Custom C++ string class

Question

I am creating a class that works similar to the string class. I am in a starting stage doing some experiments.

Assume all header files are added

class String
{
      public: 
          int len; 
          char *str = NULL;                                                                       

      // Default constructor 
      String()
      {                                             
           len = 0;
           str = new char[1];             
           str[0] = '\0';
           printf("New memory: %p\n", str);
      }

    // Copy constructor
    String(const char *ch)
    {
        len = strlen(ch);  

        if (str != NULL)
        {
            printf("Old memory: %p\n", str);
            delete str; 
            printf("Deleted...\n");               
        }

        str = new char[len + 1];
        printf("New memory: %p\n", str);

        for (int i = 0; i <= len; i++)
            str[i] = ch[i];
    }
};


int main()
{
    String msg;

    std::cout << msg.len << std::endl;
    std::cout << msg.str << std::endl;

    msg = "Hello";
    std::cout << msg.len << std::endl;
    std::cout << msg.str << std::endl;
}

When I create a String object 1 byte of memory is allocated to store string and initialises with \0 on line String msg;

When copy constructor is called on line msg = "Hello"; it first checks for any previous allocation of memory and to avoid memory wastage it will delete that and recreates a new memory block.

The issue is when copy constructor is called line if (str != NULL) is not getting executed. It should execute because str is allocated a byte of memory earlier in default constructor and it is not NULL.

What may be the thing that I missed out?

That's not a copy constructor. That's a constructor that takes a const char*. A copy constructor would have the signature String(const String& other) — JohnFilleau
– JohnFilleau, Commented May 9, 2020 at 14:10
Doesn't C++ (including its various libraries) have more than enough string classes already? Why create yet another one? — Jesper Juhl
– Jesper Juhl, Commented May 9, 2020 at 14:10
When you call a non-default constructor for a class, the default constructor is not called beforehand. When you enter String(const char*) your String is in the same state as if you just entered String(). That means str == NULL. Also use nullptr instead. — JohnFilleau
– JohnFilleau, Commented May 9, 2020 at 14:12
@JesperJuhl Learning? Practice? It's a pretty common exercise, is it not? — Asteroids With Wings
– Asteroids With Wings, Commented May 9, 2020 at 14:16

Asteroids With Wings · Accepted Answer · 2020-05-09 14:20:46Z

2

Your code does this:

Default-construct a String "A"
Convert-construct a temporary String "B" from the expression "Hello"
Copy-assign "B" to "A"

The assignment does not invoke the constructor on "A". You only get one construction per object.

Unless you specify otherwise (with delegating constructors), your default constructor is not invoked before the "copy constructor" (or a conversion constructor like yours) is run.

So this:

It should execute because str is allocated a byte of memory earlier in default constructor and it is not NULL.

is wrong.

String(const char *ch) needs to allocate.

Also, because you're performing assignment, you're going to need to implement a copy assignment operator to make that safe.

edited May 9, 2020 at 14:20

answered May 9, 2020 at 14:15

Asteroids With Wings

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1 Comment

Asteroids With Wings Over a year ago

Ah, now I see where your misconception lies.

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