C++ vector of strings, why assignment results in same char references? - c++

Question

I am currently learning C++, and I wanted to manually input values to a vector of strings, processing each character before storing it. This is the code I used:
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
int i, height = 5, width = 5;
vector<string> lab;
lab.resize(height, "");
string s;
//For input reference
cout << endl << "Input data:" << endl << "01234" << endl;
//Input values
for (i = 0; i < height; i++)
{
getline (cin, s);
for (int j = 0; j < width; j++)
{
//Process char [...]
lab[i][j] = s.at(j);
}
}
//Show Matrix
cout << endl << endl;
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
cout << lab[i][j] << " ";
}
cout << endl;
}
//Show Addresses
cout << endl << endl;
for (int i = 0; i < height; i++)
{
cout << "String Addr: " << (void*) &lab[i] << " | Chars Addr: ";
for (int j = 0; j < width; j++)
{
//cout << lab[i][j] << " ";
cout << (void*) &lab[i][j] << " ";
}
cout << endl;
}
cout << endl;
return 0;
}
The problem was that the characters inside the strings have the same address (I don't know why). Here is an example of one execution of the program:
Input data:
01234
aaaaa
bbbbb
ccccc
ddddd
abcde
a b c d e
a b c d e
a b c d e
a b c d e
a b c d e
String Addr: 0x8fe8008 | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610
String Addr: 0x8fe800c | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610
String Addr: 0x8fe8010 | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610
String Addr: 0x8fe8014 | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610
String Addr: 0x8fe8018 | Chars Addr: 0xb773260c 0xb773260d 0xb773260e 0xb773260f 0xb7732610
At the end you can see that the addresses of the chars are the same, while the ones for the strings are different. The final matrix in the example should have been:
a a a a a
b b b b b
c c c c c
d d d d d
a b c d e
But instead, it is:
a b c d e
a b c d e
a b c d e
a b c d e
a b c d e
While I already solved the problem, using the += operator instead of assigning using [], I still don't know exactly what hapened with the code above. Why does the chars have the same reference?

Answer 1

You are going out of bounds on your assignments:
lab[i][j] = s.at(j);
lab[i] is valid but lab[i][j] for any j is out of bounds, because all the strings in your vector were initialized to "", an empty string.

Answer 2

The problem is that although you've resized the vector you have not resized any of the strings inside it, they are all still zero length. So when you do
lab[i][j] = s.at(j);
you are accessing outside of the bounds of the string at lab[i].
This should work
for (int j = 0; j < width; j++)
{
//Process char [...]
lab[i].push_back(s.at(j));
}
This version adds each char to the string as it's processed.

Answer 3

This code is invalid
//Input values
for (i = 0; i < height; i++)
{
getline (cin, s);
for (int j = 0; j < width; j++)
{
//Process char [...]
lab[i][j] = s.at(j);
}
}
Each string of the vector is empty so you may not access its elements by the subscript operator.

Answer 4

Avoid working on separater characters in string. You havent event set the size before calling operator[] on your string, it does not even work when I check with g++/mingw.
This line is bad:
lab[i][j] = s.at(j);
lab[i] as not been constructed with a size, so it is not required to be able to contains anything.
Use (without the j for loop):
lab[i] = s;
Or even better:
//Input values
for (i = 0; i < height; i++)
{
getline (cin, lab[i]);
}