C++ strings and pointers

Question

I'm learning C++ and currently I'm working with strings and pointers.

I'm following an exercise book and for one of the questions I've created the following:

#include <iostream>
#include <string>

using namespace std;

int main(void){
    string * firstName=nullptr;
    string * lastName=nullptr;
    string * displayName=nullptr;

    cout << "Enter your first name: " << endl;
    getline(cin,*firstName);

    cout << "Enter your last name: " << endl;
    getline(cin,*lastName);

    displayName=new string;
    *displayName= *lastName + ", " + *firstName;

    cout << "Here's the information in a single string: " << displayName;
    cin.get();
    return 0;
}

In a bid to use more of pointers I've tried to mix it together with strings and have made the solution more complex for this reason. When I run this I get a "Unhandled Exception: Access violation reading location xxxxxxxxx".

Can someone please suggest a solution to this by still using pointers and strings instead of char arrays (which I've already figured out how to do)?

Solution 1

This is because you have not allocate your objects prior to using them:

string * firstName = new string();
//...
delete firstName;

It's worth adding that using pointers in this situation is, well, pointless: string objects in the standard C++ library allocate the data for the string from the heap; strings are usually not much more than a pair of pointers anyway.

Solution 2

I think, you don't want to use pointers at all. You can work with strings without pointers.

#include <iostream>
#include <string>

using namespace std;

int main(void){
  string firstName;
  string lastName;
  string displayName;

  cout << "Enter your first name: " << endl;
  getline(cin,firstName);

  cout << "Enter your last name: " << endl;
  getline(cin,lastName);

  displayName= lastName + ", " + firstName;

  cout << "Here's the information in a single string: " << displayName;
  cin.get();
  return 0;
}

Othewise, if you need pointers, you have to allocate memory for variables:

  cout << "Enter your first name: " << endl;
  firstName = new string();
  getline(cin,*firstName);

...and print result with dereference operator (*):

cout << "Here's the information in a single string: " << *displayName;

Solution 3

It would look like this:

int main()
{
    std::string* s = new std::string;
    std::getline(std::cin, *s);
    std::cout << *s;
    delete s;
}

But there is really no reason to do so, just define a normal string variable on the stack.

Solution 4

You are getting errors because you are using strings as pointers and you are not initializing them. A correct way of doing this would be:

#include <iostream>
#include <string>

using namespace std;

int main(void){
    string firstName;
    string lastName;
    string displayName;

    cout << "Enter your first name: " << endl;
    cin >> firstName;

    cout << "Enter your last name: " << endl;
    cin >> lastName;

    displayName = firstname + ' ' + lastName;


    cout << "Here's the information in a single string: " << displayName << endl;
    return 0;
}

You may actually use pointers to strings, but they are meant to be used as local object and passed around as references (or const references, if you wish).

Solution 5

The access violation is because you are dereferencing a null pointer.

Null pointer is set here

string * firstName=nullptr;

and then dereferenced here

getline(cin,*firstName)

You need to have firstname 'point' to something ( a string in this case ). Here's a modified version without the exceptions.

int main(void){
    string * firstName= new string();
    string * lastName=new string();
    string * displayName=new string();

    cout << "Enter your first name: " << endl;
    getline(cin,*firstName);

    cout << "Enter your last name: " << endl;
    getline(cin,*lastName);

    //displayName=new string;
    *displayName= *lastName + ", " + *firstName;

    cout << "Here's the information in a single string: " << displayName->c_str();
    cin.get();
    return 0;
}

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