Dynamically allocated string array, then change it's value?

Question

I'm trying to create a string array and use a pointer to modify it. I'm not sure how to declare the pointer since strings can vary in length, and I think this is what causes the error.

My code looks something like this:

#includes <string>
#includes <iostream>
using namespace std;

string *users = NULL;
int seatNum = NULL;
cin >> seatNum;
users = new string[seatNum];
string name;
cin >> name;
users[seatNum] = name;

It throws me an Write Access Violation when I try to change its value. From what I've read it's because strings are compiled as read-only, so my question is how would I/what would I do to change it? Easy-to-understand explanations would be preferable.

users = new string[seatNum]; creates an array of strings indexed from 0 to seatNum - 1, so users[seatNum] accesses past the bounds of the array. — Retired Ninja
– Retired Ninja, Commented Nov 26, 2013 at 2:29

bvj · Accepted Answer · 2013-11-26 02:38:49Z

4

You're accessing memory beyond the range of the allocated array

users = new string[seatNum];
users[seatNum] = name;

The first element is [0]. The last is [seatNum-1]

answered Nov 26, 2013 at 2:38

bvj

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1 Comment

PeppeJ Over a year ago

Doh, I have no idea how I could overlook something as simple as this. Sometimes I guess you just have to ask someone, thanks anyway!

sh03 · Accepted Answer · 2013-11-26 02:47:10Z

1

You have created an array of seatNum elements. Array element indexing starts at 0 therefore the range of valid indexes is [0, seatNum - 1]. By accessing users[seatNum] = ... you are effectively going past the last valid element of the array. This invokes UB (undefined behavior).

I see you have already made the right choice of using std::string instead of C-style strings. Why not make the same choice over arrays?

#include <string>
#include <array> 
#include <iostream>

int main(int, char*[]) {
    int seatNum = 0;
    std::cin >> seatNum;
    std::vector<std::string> users(seatNum);
    std::cin >> users[0];
    return 0;
}

Try to avoid pointers and C-style arrays, especially dynamic ones.

answered Nov 26, 2013 at 2:47

sh03

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Comments

Pepe · Accepted Answer · 2013-11-26 02:56:45Z

0

A few things:

int seatNum will be allocated on the stack and will never be NULL. You should set it to 0.
You are setting users[seatNum] which is out of bounds causing your program to crash. You can only use indices from 0 to seatNum-1.

Updated: Chris is correct. I looked into it and strings are indeed mutable in C++.

edited Nov 26, 2013 at 2:56

answered Nov 26, 2013 at 2:30

Pepe

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1 Comment

Qaz Over a year ago

The last point isn't really correct. Strings are mutable in C++, so there's no reason to create a copy. It just modifies the object in question. Whether that means doing a deep copy of the contained characters varies.

Martin Brisiak · Accepted Answer · 2021-02-16 09:54:07Z

0

firstly you cannot set null value to int type data

int seatNum = NULL; // wrong 
int seatNum = 0;  // right

secondly string bounds from 0 to seatnum -1

users[seatNum-1] = name; // right

edited Feb 16, 2021 at 9:54

Martin Brisiak

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answered Feb 16, 2021 at 9:21

anitha shambamurthy

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