Guaranteed Copy Elision Does Not Elide Copies

Sy Brand

Sy

This post is also available on Simon Brand’s blog

C++17 merged in a paper called Guaranteed copy elision through simplified value categories. The changes mandate that no copies or moves take place in some situations where they were previously allowed, e.g.:

You can see this behavior in compiler versions Visual Studio 2017 15.6, Clang 4, GCC 7, and above.

Despite the name of the paper and what you might read on the Internet, the new rules do not guarantee copy elision. Instead, the new value category rules are defined such that no copy exists in the first place. Understanding this nuance gives a deeper understanding of the current C++ object model, so I will explain the pre-C++17 rules, what changes were made, and how they solve real-world problems.

Value Categories

To understand the before-and-after, we first need to understand what value categories are (I’ll explain copy elision in the next section). Continuing the theme of C++ misnomers, value categories are not categories of values; they are characteristics of expressions. Every expression in C++ has one of three value categories: lvalue, prvalue (pure rvalue), or xvalue (eXpiring value). There are then two super-categories, as shown in the diagram below.

diagram expression the taxonomy described above

For an explanation of what these are, we can look at the standard (C++17 [basic.lval]/1):

  1. A glvalue [(generalized lvalue)] is an expression whose evaluation determines the identity of an object, bit-field, or function.
  2. A prvalue is an expression whose evaluation initializes an object or a bit-field, or computes the value of an operand of an operator, as specified by the context in which it appears.
  3. An xvalue is a glvalue that denotes an object or bit-field whose resources can be reused (usually because it is near the end of its lifetime).
  4. An lvalue is a glvalue that is not an xvalue.
  5. An rvalue is a prvalue or an xvalue.

Some examples:

C++11

What are the properties of the expression std::string{"a pony"}?

It’s a prvalue. Its type is std::string. It has the value "a pony". It names a temporary.

That last one is the key point I want to talk about, and it’s the real difference between the C++11 rules and C++17. In C++11, std::string{"a pony"} does indeed name a temporary. From C++11 [class.temporary]/1:

Temporaries of class type are created in various contexts: binding a reference to a prvalue, returning a prvalue, a conversion that creates a prvalue, throwing an exception, entering a handler, and in some initializations. […]

Let’s look at how this interacts with this code:

make() results in a temporary. This temporary will be moved into x. Since copyable has no move constructor, this calls the copy constructor. However, this copy is unnecessary since the object constructed on the way out of make will never be used for anything else. The standard allows this copy to be elided by constructing the return value at the call-site rather than in make (C++11 [class.copy]/31). This is called copy elision.

The unfortunate part is this: even if all copies of the type are elided, the constructor still must exist.

This means that if we instead have:

then we get a compiler error:

Aside from returning non-moveable types by value, this presents other issues:

  1. Use of Almost Always Auto style is prevented for immobile types:

  2. The language makes no guarantees that the constructors won’t be called (in practice this isn’t too much of a worry, but guarantees are more convincing than optional optimizations).
  3. If we want to support some of these use-cases, we need to write copy/move constructors for types which they don’t make sense for (and do what? Throw? Abort? Linker error?)
  4. You can’t pass non-moveable types to functions by value, in case you have some use-case which that would help with.

What’s the solution? Should the standard just say “oh, if you elide all copies, you don’t need those constructors”? Maybe, but then all this language about constructing temporaries is really a lie and building an intuition about the object model becomes even harder.

C++17

C++17 takes a different approach. Instead of guaranteeing that copies will be elided in these cases, it changes the rules such that the copies were never there in the first place. This is achieved through redefining when temporaries are created.

As noted in the value category descriptions earlier, prvalues exist for purposes of initialization. C++11 creates temporaries eagerly, eventually using them in an initialization and cleaning up copies after the fact. In C++17, the materialization of temporaries is deferred until the initialization is performed.

That’s a better name for this feature. Not guaranteed copy elision. Deferred temporary materialization.

Temporary materialization creates a temporary object from a prvalue, resulting in an xvalue. The most common places it occurs are when binding a reference to or performing member access on a prvalue. If a reference is bound to the prvalue, the materialized temporary’s lifetime is extended to that of the reference (this is unchanged from C++11, but worth repeating). If a prvalue initializes a class type of the same type as the prvalue, then the destination object is initialized directly; no temporary required.

Some examples:

That covers the most important points of the new rules. Now on to why this is actually useful past terminology bikeshedding and trivia to impress your friends.

Who cares?

I said at the start that understanding the new rules would grant a deeper understanding of the C++17 object model. I’d like to expand on that a bit.

The key point is that in C++11, prvalues are not “pure” in a sense. That is, the expression std::string{"a pony"} names some temporary std::string object with the contents "a pony". It’s not the pure notion of the list of characters “a pony”. It’s not the Platonic ideal of “a pony”.

In C++17, however, std::string{"a pony"} is the Platonic ideal of “a pony”. It’s not a real object in C++’s object model, it’s some elusive, amorphous idea which can be passed around your program, only being given form when initializing some result object, or materializing a temporary. C++17’s prvalues are purer prvalues.

If this all sounds a bit abstract, that’s okay, but internalizing this idea will make it easier to reason about aspects of your program. Consider a simple example:

In the C++11 model, the prvalue foo{} creates a temporary which is used to move-construct x, but the move is likely elided by the compiler.

In the C++17 model, the prvalue foo{} initializes x.

A more complex example:

In the C++11 model, return "a pony"; initializes the temporary return object of a(), which move-constructs the temporary return object of b(), which move-constructs x. All the moves are likely elided by the compiler.

In the C++17 model, return "a pony"; initializes the result object of a(), which is the result object of b(), which is x.

In essence, rather than an initializer creating a series of temporaries which in theory move-construct a chain of return objects, the initializer is teleported to the eventual result object.

Closing

The “guaranteed copy elision” rules do not guarantee copy elision; instead they purify prvalues such that the copy doesn’t exist in the first place. Next time you hear or read about guaranteed copy elision, think instead about deferred temporary materialization. Even if you don’t find the terminology important, this knowledge will help reason about the behavior of your code more easily.

Deferred temporary materialization/guaranteed copy elision has been supported in MSVC since Visual Studio 2017 version 15.6. We’d love for you to download the latest version and try it out. As always, we welcome your feedback. We can be reached via the comments below or via email (visualcpp@microsoft.com). If you encounter other problems with MSVC or have a suggestion for Visual Studio 2017 please let us know through Help > Send Feedback > Report A Problem / Provide a Suggestion in the product, or via Developer Community. You can also find us on Twitter (@VisualC) and Facebook (msftvisualcpp).

If you have any questions about this article, please use the comments below or send them directly to Simon at simon.brand@microsoft.com or @TartanLlama.

Sy Brand
Sy Brand

Follow Sy   

0 comments

    Leave a comment