Copy elision

Optimizes out copy- and move-constructors, resulting in zero-copy pass-by-value semantics.

If a function returns a class type by value, and the return statement's expression is the name of a non-volatile object with automatic storage duration, which isn't the function parameter, or a catch clause parameter, and which has the same type (ignoring top-level cv-qualification) as the return type of the function, then copy/move is omitted. When that local object is constructed, it is constructed directly in the storage where the function's return value would otherwise be moved or copied to. This variant of copy elision is known as NRVO, "named return value optimization".

When a nameless temporary, not bound to any references, would be moved or copied into an object of the same type (ignoring top-level cv-qualification), the copy/move is omitted. When that temporary is constructed, it is constructed directly in the storage where it would otherwise be moved or copied to. When the nameless temporary is the argument of a return statement, this variant of copy elision is known as RVO, "return value optimization".

In a throw-expression, if the operand is the name of a non-volatile object with automatic storage duration, which isn't a function parameter or a catch clause parameter, and whose scope does not extend past the innermost try-block (if there is a try-block), then copy/move is omitted. When that local object is constructed, it is constructed directly in the storage where the exception object would otherwise be moved or copied to.

When handling an exception, if the argument of the catch clause is of the same type (ignoring top-level cv-qualification) as the exception object thrown, the copy is omitted and the body of the catch clause accesses the exception object directly, as if caught by reference. 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 catch clause's parameter.

(since C++11)

When copy elision occurs, the implementation treats the source and target of the omitted copy/move 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 C++17).

Multiple copy elisions may be chained to eliminate multiple copies.

[edit] Notes

Copy elision is the only allowed form of optimization that can change the 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.

Even when copy elision takes place and the copy-/move-constructor is not called, it must be present and accessible (as if no optimization happened at all), otherwise the program is ill-formed.

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 object is designated by an lvalue; see return statement for details.

(since C++11)

[edit] Example

Run this code

#include <iostream>
#include <vector>
 
struct Noisy
{
    Noisy() { std::cout << "constructed\n"; }
    Noisy(const Noisy&) { std::cout << "copy-constructed\n"; }
    Noisy(Noisy&&) { std::cout << "move-constructed\n"; }
    ~Noisy() { std::cout << "destructed\n"; }
};
 
std::vector<Noisy> f()
{
    std::vector<Noisy> v = std::vector<Noisy>(3); // copy elision from temporary to v
    return v; // NRVO from v to the returned nameless temporary
}             // or the move constructor is called if optimizations are disabled
 
void g(std::vector<Noisy> arg)
{
    std::cout << "arg.size() = " << arg.size() << '\n';
}
 
int main()
{
    std::vector<Noisy> v = f(); // copy elision from the returned temporary to v
    g(f());                     // and from the returned temporary to the argument of g
}

Possible output:

constructed
constructed
constructed
constructed
constructed
constructed
arg.size() = 3
destructed
destructed
destructed
destructed
destructed
destructed

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Technical Specifications

Copy elision

Contents

[edit] Explanation

[edit] Notes

[edit] Example

[edit] See also