How can I create objects using a vector, for loop & unique pointers?

Question

I have a project for uni and I must create 3 objects(unique pointers) using a vector. What do I do wrong with this code? Would be really happy if you could help me out with this.

void vAufgabe_1a() {
    const int num_obj = 3;
    
        for (int i = 0; i != num_obj; i++) {
        auto FZ[i] = make_unique<Fahrzeug>(); // FZ[i] doesn't work here. 
        FZ[i].push_back(move(make_unique <Fahrzeug>));
        cout << "Der Name des Fahrzeugs: " << endl;
        cin << FZ[i]->p_sName;

    }
}

std::vector<std::unique_ptr<Fahrzeug>> FZ; should be ouside the loop. — Jarod42
– Jarod42, Commented Dec 1, 2021 at 13:11
std::move is useless in std::move(std::make_unique<Fahrzeug>()). (it might be needed for std::move(FZ[i]) though). — Jarod42
– Jarod42, Commented Dec 1, 2021 at 13:13

Pepijn Kramer · Accepted Answer · 2021-12-01 13:39:39Z

1

I wouldn't make a vector of unique pointers to objects at all (it will A: be harder, B: a double indirection). A vector will already do all the memory managment for you. Live demo here : https://onlinegdb.com/SeLMTl2Zz

#include <iostream>
#include <memory>
#include <vector>

//-----------------------------------------------------------------------------
// (Fahrzeug, but then in English)

class Vehicle
{
public:
    // give each vehicle a unique instance number
    Vehicle() :
        m_id{ ++s_id }
    {
        std::cout << "Constructed vehicle : " << m_id << "\n";
    }

    // show when object is destructed (and why unique_ptr is not needed)
    ~Vehicle()
    {
        std::cout << "Destructed vehicle : " << m_id << "\n";
    }

    std::size_t get_id() const
    {
        return m_id;
    }

private:
    static std::size_t s_id;
    std::size_t m_id;
};

//-----------------------------------------------------------------------------
// initialize id generating number
std::size_t Vehicle::s_id{ 0ul };

// don't use magic numbers in code, give your constants names
const std::size_t number_of_vehicles = 3ul;

//-----------------------------------------------------------------------------

int main()
{
    // create a scope to manage lifecycle of the vector of vehicles
    // so I can show the destruction phase more clearly
    {
        // you can initialize vectors from the constructor
        std::cout << "Creating vector with vehicles\n";
        std::vector<Vehicle> vehicles(number_of_vehicles);

        for (const auto& vehicle : vehicles)
        {
            std::cout << vehicle.get_id() << "\n";
        }

        std::cout << "Destructing vector with vehicles\n";
    }

    return 0;
}

answered Dec 1, 2021 at 13:39

Pepijn Kramer

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2 Comments

Eric Over a year ago

A vector<object> and a vector<unique_ptr<object>> are two very different things. Which one makes more sense depends entirely on the use case and how your object looks like. You shouldn't make absolute statements like "never make a vector of unique pointers to objects at all". There can be reasons to do this.

Pepijn Kramer Over a year ago

What I meant is there should be a good reason to add the extra level of indirection. It must be worth the extra effort and complexity.

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