How do you add new objects to a user-defined Class vector?

Question

I'm a C++ beginner (no prior programming experience). I'm writing a text-based game and I have a core module to develop for the "Population" of the game. So far I've established the population growth rates (based on pre-defined natality and mortality rates) and what I'm seeking to do now is to create a unique object for each citizen.

I've created the Citizen class, and I use a vector to generate the initial population of citizens:

vector<Citizen> myCitizens (100);

There is a function that sets several initial values for each of these 100 citizens. No problems there.

Every "year" the program calculates the births and deaths for that year. I want to add new objects to the myCitizens vector based on the number of births for that year.

I'm stuck on this function:

Declaration:

int new_citizens(int newBirths);

Definition:

int new_citizens(int newBirths)
{
myCitizens.push_back(newBirths);
}

Compiler build messages:

error: no matching function for call to 'std::vector<Citizen>::push_back(int&)'
note: candidate is:
note: void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = Citizen;  _Alloc = std::allocator<Citizen>; std::vector<_Tp, _Alloc>::value_type = Citizen]

I've searched for the issue, looked at docs, messed around with changing the types to no avail. I've compiled examples where push_back did work. I think I'm missing a fundamental piece of the puzzle when it comes to creating class objects through a vector.

My current hypothesis at the time is that I'm declaring type information wrong or not correctly passing information to the vector. I'm going to keep trying. Any pointers in the right direction would be appreciated!

Thank you,

Optimae

Answer 1

vector<Citizen> myCitizens (100);

this defines a vector named myCitizens comprised of 100 objects of Citizen class, and each of them is initialized with Citizen(). Then if I correctly get that what you want to do is to resize vector to contain total number of newBirths of Citizen objects you can do it this way:

int new_citizens(int newBirths)
{
    // first we need to remove items, after this size() is 0
    myCitizens.clear();

    // and populate vector now
    for(unsigned int i = 0; i<newBirths; i++)
        myCitizens.push_back(Citizen());
}

you can also use std::vector::resize to achieve this:

int new_citizens(int newBirths){
    myCitizens.resize(newBirths);
}

note: using resize the size of container is adjusted to be exactly what you specify in call to this function: container size will be reduced if current size is greater than what you specified in call to resize or increased if current size is less than newBirths.

if however you want to add this number of new items to an existing vector, then you can do it this way:

int new_citizens(int newBirths)
{
    // just add new items
    for(unsigned int i = 0; i<newBirths; i++)
        myCitizens.push_back(Citizen());
}

or again, using resize:

int new_citizens(int newBirths){
    myCitizens.resize(myCitizens.size() + newBirths);
}

Answer 2

If you want to add newBirths news citizens, then do a loop, like this one

int new_citizens(int newBirths)
{
    for(unsigned int it = 0; it<newBirths; ++it)
        myCitizens.push_back(Citizen()); // <-- uses the default constructor
}

Answer 3

You defined your vector to hold objects of class Citizen

vector<Citizen>

Function push_back() accepts objects of type value_type, that is Citizen in your case.

void push_back (const value_type& val);

You could pass a number in a constructor

vector<Citizen> myCitizens (100);

because there is a constructor that accepts numbers:

explicit vector (size_type n, const value_type& val = value_type(),
                 const allocator_type& alloc = allocator_type());

To make your function work you could write a loop instead:

for (int i = 0; i < newBirths; ++i)
{
    myCitizens.push_back(Citizen());
}

As chris noted you may use resize() function. In fact, that would be more elegant than the loop.

myCitizens.resize(myCitizens.size() + newBirths)

See std::vector resize()