emplaceback

Emplaceback is a member function of standard library sequence containers that constructs a new element at the end of the container in place, using the provided arguments as constructor parameters. Introduced in C++11, it constructs the element directly within the container’s storage, avoiding the creation of a temporary object.

Mechanism: Emplaceback uses perfect forwarding to forward the given arguments to the element type’s constructor. This enables in-place construction and can reduce or eliminate copies or moves compared with push_back, which typically requires a pre-constructed object.

Usage: Typical usage is to call emplace_back with the constructor arguments of the element type. For example, for a vector<Point> where Point has a constructor Point(int x, int y), you can write v.emplace_back(3, 4).

Comparison with push_back: push_back adds an existing object to the end of the container, possibly requiring a copy or move. Emplaceback builds the object directly from the provided arguments, which can be more efficient for complex or non-copyable types.

Notes and caveats: Reallocation of the container can move existing elements, so performance depends on the move/copy behavior of the element type. If the type has a nonnoexcept move constructor, reallocation may be more expensive. As with other in-place constructions, you should ensure the provided constructor arguments are valid for the element type.

Availability: Emplaceback is provided by standard sequence containers such as std::vector, std::deque, std::list, and std::forward_list, and is part of the C++11 standard and later. It is commonly used to construct elements directly in the container in scenarios where creating a temporary object would be undesirable.