Can't manage to use char pointers

[Reiz]

Hi,

I am new to C++. I'm trying to create a string class for homework. Interface was given to me. I only need to code implemetation. But I couldn't figure out how to use char pointers. Here is my constructor. I am just trying to see outputs here.

  MyString::MyString(const char* s, int string_length){
    str = new char();
    *str = *s;
    cout << s << endl;
    cout << str << endl;
    length = string_length;
 }

And my Main:

int main()
{
    char *ss = "asdasd";
    MyString ms(ss, 6);
    return 0;
}

First cout works as expected. Prints out: "asdasd" .But the second one writes compeletely different things to screen. Why is it acting like this? What is wrong here? Second one probably writing its address to screen. But why?

[keskiverto]

Please, explain your lines 2 and 3 (of the constructor). What do they supposedly do?


(It would be nice to see the given interface. The main() has no MyString objects.)

[Peter87]

new char() creates a single char.

*str = *s; assigns the char pointed to by s (the first char in the string) to the char pointed to by str (the char that you created on the previous line).

The << operator expects char pointers to point to the first character in a a char array and that the end of the string is marked using the null termination character '\0'. This is not the case in your problem. It will print *str and then continue printing the bytes in memory until it finds a null character (zero byte). This could cause a crash and/or garbage to be printed to the screen, or something else to happen.

[Reiz]

I edited Main(). Sorry.

Thanks Peter87. I now understand the problem.

So how can I copy the value inside s variable to str ?? How can I make str exactly like s ? And do I need new char() ?

[LB]

Reiz wrote:

How can I make str exactly like s ?

str = s;
I don't know if that's what you want though.

[Reiz]

I figured it out:

MyString::MyString(const char* s, int string_length){
    str = new char[string_length]();
    for(int i = 0; i < string_length; i++)
            str[i] = s[i];
    length = string_length;
 }

[keskiverto]

Pointer is a variable that holds one value. A number. The number means a memory address.

In const char * gaz = "fubar"; the gaz stores the address of the 'f'.
The 'f' is actually in read-only memory, and the following five consecutive memory locations do hold values 'u', 'b', 'a', 'r', '\0'.

An array. You do need an array to hold characters. You do need a dynamically allocated array, not just one character. How large should that array be?

Copying values from one array to an another array usually involves a loop, although you might be able to use functions from header <cstring>.

What should happen on:
MyString ms( "Slartibartfast has Bistro-Math", 7);