Question

I have a the following code:

#include <iostream>
using namespace std;

void func(char * aString)
{
    char * tmpStr= new char[100];
    cin.getline(tmpStr,100);
    delete [] aString;
    aString = tmpStr;
}

int main()
{
    char * str= new char[100];
    cin.getline(str,100);
    cout<< str <<endl;
    func(str);
    cout<< str <<endl;
    return 0;
}

Why the second cout does not print the second input string? How can I change this code to work it?

Answer 1

Because the second cout will print what is pointed by str. And str, the pointer, in your main function will have the same value before and after the call to func.

Indeed, in the func function, you are changing the value of the aString variable. But this is another variable than str in main.

If you want the value of str to be changed, you have to pass it to func by reference or by pointer. (Note that what you write, is to pass the characters by pointer. I mean you have to pass the pointer by pointer: void func(char **str_ptr), or by reference void func(char *&str_ref))

If you’re really doing C++, you should use std::string instead of the old C strings.

An example of passing the pointer by pointer:

func(char ** aString)
{
    char * tmpStr= new char[100];
    cin.getline(tmpStr,100);
    delete [] *aString;
    *aString = tmpStr;
}

Plus you should call it like this: func(&str);