Creating a vector of objects - c++

Question

vector <Population> obj;
int num_of_cities = 0;
cout<<"Enter the number of cities"<<endl;
cin>>num_of_cities;
for( int x = 0; x < num_of_cities ; x++)
{
cout<<"Enter population for city #"<< x + 1 <<endl;
cin>>populate;
obj[x].setPopulation(populate);
.....
Im trying to make a vector of objects. Basically the user will input the amount of cities and the program should create an object for each city. That way information on each city can be entered. I believe there is a problem with the syntax because once I put a value for populate, the program crashes. Any one can help ?

Answer 1

The following line is the problem:
obj[x].setPopulation(populate);
You are trying to access the object at index x but your vector is actually empty.
To solve this, there is 2 solutions:
You should create a Population object and push_back to your vector at each loop iteration
Example:
//...
for( int x = 0; x < num_of_cities ; x++)
{
cout<<"Enter population for city #"<< x + 1 <<endl;
cin>>populate;
Population pop;
pop.setPopulation(populate)
obj.push_back(pop);
}
//...
Or you initialize your vector with a size only when you know the number of cities: vector <Population> obj (num_of_cities);
Example:
//...
cin>>num_of_cities;
vector <Population> obj (num_of_cities); // move your vector declaration here
for( int x = 0; x < num_of_cities ; x++)
//...

Answer 2

obj[x].setPopulation(populate);
here you are trying to access an element which is out-of-bound, since the vector obj has no content yet.
A way to make this work would be to call
obj.resize(num_of_cities)
before the for loop.
A more general approach (which would work if you do not know beforehand how many elements you will have, but which is slower in this case), would be creating a Population object, and then pushing it to the vector:
Population p;
p.setPopulation(populate);
obj.push_back(p);

Answer 3

While currently available answers correctly identify the source of the problem at hand - access to empty vector - they fail to set the right example of using the vector in a given scenario. They either suggest a push_back of pre-constructed object - thus calling a copy-construcor unnecessarily - or demand a presence of default constructor, with a side effect of exra assignment.
The most elegant solution would be following:
Make sure your Population has a constructor which accepts population size and sets internal member accordingly (likely to be const int):
Population::Population(int habitants) : habitants(habitants) { }
Than, use emplace_back to insert an object without calling any other constructors:
obj.emplace_back(populate);

Answer 4

The following code will let you make a vector of city objects that you can then populate with meaningful data. First, you need to define a City class that can set and get the various data items. The code will compile and run but it is a limited version of what you might want to accomplish.
#include<iostream>
#include<vector>
#include<string>
using namespace std;
class City {
public:
City() {}
~City() {}
void setPopulation(int& pop) { m_pop = pop; }
int getPopulation() const { return m_pop; }
void setName(const string& name) { m_name = name; }
string getName() const { return m_name; }
private:
string m_name{ "" };
int m_pop{ 0 };
};
vector<City> cities(0);
int num_of_cities = 0;
int main() {
do {
cout << "Enter number of cities: ";
cin >> num_of_cities;
} while (num_of_cities < 1);
cities.resize(num_of_cities);
int val{ 0 };
string nam{ "" };
for (int i = 0; i < cities.size(); i++) {
cout << "Enter name of city: ";
cin >> nam;
cities[i].setName(nam);
cout << "Enter population for city: ";
cin >> val;
cities[i].setPopulation(val);
}
system("pause");
return 0;
}