15 Special member functions [special]

15.8 Copying and moving class objects [class.copy]

15.8.3 Copy/move elision [class.copy.elision]

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side effects.
In such cases, the implementation treats the source and target of the omitted copy/move operation as simply two different ways of referring to the same object.
If the first parameter of the selected constructor is an rvalue reference to the object's type, the destruction of that object occurs when the target would have been destroyed; otherwise, the destruction occurs at the later of the times when the two objects would have been destroyed without the optimization.122
This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):
  • in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function parameter or a variable introduced by the exception-declaration of a handler ([except.handle])) with the same type (ignoring cv-qualification) as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function call's return object
  • in a throw-expression, when the operand is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one), the copy/move operation from the operand to the exception object can be omitted by constructing the automatic object directly into the exception object
  • when the exception-declaration of an exception handler (Clause [except]) declares an object of the same type (except for cv-qualification) as the exception object, the copy operation can be omitted by treating the exception-declaration as an alias for the exception object if the meaning of the program will be unchanged except for the execution of constructors and destructors for the object declared by the exception-declaration.
    [Note
    :
    There cannot be a move from the exception object because it is always an lvalue.
    end note
    ]
Copy elision is required where an expression is evaluated in a context requiring a constant expression and in constant initialization.
[Note
:
Copy elision might not be performed if the same expression is evaluated in another context.
end note
]
[Example
:
class Thing {
public:
  Thing();
  ~Thing();
  Thing(const Thing&);
};

Thing f() {
  Thing t;
  return t;
}

Thing t2 = f();

struct A {
  void *p;
  constexpr A(): p(this) {}
};

constexpr A g() {
  A a;
  return a;
}

constexpr A a;          // well-formed, a.p points to a
constexpr A b = g();    // well-formed, b.p points to b

void g() {
  A c = g();            // well-formed, c.p may point to c or to an ephemeral temporary
}
Here the criteria for elision can eliminate the copying of the local automatic object t into the result object for the function call f(), which is the global object t2.
Effectively, the construction of the local object t can be viewed as directly initializing the global object t2, and that object's destruction will occur at program exit.
Adding a move constructor to Thing has the same effect, but it is the move construction from the local automatic object to t2 that is elided.
end example
]
In the following copy-initialization contexts, a move operation might be used instead of a copy operation: overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue.
If the first overload resolution fails or was not performed, or if the type of the first parameter of the selected constructor is not an rvalue reference to the object's type (possibly cv-qualified), overload resolution is performed again, considering the object as an lvalue.
[Note
:
This two-stage overload resolution must be performed regardless of whether copy elision will occur.
It determines the constructor to be called if elision is not performed, and the selected constructor must be accessible even if the call is elided.
end note
]
[Example
:
class Thing {
public:
  Thing();
  ~Thing();
  Thing(Thing&&);
private:
  Thing(const Thing&);
};

Thing f(bool b) {
  Thing t;
  if (b)
    throw t;            // OK: Thing(Thing&&) used (or elided) to throw t
  return t;             // OK: Thing(Thing&&) used (or elided) to return t
}

Thing t2 = f(false);    // OK: no extra copy/move performed, t2 constructed by call to f

struct Weird {
  Weird();
  Weird(Weird&);
};

Weird g() {
  Weird w;
  return w;             // OK: first overload resolution fails, second overload resolution selects Weird(Weird&)
}
end example
]
Because only one object is destroyed instead of two, and one copy/move constructor is not executed, there is still one object destroyed for each one constructed.