Why does this usage of std::reference_wrapper not modify the original?

Question

I have a problem with using the inbuilt & references in C++ correctly. I have an instance of a class named "Entity" it was created in the main() function. I also have an instance of a class named "Simulation" the Simulation class has a function named: addEntity(Entity e) I think that passing an instance into this function just creates a copy of it since the Simulation class then uses this instance to change it's data. After the data got changed the instance in the main() keeps it's default values.

Code:

This is the addEntity function:

void Simulation::addEntity(Entity& entity)
{
    simulation_entities.push_back(std::reference_wrapper<Entity>{entity});

}

This function uses the Entity instances to call their public functions.

void Simulation::simThread()
{

    while(simulation_state == SimulationState::Running)
    {
        auto start = std::chrono::system_clock::now();
        for (int i = 0; i<simulation_entities.size(); i++)
        {
            Entity e = simulation_entities[i];
            e.tick(simulation_entities);
        }
        auto end = std::chrono::system_clock::now();

        auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(end - start);
        std::this_thread::sleep_for(simulation_speed - elapsed);
    }

}

This is the tick method which changes internal variables of the Entity instance

void Entity::tick(vector<std::reference_wrapper<Entity>> e)
{
    std::cout << Entity::x << std::endl;
    Entity::x += entity_velocity[0];
    x += entity_velocity[0];
    y += entity_velocity[1];
    z += entity_velocity[2];
    std::cout << "after " << Entity::x << std::endl;
}

But when i try to use the instance after it was supposed to be edited by the code above it still retains it's default values...

fwiw, `std::reference_wrapper` tries to mimic built in references, but they are fundamentally different things. — 463035818_is_not_an_ai, Oct 18 '21 at 16:18
In any case, see https://en.cppreference.com/w/cpp/utility/functional/reference_wrapper — and note that assigning as `Entity e = ..` is a *copy* operation. — user2864740, Oct 18 '21 at 16:20
`Entity e = simulation_entities[i];` in `simThread` makes a copy. It doesn't matter what's in the vector, `e` is a separate object because you've declared it to be an `Entity`, not an `Entity&` or `Entity*`. — Miles Budnek, Oct 18 '21 at 16:22
Agreed but may I ask you to tell me in which places should I put the &? — Jan Ráček, Oct 18 '21 at 16:33

score 0 · Answer 1 · answered Oct 18 '21 at 18:25

0

Ok the problem was resolved by making the e variable a reference. Thanks goes to @MilesBudnek who pointed it out in the comments.

answered Oct 18 '21 at 18:25

Jan Ráček

21
2

Why does this usage of std::reference_wrapper not modify the original?

1 Answers1