Understanding `push_back` vs. `emplace_back` in C++ STL Containers

When working with C++ Standard Template Library (STL) containers, two commonly used member functions for appending elements are push_back and emplace_back. While both serve the purpose of adding elements to containers like vectors, understanding their differences is crucial for optimizing performance.

Introduction

The methods push_back and emplace_back allow us to add new elements at the end of a container. However, they differ significantly in how they handle object creation and insertion, impacting performance, especially when dealing with complex data types or temporary objects.

`push_back`

The push_back method is used to append an element to a container by copying or moving it. It comes in three overloads:

By Const Reference:
```
void push_back(const Type& _Val);
```
By Rvalue Reference:
```
void push_back(Type&& _Val);
```

The const reference overload is used for lvalues, while the rvalue reference overload utilizes move semantics to efficiently transfer ownership of objects.

`emplace_back`

emplace_back, on the other hand, constructs an element directly in place at the end of a container. It uses perfect forwarding with variadic templates:

template<class... Args>
void emplace_back(Args&&... args);

This means that instead of creating a temporary object and then moving or copying it into the container, emplace_back constructs the element directly in the storage space managed by the container using the provided arguments. This can eliminate unnecessary copies or moves.

Key Differences

Object Construction:
- push_back: Inserts a fully constructed object (copy/move).
- emplace_back: Constructs the object in place with given constructor parameters.
Argument Handling:
- push_back takes either a const reference or an rvalue reference to an existing object.
- emplace_back accepts variadic arguments, forwarding them to construct the object directly within the container.
Performance:
- For simple data types or when move semantics are sufficient, both methods may perform similarly.
- With complex objects or cases where temporary objects need construction (e.g., pairs in maps), emplace_back can be more efficient by avoiding extra copies or moves.

Practical Example

Consider adding a pair to a std::vector<std::pair<int, std::string>>. Using push_back, you create a pair and then copy it into the vector:

std::vector<std::pair<int, std::string>> vec;
vec.push_back(std::make_pair(1, "example"));

Using emplace_back directly constructs the pair within the container:

vec.emplace_back(1, "example");

In this case, emplace_back avoids creating a temporary std::pair object, making it more efficient.

Use Cases

Temporary Objects: When you need to create and insert temporary objects into containers, prefer emplace_back.
Complex Constructions: For classes with expensive copy/move constructors or destructors, use emplace_back to construct directly in the container.
General Usage: In scenarios where simplicity outweighs performance concerns, either method can be used, but emplace_back offers greater flexibility and potential efficiency benefits.

Conclusion

Choosing between push_back and emplace_back depends on your specific needs regarding object construction and insertion into containers. While both methods achieve similar outcomes in terms of adding elements to a container, understanding their differences allows you to optimize for performance, particularly when dealing with complex data types or temporary objects. By leveraging the power of emplace_back, you can write more efficient and expressive C++ code.