Emplace an aggregate in std::vector

Question

I tried to initialize the std::vector

std::vector<Particle> particles;

with instances of the simple struct

struct Particle {
    int id;
    double x;
    double y;
    double theta;
    double weight;
};

by using emplace with an initializer list:

num_particles = 1000;
for (int i = 0; i < num_particles; i++)
{
    particles.emplace_back({ i,0.0,0.0,0.0,1 });
}

But I get the error

C2660 "std::vector>::emplace_back": Function doesn't accept one argument

How can I fix that?

Answer 1

std::vector::emplace expects an iterator as argument too, because it inserts the element before that iterator's position.

Another problem is that your {i, 0.0, 0.0, 1} initialization doesn't work because it isn't in a context which tells what type it needs to instantiate. The reason there isn't any context is due to emplace and emplace_back member functions having generic parameters.

If you just want to append elements to the vector, use emplace_back.

However, emplace_back depends on the element type having a valid constructor in order to work, as the element is initialized through parentheses. That changed in C++20, which now allows aggregate-initialization through parentheses without the need to define a valid constructor.

So, up until C++17, your example would be changed to:

for (int i = 0; i < num_particles; ++i)
    particles.push_back({i, 0.0, 0.0, 1});

And in C++20 and later, you may do this instead:

for (int i = 0; i < num_particles; ++i)
    particles.emplace_back(i, 0.0, 0.0, 1);

Answer 2

You have several issues with your code:

• Emplace takes an iterator as insertion point, and then a list of values which serve as arguments to a constructor.

• Your struct must have a constructor which takes the values you pass to emplace.

• You only have 4 argument values in your code, but your Particle struct has 5 fields.

Try this code instead:

struct Particle {
    int id;
    double x;
    double y;
    double theta;
    double weight;

    Particle(int id, double x, double y, double theta, double weight) 
        : id(id), x(x), y(y), theta(theta), weight(weight)
    {
    }
};

Notice the constructor there. And then emplace, for instance in the beginning [just an example which is not inserting at the back (see below)]:

std::vector<Particle> particles;

auto num_particles = 1000;
for (int i = 0; i < num_particles; i++)
{
    particles.emplace(particles.begin(), i, 0.0, 0.0, 1.0, 0.0);
}

As others have noted, if you just want to insert without specifying a specific position in the vector, you can use emplace_back:

std::vector<Particle> particles;

auto num_particles = 1000;
for (int i = 0; i < num_particles; i++)
{
    particles.emplace_back(i, 0.0, 0.0, 1.0, 0.0);
}

This inserts the elements at the end of the vector.