`emplace_back` vs. `push_back` in C++

Overview

Both push_back and emplace_back are member functions of C++ standard library containers (e.g., std::vector, std::deque) used to add elements to the end of the container. However, they differ in how they construct and insert these elements.

`push_back`

The push_back function adds an existing object to the end of the container. It requires the object to be constructed before being passed to the function.

Usage:

std::vector<MyClass> vec;
MyClass obj(args);
vec.push_back(obj); // Adds a copy of 'obj' to the vector

If the object is movable, push_back can utilize move semantics:

vec.push_back(std::move(obj)); // Moves 'obj' into the vector

`emplace_back`

The emplace_back function constructs a new element in place at the end of the container. It forwards the provided arguments to the constructor of the element, eliminating the need for a temporary object.

Usage:

std::vector<MyClass> vec;
vec.emplace_back(args); // Constructs 'MyClass' directly in the vector

This approach can improve performance by avoiding unnecessary copy or move operations, especially for complex objects.

Key Differences

Object Construction:
- push_back: Requires a fully constructed object.
- emplace_back: Constructs the object in place using provided arguments.
Performance:
- push_back: May involve copy or move operations, depending on whether the object is passed by value or moved.
- emplace_back: Potentially more efficient for complex objects, as it avoids extra copy or move operations.

When to Use

Use push_back when you have an existing object that you want to add to the container.
Use emplace_back when you want to construct a new object directly in the container, especially if the object's construction is complex or resource-intensive.

Example

#include <vector>
#include <string>

class MyClass {
public:
    MyClass(int id, const std::string& name) : id_(id), name_(name) {}
private:
    int id_;
    std::string name_;
};

int main() {
    std::vector<MyClass> vec;

    // Using push_back
    MyClass obj(1, "Object1");
    vec.push_back(obj); // Adds a copy of 'obj'

    // Using emplace_back
    vec.emplace_back(2, "Object2"); // Constructs 'MyClass(2, "Object2")' in place

    return 0;
}

In this example, emplace_back constructs the MyClass object directly within the vector, potentially reducing overhead compared to push_back, which adds a copy of an existing object.

For a visual explanation and further insights, consider watching the following video:

Benchmarks

Compile and run these 6 programs to see the performance difference.

push_back.cpp push_back_move.cpp push_back_reserve.cpp emplace_back_slower.cpp emplace_back_faster.cpp emplace_back_fastest.cpp

$g++ push_back.cpp -o push_back
$g++ push_back_move.cpp -o push_back_move
$g++ push_back_reserve.cpp -o push_back_reserve
$g++ emplace_back_slower.cpp -o emplace_back_slower
$g++ emplace_back_faster.cpp -o emplace_back_faster
$g++ emplace_back_fastest.cpp -o emplace_back_fastest

$time ./push_back

real	0m0.496s
user	0m0.236s
sys	0m0.260s

$time ./push_back

real	0m0.497s
user	0m0.257s
sys	0m0.240s

$time ./push_back_move

real	0m0.353s
user	0m0.072s
sys	0m0.280s

$time ./push_back_move

real	0m0.364s
user	0m0.104s
sys	0m0.260s

$time ./push_back_reserve

real	0m0.340s
user	0m0.088s
sys	0m0.252s

$time ./push_back_reserve

real	0m0.338s
user	0m0.108s
sys	0m0.231s

$time ./emplace_back_slower 

real	0m0.487s
user	0m0.248s
sys	0m0.239s

$time ./emplace_back_slower 

real	0m0.483s
user	0m0.256s
sys	0m0.228s

$time ./emplace_back_faster

real	0m0.360s
user	0m0.096s
sys	0m0.264s

$time ./emplace_back_faster

real	0m0.351s
user	0m0.112s
sys	0m0.239s

$time ./emplace_back_fastest

real	0m0.336s
user	0m0.104s
sys	0m0.232s

$time ./emplace_back_fastest

real	0m0.336s
user	0m0.101s
sys	0m0.235s

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emplace_back vs. push_back in C++