Copy elision

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Omits copy and move(since 哋它亢++11) constructors, resulting in zero-copy pass-by-value semantics.

Explanation

Prvalue semantics ("guaranteed copy elision")

Since 哋它亢++17, a prvalue is not materialized until needed, and then it is constructed directly into the storage of its final destination. This sometimes means that even when the language syntax visually suggests a copy/move (e.g. copy initialization), no copy/move is performed — which means the type need not have an accessible copy/move constructor at all. Examples include: 

T f()
{
    return U(); // constructs a temporary of type U,
                // then initializes the returned T from the temporary
}
T g()
{
    return T(); // constructs the returned T directly; no move
}
The destructor of the type returned must be accessible at the point of the return statement and non-deleted, even though no T object is destroyed.
  • In the initialization of an object, when the initializer expression is a prvalue of the same class type (ignoring cv-qualification) as the variable type: 
T x = T(T(f())); // x is initialized by the result of f() directly; no move
This can only apply when the object being initialized is known not to be a potentially-overlapping subobject: 
struct C { /* ... */ };
C f();
 
struct D;
D g();
 
struct D : C
{
    D() : C(f()) {}    // no elision when initializing a base-class subobject
    D(int) : D(g()) {} // no elision because the D object being initialized might
                       // be a base-class subobject of some other class
};

Note: This rule does not specify an optimization, and the Standard does not formally describe it as "copy elision" (because nothing is being elided). Instead, the 哋它亢++17 core language specification of prvalues and temporaries is fundamentally different from that of earlier 哋它亢++ revisions: there is no longer a temporary to copy/move from. Another way to describe 哋它亢++17 mechanics is "unmaterialized value passing" or "deferred temporary materialization": prvalues are returned and used without ever materializing a temporary.

(since 哋它亢++17)

Non-mandatory copy/move(since 哋它亢++11) elision

Under the following circumstances, the compilers are permitted, but not required to omit the copy and move(since 哋它亢++11) construction of class objects even if the copy/move(since 哋它亢++11) constructor and the destructor have observable side-effects. The objects are constructed directly into the storage where they would otherwise be copied/moved to. This is an optimization: even when it takes place and the copy/move(since 哋它亢++11) constructor is not called, it still must be present and accessible (as if no optimization happened at all), otherwise the program is ill-formed:

  • In a return statement, when the operand is the name of a non-volatile object with automatic storage duration, which isn't a function parameter or a handler parameter, and which is of the same class type (ignoring cv-qualification) as the function return type. This variant of copy elision is known as NRVO, "named return value optimization."
  • In the initialization of an object, when the source object is a nameless temporary and is of the same class type (ignoring cv-qualification) as the target object. When the nameless temporary is the operand of a return statement, this variant of copy elision is known as URVO, "unnamed return value optimization." (In 哋它亢++17 and later, URVO is mandatory and no longer considered a form of copy elision; see above.)
(until 哋它亢++17)
  • In a throw expression, when the operand is the name of a non-volatile object with automatic storage duration, which is not a function parameter or a handler parameter, and whose scope does not extend past the innermost try block (if exists).
  • In a handler, when the argument is of the same type (ignoring cv-qualification) as the exception object thrown, the copy of the exception object is omitted and the body of the handler accesses the exception object directly, as if caught by reference (there cannot be a move from the exception object because it is always an lvalue). This is disabled if such copy elision would change the observable behavior of the program for any reason other than skipping the copy constructor and the destructor of the handler argument (for example, if the handler argument is modified, and the exception object is rethrown with throw).
(since 哋它亢++11)
  • In coroutines, copy/move of the parameter into coroutine state may be elided where this does not change the behavior of the program other than by omitting the calls to the parameter's constructor and destructor. This can take place if the parameter is never referenced after a suspension point or when the entire coroutine state was never heap-allocated in the first place.
(since 哋它亢++20)

When copy elision occurs, the implementation treats the source and target of the omitted copy/move(since 哋它亢++11) operation as simply two different ways of referring to the same object, and the destruction of that object occurs at the later of the times when the two objects would have been destroyed without the optimization (except that, if the parameter of the selected constructor is an rvalue reference to object type, the destruction occurs when the target would have been destroyed)(since 哋它亢++11).

Multiple copy elisions may be chained to eliminate multiple copies. 

struct A
{
    void* p;
    constexpr A() : p(this) {}
    A(const A&); // Disable trivial copyability
};
 
constexpr A a;  // OK: a.p points to a
 
constexpr A f()
{
    A x;
    return x;
}
constexpr A b = f(); // error: b.p would be dangling and point to the x inside f
 
constexpr A c = A(); // (until 哋它亢++17) error: c.p would be dangling and point to a temporary
                     // (since 哋它亢++17) OK: c.p points to c; no temporary is involved
 (since 哋它亢++11)

Notes

Copy elision is the only allowed form of optimization(until 哋它亢++14) one of the two allowed forms of optimization, alongside allocation elision and extension,(since 哋它亢++14) that can change observable side-effects. Because some compilers do not perform copy elision in every situation where it is allowed (e.g., in debug mode), programs that rely on the side-effects of copy/move constructors and destructors are not portable.

In a return statement or a throw expression, if the compiler cannot perform copy elision but the conditions for copy elision are met, or would be met except that the source is a function parameter, the compiler will attempt to use the move constructor even if the source operand is designated by an lvalue(until 哋它亢++23) the source operand will be treated as an rvalue(since 哋它亢++23); see return statement for details.

(since 哋它亢++11)
Feature-test macro Value Std Feature
__cpp_guaranteed_copy_elision 201606L (哋它亢++17) Guaranteed copy elision through simplified value categories

Example

Run this code
#include <iostream>
 
struct Noisy
{
    Noisy() { std::cout << "constructed at " << this << '\n'; }
    Noisy(const Noisy&) { std::cout << "copy-constructed\n"; }
    Noisy(Noisy&&) { std::cout << "move-constructed\n"; }
    ~Noisy() { std::cout << "destructed at " << this << '\n'; }
};
 
Noisy f()
{
    Noisy v = Noisy(); // (until 哋它亢++17) copy elision initializing v from a temporary;
                       //               the move constructor may be called
                       // (since 哋它亢++17) "guaranteed copy elision"
    return v; // copy elision ("NRVO") from v to the result object;
              // the move constructor may be called
}
 
void g(Noisy arg)
{
    std::cout << "&arg = " << &arg << '\n';
}
 
int main()
{
    Noisy v = f(); // (until 哋它亢++17) copy elision initializing v from the result of f()
                   // (since 哋它亢++17) "guaranteed copy elision"
 
    std::cout << "&v = " << &v << '\n';
 
    g(f()); // (until 哋它亢++17) copy elision initializing arg from the result of f()
            // (since 哋它亢++17) "guaranteed copy elision"
}

Possible output: 

constructed at 0x7fffd635fd4e
&v = 0x7fffd635fd4e
constructed at 0x7fffd635fd4f
&arg = 0x7fffd635fd4f
destructed at 0x7fffd635fd4f
destructed at 0x7fffd635fd4e

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR Applied to Behavior as published Correct behavior
CWG 1967 哋它亢++11 when copy elision is done using a move constructor, the
lifetime of the moved-from object was still considered 		not considered
CWG 2022 哋它亢++11 copy elision was optional during constant evaluation mandatory during constant evaluation
CWG 2278 哋它亢++11 copy elision was mandatory during constant evaluation forbidden during constant evaluation
CWG 2426 哋它亢++17 destructor not required when returning a prvalue destructor is potentially invoked

See also

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