Custom string class (C++)

Question

I\'m trying to write my own C++ String class for educational and need purposes.
The first thing is that I don\'t know that much about operators and that\'s why I want to

Answer 1

Here's what's going on:

1. The constructor is indeed called twice. Once for "hello" and once for " world". The order is undefined.
2. The CString::operator + is called on the 1st CString ("hello"), passing the second CString (" world") as it's argument. The return value of CString::operator + is a new CString
3. Since you're assigning in initialization, i.e: CString a = [CString result of operator +], c++ will just call you're copy constructor. Hence the call to CString(CString& ) that you're seeing in the debugger.

Now, that's 4 total objects just created, one for each string literal ("hello", and " world"), one for the concatenation of those (the result of the CString::operator + call, and one to hold the result (CString a = ...). Each one of those temporary objects will have it's destructor called.

As for why you're not getting the printf, I have no idea. I just copy pasted your code in this file:

#include <cstdio>
#include <cstring>

[your code]

int main(int argc,char* argv[]) {
  CString a = CString("hello") + CString(" world");
  printf(a);
}

And when I ran the resulting executable, I got hello world as the output. This is on Ubuntu with g++ 4.4. Not exactly sure why under the VS debugger it's not printing anything.

Answer 2

The line:

cstr=new char[strlen(str)];

should be:

cstr=new char[strlen(str) + 1];

Also, the test for self-assignment does not make sense in the copy constructor - you are creating a new object - it cannot possibly have the same address as any existing object. And the copy constructor should take a const reference as a parameter,

If in your code, you were expecting the assignment operator to be used, you would be expectining wrong. This code:

CString a = CString("Hello") + CString(" World");

is essentially the same as:

CString a( CString("Hello") + CString(" World") );

which is copy construction, not assignment. The temporary CString "Hello world" will be destroyed (invoking the destructor) after a has been constructed.

Basically, it sounds as if your code is working more or less as expected.

Answer 3

A few mistakes you made:

1. Copy constructor signature is wrong. It must be:

CString(const CString& q)

2. op= signature is wrong. It must be:

CString& operator=(const CString& q)

Btw., that was also the reason that the copy constructor was called. You did a return *thisin the end which copied the object (with your op= signature).

3. You allow CString instances with cstr == NULL (your default constructor will result in such an instance). Though, in almost all functions (copy constructor, operator +, operator =) you don't handle that case well (q.cstr == NULL).

Maybe the easiest and safest way would be to just disallow that case and change your default constructor to:

CString::CString()
{
   cstr = new char[1];
   cstr[0] = 0;
}

Answer 4

Don't use strlen, store your own string length. The string should not be depended on to have a null terminator. It's ok to use such if you're passed in a random const char*, but for internal operations, you should use the size.

Also, you forgot to make your operator const char* a const overload.