How Can I Force std::vector to Use Move Semantics During Expansion?

Enforcing Move Semantics during Vector Expansion

In situations where a std::vector contains objects with both copy and move constructors, it may be desirable to enforce the use of the move constructor as the vector expands. This ensures efficient memory management and prevents unnecessary copying.

Problem:

A std::vector of objects of class A will utilize the copy constructor A( const A&) when growing its size via push_back. However, it is desired to leverage the move constructor A(A&&) instead.

Solution:

To enable the use of the move constructor during vector expansion, the following steps are necessary:

1. Declare a Noexcept Move Constructor: The move constructor must be declared as noexcept to guarantee that it will not throw any exceptions. This is essential because std::vector relies on this property to ensure exception safety.
2. Implement the Noexcept Move Constructor: The move constructor must be implemented with the noexcept specifier. This ensures that C (and specifically std::vector) understands that the move operation is exception-safe.

Example:

The following code demonstrates a move constructor implementation that is recognized by std::vector:

A(A &&rhs) noexcept { 
  std::cout << "i am the move constr" << std::endl;
  ... some code doing the move ...  
  m_value=std::move(rhs.m_value) ; // etc...
}

By declaring and implementing the move constructor as noexcept, std::vector will be able to use it when growing its size.

Additional Considerations:

• Use emplace_back: Consider using emplace_back instead of push_back when possible. emplace_back directly constructs objects in place within the vector, potentially providing performance improvements.
• Default Move Constructor: In many cases, the default move constructor generated by the compiler may be sufficient. To explicitly request the default move constructor, declare it as A(A&&) = default;. This will ensure that it is noexcept when possible.

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