Dynamically allocate memory for vectors - c++

Question

I am trying to replace arrays with vectors but I can't figure out how.
Replace this function to dynamically allocate memory for vectors:
string readFile(string filename, string** list, int size){
*list = new string[size];
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
*(*list + i) = line;
}
file.close();
return **list;
}
And here's my attempt to change it to vectors with no luck. Any feedback is greatly appreciated:
string processFile(string filename, vector<string>** list, int size){
*list = new vector<string>(size);
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
*list[i] = line; // error
}
file.close();
return **list; // error
}

Answer 1

You will need some proper error handling, but basically, you need neither pointers nor fixed sizes if you use containers:
std::vector<std::string> readLinesFromFile(const std::string& filename)
{
std::vector<std::string> result;
std::ifstream file(filename);
for (std::string line; std::getline(file, line); )
{
result.push_back(line);
}
return result;
}

Answer 2

There are several problems:
You don't need to use vector**, vector is equivalent to the list in previous code.
The return type is string, but you are returning vector**
This code should work, not tested though:
void processFile(string filename, vector<string>& list, int size){
//list = new vector<string>(size); // no need if you get a vector reference
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
list.push_back(line); //the error was because you are assigning string to a vector<string>*
}
file.close();
// you dont have to return, as vector is passed by reference
}
If you still need to use pointer of vector
void processFile(string filename, vector<string>** list, int size){
*list = new vector<string>(size); // bad practice
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
(*list)->push_back(line);
}
file.close();
// you dont have to return, as vector is passed by pointer
}

Answer 3

Change *list[i] = line to *list->push_back(line) and you should be okay for the first error.
The second error is going to depend on what your intent is for the return value. I think return *list->front(); will give the same result as the first example, but if you are planning on returning more than just the first line then you will need to do some concatenation. You can just create a local string and append each line as you read them.
Hopefully your teacher knows using new vector is almost always a code smell in C++ and is using this for a specific reason with a plan to fix it later.

Answer 4

here is a working example. enjoy :)
BTW - you don't need to pass the length, just instantiate the memory and use the push_back method.
#include <vector>
#include <fstream>
#include <string>
using namespace std;
void processFile(string filename, vector<string>** list, int size);
void main()
{
vector<string>* list = NULL;
processFile("C:\\temp.txt", &list, 13);
int i = 1;
}
void processFile(string filename, vector<string>** list, int size){
*list = new vector<string>();
ifstream file(filename);
string line;
for (int i = 0; i < size; i++){
getline(file, line);
(**list).push_back(line); // error
}
file.close();
}