MyString's copy assignment operator can't handle self-assignment correctly and not exception safe

[ltimaginea]

CPP/week11/examples/example2/mystring.hpp

Lines 29 to 48 in 8d45044

	MyString & operator=(const MyString &ms)
	{
	create(ms.buf_len, ms.characters);
	return *this;
	}
	bool create(int buf_len, const char * data)
	{
	release();
	this->buf_len = buf_len;
	if( this->buf_len != 0)
	{
	this->characters = new char[this->buf_len]{};
	}
	if(data)
	strncpy(this->characters, data, this->buf_len);
	return true;
	}

Issue1: can't handle self-assignment

See: Assignment Operators, C++ FAQ (isocpp.org)
If x = x , bad errors will occur.
We can handle self-assignment by explicitly testing for self-assignment:

MyString& operator=(const MyString& ms)
{
    if (this != &ms)
    {
        create(ms.buf_len, ms.characters);
    }
    return *this;
}

Issue2: not exception safe

In create function, if new char[this->buf_len]{}; throws an exception, *this won't keep a valid state. So, copy assignment operator is not exception safe.
The solution is copy the underlying data firstly, then delete *this's old resource:

bool create(int buf_len, const char* data)
{
    char* new_characters = NULL;
    if (buf_len != 0)
    {
        new_characters = new char[buf_len] {};
    }
    if ((new_characters != NULL) && (data != NULL))
        strncpy(new_characters, data, buf_len);

    release();

    this->buf_len = buf_len;
    this->characters = new_characters;

    return true;
}

See also

• C++ Primer (5th Edition) Section 13.2 Copy Control and Resource Management
• C.62: Make copy assignment safe for self-assignment - C++ Core Guidelines
• C.65: Make move assignment safe for self-assignment - C++ Core Guidelines