) : mPtr( & r ) {}
    void operator = ( int n ) {
    	if ( n > 1 ) {
    		throw "not binary digit";
    	}
    	*mPtr = n;
    }
    int * mPtr;
};

struct array {
    int mArray[10];
    aproxy operator[]( int i) {
    	return aproxy( mArray[i] );
    }
};

int main() {
    try {
    	array a;
    	a[0] = 1;	// ok
    	a[0] = 42;      // throws exception
    }
    catch( const char * e ) {
    	cout << e << endl;
    }
}

代理类现在检查二进制数字我们使数组的operator []返回一个代理的实例，它对数组的内部访问有限。

The proxy class now does our checking for a binary digit and we make the array's operator[] return an instance of the proxy which has limited access to the array's internals.

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