C++ 字符 class function append

Question

void CMPT135_String::append(const CMPT135_String &s) /*12*/
{
    char *temp = new char[this->getLength()+1];

    int len = CMPT135_String::cstrlen(buffer);

    for (int i =0; i < len; i++)
        temp[i] = this->operator[](i);

    for (int i = 0; i < s.getLength()+1; i++)
        temp[len+i] = s[i];

    if(this->getLength() >0) 
        delete[] this->buffer;

    this->buffer = temp;
}

I have been working with this append() member function for a custom string class.

The function works fine, but after it runs I get a popup window showing up:

Windows has triggered a breakpoint in CMPT135_Assignment1.exe.

This may be due to a corruption of the heap, which indicates a bug in Assignment1.exe or any of the DLLs it has loaded.

This may also be due to the user pressing F12 while CMPT135_Assignment1.exe has focus.

The output window may have more diagnostic information.

Please tell me what is going wrong with this.

Answer 1

You are not allocating enough memory for the new buffer. You are only allocating enough memory to copy this->getLength() number of characters from this , there is no room to also copy s.getLength() number of characters from s into that new buffer, so your second loop is corrupting random memory past the end of the new buffer.

Try something more like this instead:

void CMPT135_String::append(const CMPT135_String &s) /*12*/
{
    int this_len = this->getLength();
    int s_len = s.getLength();
    int new_len = this_len + s_len;

    char *temp = new char[new_len + 1];

    for (int i = 0; i < this_len; ++i)
        temp[i] = this->operator[](i);

    for (int i = 0; i < s_len; i++)
        temp[this_len + i] = s[i];

    temp[new_len] = '\0';

    delete[] this->buffer;
    this->buffer = temp;
}