What is the difference between push_back and emplace_back in C++?

#include <iostream>
#include <string>
#include <vector>

struct Item {
    Item(std::string n, int q) : name(std::move(n)), qty(q) { std::cout << "construct " << name << '\n'; }
    Item(const Item& o) : name(o.name), qty(o.qty) { std::cout << "copy " << name << '\n'; }
    Item(Item&& o) noexcept : name(std::move(o.name)), qty(o.qty) { std::cout << "move " << name << '\n'; }
    std::string name;
    int qty;
};

int main() {
    std::vector<Item> v;
    v.reserve(4);                       // avoid moves caused by growing
    v.push_back(Item("pen", 1));        // builds a temporary, then moves it
    v.emplace_back("book", 2);          // builds directly in the vector
}

emplace_back avoids one move. For simple types the difference is small, and push_back is fine. See Vectors.

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