2.写一个函数完成内存之间的拷贝

void* mymemcpy(void *dest, const void *src, size_t count)
{
    char* pdest = static_cast(dest);
    const char* psrc = static_cast(src);
    if (pdest > psrc && pdest < (char*)psrc + count ) //能考虑到这种情况就行了
    {
        for (size_t i = count - 1; i != -1; --i)
            pdest[i] = psrc[i];
    }
    else
    {
        for (size_t i = 0; i < count; ++i)
            pdest[i] = psrc[i];
    }
    return dest;
}