vector pitfalls

vector_pitfalls.cc

//file: optm_vec.cc
#include <iostream>
#include <vector>

struct Vertex {
    float x, y, z;

    Vertex(float x, float y, float z)
        : x(x), y(y), z(z)
    {
        std::cout << "normal construction!" << std::endl;
    }

    Vertex(const Vertex & other)
        : x(other.x), y(other.y), z(other.z)
    {
        std::cout << "copy!" << std::endl;
    }
};

int main(void) {
    std::vector<Vertex> vertices;

    // avoid resizing each time we push_back a new vertex.
    vertices.reserve(3); // comment this to see what happens

    // if we instead did push_back, then we'd first have
    // to place the Vertex on the stack and later add it to
    // vector with a copy for each push_back. emplace_back recieves
    // Vertex's parameters and instantiates it directly inside vector.
    vertices.emplace_back(1,2,3);
    vertices.emplace_back(4,5,6);
    vertices.emplace_back(7,8,9);

    // swap comments with this one to see what happens
    // vertices.push_back(Vertex(1,2,3));
    // vertices.push_back(Vertex(4,5,6));
    // vertices.push_back(Vertex(7,8,9));
}