When should you use emplace_back over push_back in vector ?

Many STL containers (vector,map etc) support both insert and emplace operations,
vector => push_back vs emplace_back
map => insert vs emplace

Emplace semantics support in-place construction, which is why they're fast.
Okay, we've introduced yet another jargon here, In-Place construction, what is it and why is it better.To understand this, We need to take a look at what push_back does to a vector

push_back
a. Create an object in stack
b. Copy the object from stack to vector memory

As you might be aware, copying is one of the primal sins when it comes to performance. We need to eliminate it as much as we can. This is where emplace_back fits right in.

emplace_back
a. Creates object directly inside vector's memory thus avoiding the need for a copy operation

Following code snippet, shows the nitty gritties of both operations,

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

struct Chocolate
{
    std::string name;
    std::string origin;

    Chocolate(std::string p_name, std::string p_origin )
        : name(std::move(p_name)), 
          origin(std::move(p_origin))
    {
        std::cout << " Chocolate Construct operation\n";
    }
    Chocolate(Chocolate&& other)
        : name(std::move(other.name)), 
          origin(std::move(other.origin)) 
    {
        std::cout << "Chocolate Move operation\n";
    }
    Chocolate& operator=(const Chocolate& other) = default;
};

int main()
{
    std::vector<Chocolate> giftbox1;
    giftbox1.emplace_back("Dark Chocolate", "Ghana");

    std::vector<Chocolate> giftbox2;
    giftbox2.push_back(Chocolate("Milk Chocolate", "Tanzania"));

    // Unbox the content of both giftboxes 
    for (const Chocolate& choco: giftbox1) 
        std::cout << choco.name << " is from  "<<choco.origin<<"\n";

    for (const Chocolate& choco: giftbox2) 
        std::cout << choco.name << " is from  "<<choco.origin<<"\n";
}

Run this example in gcc, observe the output to infer the difference between push_back and emplace_back

That all for today, Have a great one ☕