(Guaranteed) Copy Elision in C++11/14/17

Compilers have become incredible optimization machines, which use loop unrolling, heap elision, devirtualization and other techniques to turn (regular) code into highly efficient machine instructions. One of these optimization techniques elides copies and moves by constructing an object directly into the target of the omitted cope/move operation. This technique is called copy/move elision and is now guaranteed by the C++17 standard to occur in certain situations.

An example of (guaranteed) copy elision occurs when initializing from a temporary.

#include <iostream>
struct Foo {
    Foo() {std::cout << "Default Constructor" << std::endl;}
    Foo(Foo const& other) {std::cout<< "Copy Constructor" << std::endl;}
    Foo(Foo &&) {std::cout << "Move Constructor" << std::endl;}
    Foo& operator=(Foo const& other) {std::cout <<" Assignment" << std::endl; return *this;}
    Foo& operator=(Foo &&) {std::cout <<"Move Assignment" << std::endl; return *this;}
    ~Foo() {std::cout << "Destructor" << std::endl;}
  auto foo = Foo();

Regardless of the C++ version, compiling and running this with a modern compiler will print:

  Default Constructor

Even in earlier versions of C++, compilers were allowed to elide copies in this situation. This is true regardless of potential side-effects of the copy/move constructor. However, when using C++11/14 Foo has to be move-constructable even though the move constructor is not actually called. Deleting the move constructor Foo (Foo &&) = delete results in a compile error when using C++11/14. This is because the program has to be well-formed even when copy elision is not performed, thus requiring the move constructor. The C++17 standard guarantees copy elision in this case.

The above case is an example of copy elision during initializing from a temporary. There are three distinct cases where copy elision can occur:

  1. Initializing from a temporary
  2. Return Value Optimization (RVO) including Named Return Value Optimization (NRVO)
  3. Throwing/catching exceptions by value

I will cover the latter two in future blog posts. As you might infer from the above results, it is hard to pass non-movable types around without guaranteed copy elision.

How guaranteed copy elision works1

To understand how guaranteed copy elision is achieved in C++17, we need to take a look at C++ value categories (which confusingly enough categorize expressions, not values). There are excellent cpp-reference sites2 and blog posts3 explaining value categories and their references. I highly recommend checking them out.

For the purpose of this post it suffices to take a look at prvalues (pure rvalues) and glvalues (generalized lvalues). The proposal for guaranteed copy elision rewords the definition of these two value catagories. Roughly speaking:

  • A glvalue is an expression whose evaluation computes the location of an object,
  • A prvalue is an expression whose evaluation initializes an object

In other words, a glvalue specifies the object location while a prvalue is its initializer. For all the details take a look at the original proposal1. With these definitions in mind, let us revisit the example from the beginning:

auto foo = Foo();

here foo is an glvalue and Foo() a prvalue. This means, that Foo() can directly initialize foo without a reachable for copy-/move- constructor.

With the above definitions gl- and pr-values copy elision occurs naturally and no special rules are required.

Philipp Jung
Philipp Jung
Software Engineer

My interest include C++, Rust, modern CMake and other programming related topics.