Segmentation fault using string pointer - c++

Question

I'm a C++ newbie, I'm trying to put in practice pointers with strings. The program I have made is just to store strings the user types in the command line. But I'm getting segfault, not sure why.
This is the code:
#include <cstdio>
#include <cstdlib>
#include <iostream>
using namespace std;
//This code is meant to learn how to use pointers and strings
//Ask the user for who are they in the family and save it in string array!
void print_string (string* Value, int const nSize);
int get_names(string* Family_input);
int main ( int nNumberofArgs, char* pszArgs[])
{
cout << "Tis program stores your family members\n";
cout<< "Type the names and write 0 to exit\n";
string familia_string;
string* familia = &familia_string;
int family_number;
family_number=get_names(familia);
cout << "The family members are: ";
print_string(familia, family_number);
cout << endl;
return 0;
}
int get_names(string* Family_input)
{
int i=0;
string input="";
string old_input="";
while (input!="0")
{
cout << "type " << i <<" member\n";
//cin >> *(Family_input+i);
//input=*(Family_input+i);
cin >> input;
*(Family_input + old_input.length()) = input;
old_input=input;
i++;
}
return i;
}
void print_string (string* Value, int const nSize)
{// I don't want to &psValue to be changed!
for (int i=0; i<nSize; i++)
{
cout << *(Value+i) << " ";
//&psValue++;
}
}
I'm not sure if it's because I'm not taking correctly the size of the string, or I'm not using correctly the pointer or is that I have to allocate memory before using the offset.

Answer 1

As #kleszcz pointed out already, the line
*(Family_input + old_input.length()) = input;
is wrong. You are accessing memory that you are not supposed to.
The easiest fix is to change get_names slightly:
int get_names(string* Family_input)
{
int i=0;
string input="";
while (input!="0")
{
cout << "type " << i <<" member\n";
cin >> input;
*Family_input += input; // Just keep on appending to the input argument.
*Family_input += "\n"; // Add a newline to separate the inputs.
i++;
}
return i;
}
Also change print_string to:
void print_string (string* Value)
{
cout << *Value;
}
Of course, print_string has become so simple, you don't need to have it at all.
You could change get_names to use a reference argument instead of a pointer argument. This is a better practice.
int get_names(string& Family_input)
{
int i=0;
string input="";
while (input!="0")
{
cout << "type " << i <<" member\n";
cin >> input;
Family_input += input; // Just keep on appending to the input argument.
Family_input += "\n"; // Add a newline to separate the inputs.
i++;
}
return i;
}
Then, change the call to get_names. Instead of using
family_number=get_names(familia);
use
family_number=get_names(familia_string);

Answer 2

You get seg fault because you haven't allocate memory for an array of strings.
*(Family_input + old_input.length()) = input;
This is total nonsense. If you'd have an array you increment index only by one not by length of string.

Answer 3

If you want to save different names in different string objects I would suggest:
#include <iostream>
#include <string>
#include <cstdlib>
using namespace std;
void print_string(string** Value, int const nSize);
void get_names(string** Family_input, int count);
int main(int nNumberofArgs, char* pszArgs[]) {
cout << "This program stores your family members\n";
int family_number;
cout << "Types the number of family members";
cin >> family_number;
/*allocate the number of string pointers that you need*/
string** familia = new string*[family_number];
cout << "Type the names\n";
get_names(familia, family_number);
cout << "The family members are: ";
print_string(familia, family_number);
cout << endl;
return 0;
}
void get_names(string** Family_input, int count) {
for(int i = 0 ; i < count; i++){
cout << "type " << i << " member\n";
/*create a string obj in the heap memory*/
string *input = new string ("");
// using that way you get only a single word
cin >> *input;
/*create a string object on the stack and put its pointer in the family_array*/
Family_input[i] = input;
}
}
void print_string(string** Value, int const nSize) {
for (int i = 0; i < nSize; i++) {
cout << *(Value[i]) << " ";
}
}