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I'm making my own string class (as an exercise) and I know I need a copy constructor and an overloaded = assignment operator but not sure where to start.

the class is simple and only stores the string e.g.

char* m_string;

I currently have a standard constructor (MyString(const char* str)) which takes a const char* string and then allocates the necessary memory via m_string = new char[numChars + 1] (after counting the number of chars there are in the argument)

Obviously I need to be able to assign one string to another with the assignment operator, but also want to be able to construct a string object from another. i.e.

MyString(const MyString& str)
{
}

In regards to the overload= assignment operator

Do I then get the length of both rhs and lhs in terms of number of characters and then resize the amount of memory depending on whether the rhs is longer or shorter than the lhs?

I don't necessarily want the whole answer as I don't believe you learn anything from just being told the answer but some advice and guidance in the right direction would be appreciated.

Thanks

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2 Answers 2

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The copy constructor has to allocate memory for the payload of str and then copy the content. This is very much like the constructor you already have, except that the char * is not given directly as the parameter but is "hidden" in str.

The assignment operator has to take the already allocated memory into account.

  • It can either reuse the existing allocated memory. This works if the new content has the same length or is shorter. The terminating \0 will make the additional memory at the end invisible to regular string handling functions.

  • If the new string is larger, you have to free the old memory and allocate a new block of sufficient size.

  • For safety it's advisable to check for self-assignment, i.e. this == &rhs. If you delete memory and then try to read from it, you'll get problems. This allows a = a to work.
  • It's also common practice to return a reference to the modified object. This allows a = b = c to work.
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12 Comments

That's what I kind of thought, do I delete the memory of the Lhs first and then totally reallocate it with another new?
@unknownSPY It is a constructor. There is no allocated memory to delete.
@unknownSPY No, since it's a constructor, there is nothing to delete.
ah ok - thanks that makes sense. What about in the case of the overloaded= assignment operator?
@unknownSPY For operator=(), you are right, you have an existing object, whose memory must be freed (or reused, if the existing buffer is large enough and you want to reduce memory allocations) to avoid memory leaks.
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Probably this will work

MyString(const MyString & str) {
    m_string = new char[str.length() + 1];
    . . .    /// same as MyString(const char * str)'s definition
}

and for the operator =

operator =(const char * str) {
    if (strcmp(mstring, str) == 0) return;    /// Optional
    delete[] mstring;
    mstring = new char[strlen(str) + 1];
    . . .    /// Same as the constructor
}
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5 Comments

should delete mstring be delete[] mstring?
@unknownSPY Yes, fixed. You can also add a check for equality but it's opinion based.
I recommend if (mstring==str) return; rather than strcmp(). The problem here is about aliasing.
@DanAllen Both are C-style strings, how will this work ?
The danger here is that delete[] mstring is deleting str because it is an alias. In fact you need to deal with str pointing to somewhere 'inside' mstring. suppose str==mstring+1 your test will be false but the code will still fail.

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