Pop "Debug Assertion Failed"

Question

I don't know what's wrong, Do I need define a constructor or just keep a copy constructor? I think It's a issue about shallow and deep copies. Please help, Thank you. When I was debugging, Pop this window

#include <cstring> 
#include<iostream> 
using namespace std;

class MyString
{
public:
    MyString(const char* s);  //constructor
    ~MyString() {  //destructor
        delete[]data; 
    }
protected:
    unsigned len;
    char* data;
    //char data[20];
};

MyString::MyString(const char* s)
{
    len = strlen(s);
    data = new char[len + 1];
    strcpy_s(data, len, s);
}

int main()
{
    MyString a("C++ Programming");
    MyString b(a);
    return 0;
}

Answer 1

Currently, you don't have a copy contructor. What you have is a constructor which takes a const char* array.

A copy constructor has the following format:

MyString(const MyString& obj)
{
   // here you will initialize the char* data array to be of the same size
   // and then copy the data to the new array using a loop or strcpy_s
}

Putting it all together you can write something like this:

#include <cstring> 
#include<iostream> 
using namespace std;

class MyString
{
public:
    MyString(const char* s);  //constructor
    MyString(const MyString& obj);  //constructor

    ~MyString() {  //destructor
        delete[] data; 
    }

protected:
    unsigned int len;
    char* data;

    void copy_cstring(const char* s)
    {
        len = strlen(s);
        data = new char[len + 1]; // len + 1 to make room for null terminate \0

        int i = 0;
        for (i = 0; i < len; ++i)
        {
            data[i] = s[i];
        }
        data[i] = '\0'; // add \0 to the back of the string
    }
};

MyString::MyString(const char* s)
{
    copy_cstring(s);
}

MyString::MyString(const MyString& obj)
{
    copy_cstring(obj.data);
}

int main()
{
    MyString a("C++ Programming");
    MyString b(a);
    return 0;
}

When I use strcpy_s(data,len+1,s) to replace strcpy_s(data,len,s). It doesn't pop that. – theprog

This is happening because when you are using strcpy_s it copies over the null terminate character as well, and if your destination cstring is not large enough it will throw an exception, but once you add 1 to len your destination cstring is going to be of sufficient size.

Answer 2

As others have mentioned, you don't have a copy constructor in your code.

A minimal copy constructor would be to delegate to your existing const char * constructor, like so (put this in the declaration of class MyString):

MyString (const MyString &s) : MyString (s.data) {}

You should also add a copy assignment operator (rule of 3), if you want to avoid nasty surprises down the line.