如何解决此C ++访问冲突问题？

Question

I'm getting an error in the following code. Visual Studio throws an access violation error when writing to _buf . How can I fix this?

The Sendn function is a socket sending function. It's not the problem, you can ignore it.

It looks like _buf points at 0x00000000

The error message I'm seeing is

 0xC0000005: 0x00000000 : access violation 

void ?????::?????(int number, string title)
{

    int titlesize = sizeof(title);
    int bufsize = 4 + 4 + 4 + titlesize;

    char *_buf = new char[bufsize];

    _buf = { 0 };

    // char _buf[bufsize] = { 0 }; (수정 내용)

    int commands = 3;

    int index = 0;
    memcpy(_buf, &commands, sizeof(int));
    index += sizeof(int);

    memcpy(_buf + index, &number, sizeof(int));
    index += sizeof(int);

    memcpy(_buf + index, &titlesize, sizeof(int));
    index += sizeof(int);
    for (int i = 0; i < titlesize; i++)
    {
        memcpy(_buf + index, &title[i], sizeof(char));
        index += sizeof(char);
    }

    Sendn(_buf, bufsize);

    delete[] _buf;

    return;
}

Answer 1

char *_buf = new char[bufsize];
_buf = { 0 };

This does not zero-fill the dynamically-allocated array pointed to by _buf . It sets the pointer _buf to be a null pointer. Since _buf is a null pointer, later attempts to dereference it lead to undefined behavior.

There's no need to zero-fill the array pointed to by _buf in this case, so you can simply remove the _buf = { 0 }; line.

---

Once you've fixed that problem, you also aren't allocating the right amount of memory. sizeof(title) will not give you the number of characters that title holds. It just gives you the static size of a std::string object, which is usually only a pointer and two integers. Use title.size() instead.

Answer 2

You're trying to copy the content of title together with 3 other integer numbers into _buf right? The problem is that sizeof(title) is not the length of the string stored in title . In order to get the length of title , you need to call the member function length on type std::string like this:

auto titlesize = title.length();

The sizeof operator only gives you the size of your std::string object on stack (in comparison, the actual string is stored on heap) and sizeof expressions are always constant expressions. On my computer, sizeof(std::string) is 24 regardless of what the actual string is.