内存拷贝（注意重叠问题）

#include "stdafx.h"
#include <iostream>
using namespace std;

char *memorycpy(char *dest, char *src, int len)
{
    if (!dest || !src ||!len || (dest == src))
    {
        return dest;
    }

    char *pdest = dest;
    char *psrc = src;
    //目标地址高于源地址，从后往前复制
    if (dest > src)
    {
        pdest = pdest + len - 1;
        psrc = psrc + len - 1;
        while(len--)
        {
            *pdest-- = *psrc--;
        }
    }
    else //反过来，从前向后复制
    {
        while(len--)
        {
            *pdest++ = *psrc++;
        }
    }

    return dest;
}

int _tmain(int argc, _TCHAR* argv[])
{
    char *s1 = new char[30];
    char *s2;

    for (int i = 0; i < 10; i++)
    {
        *(s1 + i) = 'a' + i;
    }

    s2 = s1 + 2;

    s2 = memorycpy(s2, s1, 3);

    cout << s2 << endl;

    delete [] s1;

    return 0;
}