`push_back` vs. `emplace_back`: When Should I Use Which in C ?

push_back vs emplace_back: A Comprehensive Explanation

In C , there are two methods for adding elements to a container: push_back and emplace_back. While both serve the same purpose, there are subtle differences in their behavior that often lead to confusion.

Overloaded push_back

The push_back method has three overloads:

void push_back(const Type& _Val);
void push_back(Type& _Val);
void push_back(Type&& _Val);

The first overload takes a constant reference, the second takes a reference, and the third takes an rvalue reference. The rvalue reference overload was introduced to facilitate move semantics, allowing for efficient insertion of temporary objects.

emplace_back

The emplace_back method has only one overload:

void emplace_back(Type& _Val);

This overload takes an rvalue reference. However, unlike the third push_back overload, emplace_back does not actually insert the value; instead, it constructs a new object directly into the container.

Purpose of emplace_back

The purpose of emplace_back becomes clear when considering the limitations of push_back. While the third push_back overload allows for efficient insertion of temporary objects, it still requires the creation of a temporary. This can lead to unnecessary copies or moves, especially when working with complex objects.

emplace_back, on the other hand, avoids the creation of a temporary by constructing the object directly into the container. This is particularly beneficial in situations where constructing the object involves multiple operations or expensive computations. For example, consider the following code:

std::vector<complicated> v;
Complicated obj(4, 5.0, "C++");
v.push_back(obj); // Creates a temporary copy
v.emplace_back(4, 5.0, "C++"); // Constructs directly into the vector</complicated>

In this case, using push_back requires the creation of a temporary copy of obj, which is then inserted into the vector. Using emplace_back eliminates the need for the temporary copy, resulting in improved efficiency.

Conclusion

While both push_back and emplace_back can be used to add elements to a container, emplace_back offers a more efficient solution when constructing complex or temporary objects directly into the container. Understanding the difference between these methods can lead to improved performance and code readability in C applications.

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