modifying a dynamic array using a function modifies the original array the second time calling the function

Question

This is a sample code of a bigger program. The modifyArray function increases the array size by one then adds a new string element which here is "eeee", calling the function once works fine but when calling it the second time it modifies the original array, i want it to modify the array that came out the first modifyArray function. here is the code

#include <iostream>
#include <string>
#include <fstream>
#include <windows.h>
#include <filesystem>
#include <iomanip>
#include <string.h>
#include <sstream>

using namespace std;

void modifyArray(string*&, int);

int main()
{
    int arraySize = 4;
    string* playlist;
    playlist = new string[arraySize];
    playlist[0] = "aaaa";
    playlist[1] = "bbbb";
    playlist[2] = "cccc";
    playlist[3] = "dddd";
    
    modifyArray(playlist, arraySize);
        modifyArray(playlist, arraySize);
    for (int i = 0; i < arraySize; i++)
    {
        cout << playlist[i] << endl;
    }
}

void modifyArray(string*& Fplaylist, int FarraySize)
{
    string* subPlaylist;
    subPlaylist = new string[FarraySize];
    copy(Fplaylist, Fplaylist + FarraySize, subPlaylist);

    delete[]Fplaylist;
    FarraySize = FarraySize + 1;
    Fplaylist = new string[FarraySize];
    copy(subPlaylist, subPlaylist + FarraySize - 1, Fplaylist);
    Fplaylist[FarraySize - 1] = "eeee";
    

}

expected output:

aaaa
bbbb
cccc
dddd
eeee
eeee

Real output:

aaaa
bbbb
cccc
dddd

ps: I can't use vectors

I tried other things like storing the array in an external text file but I get the same results. I am a beginner at C++ so I really don't understand pointers quite well.

ps: I can't use vectors -- The std::vector has been officially part of C++ for 24 years now. When will this silly restriction of not using vector be put in the dustbin of history.? — PaulMcKenzie
– PaulMcKenzie, Commented Dec 23, 2022 at 14:26
Ditch the stupid manual memory management as step 0 towards fixing this. — Jesper Juhl
– Jesper Juhl, Commented Dec 23, 2022 at 14:30
Compare both parameters to the function. Keep staring at them until you see the difference. There's something radically different between them. Therein lies the explanation for your bug. — Sam Varshavchik
– Sam Varshavchik, Commented Dec 23, 2022 at 14:34

john · Accepted Answer · 2022-12-23 14:55:55Z

2

There are two problems with the modifyArray function.

The first problem is that FarraySize should be a reference so that changes to it affect the variable used to call the function (exactly like Fplaylist).

The second problem is that the function makes two allocations and two copies, when it should be one of each. Making two allocations leaks memory, and make two copies is needlessly inefficient.

Here's a version that works

void modifyArray(string*& Fplaylist, int& FarraySize)
{
    string* newPlaylist = new string[FarraySize + 1];
    copy(Fplaylist, Fplaylist + FarraySize, newPlaylist);
    newPlaylist[FarraySize++] = "eeee";
    delete[] Fplaylist;
    Fplaylist = newPlaylist;
}

answered Dec 23, 2022 at 14:55

john

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