rspec/rules/S5018/rule.adoc

Move operations (move constructor, move assignment operator) are all about efficient resource stealing. When stealing resources from the source, you don't have to allocate any memory or perform any other operation that might fail. This is why most people will expect move operation to be non-throwing.


Additionally, if a move operation fails, the source object can have been partially altered by the move, making recovery very tricky, or just impossible. Therefore, to ensure robustness, some functions (for instance, ``++std::move_if_noexcept++``, used by ``++std::vector++``) will decide to copy your object if its move operations are not decorated with ``++noexcept++``. This can significantly slow down your program.


If you can not implement your move operations so that they never throw, you may as well only provide copy operations that will be safer to use.


Swap operations are very similar to move operations, in that they should be equivalent to moving two objects into each other. So if you are adding a swap function to your type, it should be _noexcept_ too.


Note that for most classes, you should not write your own move operations, but rely on the "Rule-of-Zero" (S4963).


This rule raises an issue when a move or swap operation is not _noexcept_, which can happen in two cases:

* The operation is user-defined, and is not unconditionally declared as ``++noexcept++``,
* The operation is implicitly defined, and one of the base classes or member variables of the class does not have ``++noexcept++`` move operations.


== Noncompliant Code Example

----
struct A {
  A (A const &a);
  A (A && a); // Noncompliant
  ~A();
  A &operator=(A const &a);
  A &operator=(A &&a); // Noncompliant
};

void swap(A& a1, A& a2); // Noncompliant
----


== Compliant Solution

----
struct A {
  A (A const &a);
  A (A && a) noexcept;
  ~A();
  A &operator=(A const &a);
  A &operator=(A &&a) noexcept;
};

void swap(A& a1, A& a2) noexcept;
----


== See

* https://github.com/isocpp/CppCoreGuidelines/blob/036324/CppCoreGuidelines.md#c66-make-move-operations-noexcept[{cpp} Core Guidelines C.66] - Make move operations ``++noexcept++``
* https://github.com/isocpp/CppCoreGuidelines/blob/036324/CppCoreGuidelines.md#c85-make-swap-noexcept[{cpp} Core Guidelines C.85] - Make swap operations ``++noexcept++``