auto(x): decay-copy in the language

Changes Since R3

• complete library wording

Changes Since R2

• dropped decltype(auto)(x) in comply with EWG’s opinion

Changes Since R1

• propose decltype(auto)(x) as well

Changes Since R0

• updated examples
• discussed decltype(auto)(x)
• added library wording

Introduction

This paper proposes auto(x) and auto{x} for casting x into a prvalue as if passing x as a function argument by value. The functionality appears as the decay-copy function in the standard for exposition only.

Motivation

Obtaining a prvalue copy is necessary

A generic way to obtain a copy of an object in C++ is auto a = x; but such a copy is an lvalue. We could often convey the purpose in code more accurately if we can obtain the copy as a prvalue. In the following example, let Container be a concept,

void pop_front_alike(Container auto& x) {
    std::erase(x.begin(), x.end(), auto(x.front()));
}

If we wrote

void pop_front_alike(Container auto& x) {
    auto a = x.front();
    std::erase(x.begin(), x.end(), a);
}

, questions arise – why this is not equivalent to

void pop_front_alike(Container auto& x) {
    std::erase(x.begin(), x.end(), x.front());
}

The problem is, the statement to obtain an lvalue copy is a declaration:

    auto a = x.front();

The declaration’s primary purpose is to declare a variable, while the variable being a copy is the declaration’s property. In contrast, the expression to obtain an rvalue copy is a clear command to perform a copy:

    auto(x.front())

One might argue that the above is indifferent from

    T(x.front())

However, there are plenty of situations that the T is nontrivial to get. We probably don’t want to write the original example as

void pop_front_alike(Container auto& x) {
    using T = std::decay_t<decltype(x.front())>;
    std::erase(x.begin(), x.end(), T(x.front()));
}

Obtaining a prvalue copy with auto(x) works always

In standard library specification, we use the following exposition only function to fulfill auto(x)'s role:

template<class T>
constexpr decay_t<T> decay_copy(T&& v) noexcept(
    is_nothrow_convertible_v<T, decay_t<T>>) {
    return std::forward<T>(v);
}

This definition involves templates, dependent constexpr, forwarding reference, noexcept, and two traits, and still has caveats if people want to use it in practice. An obvious issue is that decay_copy(x.front()) copies x.front() even if x.front() is a prvalue, in other words, a copy.

There is a less obvious issue that needs a minimal reproduce:

class A {
    int x;

public:
    A();

    auto run() {
        f(A(*this));           // ok
        f(auto(*this));        // ok as proposed
        f(decay_copy(*this));  // ill-formed
    }

protected:
    A(const A&);
};

The problem is that decay_copy is nobody’s friend. We can use A directly in this specific example. However, in a more general setting, where a type has access to a set of type T's private or protected copy/move constructors, decay-copy an object of T fails inside that type’s class scope, but auto(x) continues to work.

Discussion

auto(x) is a missing piece

Replacing the char in char('a') with auto, we obtain auto('a'), which is a function-style cast. Such a formula also supports injected-class-names and class template argument deduction in C++17. Introducing auto(x) and auto{x} significantly improves the language consistency:

** The type of x is a specialization of ClassTemplate.

With this proposal, all the cells in the table copy construct form x (due to CTAD’s default behavior) to obtain lvalues, prvalues, and pointers to objects, categorized by their columns. Defining auto(x) as a library^[1] facility loses orthogonality.

Introducing auto(x) into the language even improves the library consistency:

Do we also miss decltype(auto){x}?

decltype(auto){arg} can forward arg without computing arg's type. It is equivalent to static_cast<decltype(arg)>(arg) . If arg is a variable of type T&&, arg is an lvalue but static_cast<T&&>(arg) is an xvalue.

EWG discussed this idea, disliked its expert-friendly nature, and concluded that adding this facility would cause the teaching effort to add up.

Implementation

Try it out: Godbolt

Wording

The wording is relative to N4861.

Part 1

Modify 7.6.1.3 [expr.type.conv]/1 as indicated:

A simple-type-specifier (9.2.8.2) or typename-specifier (13.8) followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer. If the type is a placeholder for a deduced class type, it is replaced by the return type of the function selected by overload resolution for class template deduction (12.4.1.8) for the remainder of this section. Otherwise, if the type is auto, it is replaced by the type deduced for the variable x in the invented declaration (9.2.8.5):

auto x init;

, where init is the initializer.

Modify 9.2.8.5 [dcl.spec.auto]/5 as indicated:

A placeholder type can also be used in the type-specifier-seq in the new-type-id or type-id of a new-expression (7.6.2.7) and as a decl-specifier of the parameter-declaration’s decl-specifier-seq in a template-parameter (13.2). The auto type-specifier can also be used as the simple-type-specifier in an explicit type conversion (functional notation) (7.6.1.3).

Part 2

Remove the first entity from 16.4.2.1 [expos.only.func]/2:

template<class T> constexpr decay_t<T> decay-copy(T&& v)
    noexcept(is_nothrow_convertible_v<T, decay_t<T>>)     // exposition only
  { return std::forward<T>(v); }

Modify 24.3.1 [range.access.begin]/2 as indicated:

Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E. Then:

• If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges::begin(E) is ill-formed.
• Otherwise, if T is an array type (6.8.2) and remove_all_extents_t<T> is an incomplete type, ranges::begin(E) is ill-formed with no diagnostic required.
• Otherwise, if T is an array type, ranges::begin(E) is expression-equivalent to t + 0.
• Otherwise, if decay-copyauto(t.begin()) is a valid expression whose type models input_or_output_iteratr, ranges::begin(E) is expression-equivalent to decay-copyauto(t.begin()).
• Otherwise, if T is a class or enumeration type and decay-copyauto(begin(t)) is a valid expression whose type models input_or_output_iterator with overload resolution performed in a context in which unqualified lookup for begin finds only the declarations
    void begin(auto&) = delete;
  void begin(const auto&) = delete;
  then ranges::begin(E) is expression-equivalent to decay-copyauto(begin(t)) with overload resolution performed in the above context.
• Otherwise, ranges::begin(E) is ill-formed.

Modify 24.3.2 [range.access.end]/2 as indicated:

Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E. Then:

• If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges::end(E) is ill-formed.
• Otherwise, if T is an array type (6.8.2) and remove_all_extents_t<T> is an incomplete type, ranges::end(E) is ill-formed with no diagnostic required.
• Otherwise, if T is an array of unknown bound, ranges::end(E) is ill-formed.
• Otherwise, if T is an array, ranges::end(E) is expression-equivalent to t + extent_v<T>.
• Otherwise, if decay-copyauto(t.end()) is a valid expression whose type models sentinel_for<iterator_t<T>> then ranges::end(E) is expression-equivalent to decay-copyauto(t.end()).
• Otherwise, if T is a class or enumeration type and decay-copyauto(end(t)) is a valid expression whose type models sentinel_for<iterator_t<T>> with overload resolution performed in a context in which unqualified lookup for end finds only the declarations
    void end(auto&) = delete;
  void end(const auto&) = delete;
  then ranges::end(E) is expression-equivalent to decay-copyauto(end(t)) with overload resolution performed in the above context.
• Otherwise, ranges::end(E) is ill-formed.

Modify 24.3.5 [range.access.rbegin]/2 as indicated:

Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E. Then:

• If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges::rbegin(E) is ill-formed.
• Otherwise, if T is an array type (6.8.2) and remove_all_extents_t<T> is an incomplete type, ranges::rbegin(E) is ill-formed with no diagnostic required.
• Otherwise, if decay-copyauto(t.rbegin()) is a valid expression whose type models input_or_output_iteratr, ranges::rbegin(E) is expression-equivalent to decay-copyauto(t.rbegin()).
• Otherwise, if T is a class or enumeration type and decay-copyauto(rbegin(t)) is a valid expression whose type models input_or_output_iterator with overload resolution performed in a context in which unqualified lookup for rbegin finds only the declarations
    void rbegin(auto&) = delete;
  void rbegin(const auto&) = delete;
  then ranges::rbegin(E) is expression-equivalent to decay-copyauto(rbegin(t)) with overload resolution performed in the above context.
• […]

Modify 24.3.6 [range.access.rend]/2 as indicated:

Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E. Then:

• If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges::rend(E) is ill-formed.
• Otherwise, if T is an array type (6.8.2) and remove_all_extents_t<T> is an incomplete type, ranges::rend(E) is ill-formed with no diagnostic required.
• Otherwise, if decay-copyauto(t.rend()) is a valid expression whose type models sentinel_for<decltype(ranges::rbegin(E)> then ranges::rend(E) is expression-equivalent to decay-copyauto(t.rend()).
• Otherwise, if T is a class or enumeration type and decay-copyauto(rend(t)) is a valid expression whose type models sentinel_for<decltype(ranges::rbegin(E)> with overload resolution performed in a context in which unqualified lookup for rend finds only the declarations
    void rend(auto&) = delete;
  void rend(const auto&) = delete;
  then ranges::rend(E) is expression-equivalent to decay-copyauto(rend(t)) with overload resolution performed in the above context.
• […]

Modify 24.3.9 [range.access.size]/2 as indicated:

Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E. Then:

• If T is an array of unknown bound (9.3.3.4), ranges::size(E) is ill-formed.
• Otherwise, if T is an array type, ranges::size(E) is expression-equivalent to decay-copyauto(extent_v<T>).
• Otherwise, if disable_sized_range<remove_cv_t<T>> (24.4.3) is false and decay-copyauto(t.size()) is a valid expression of integer-like type (23.3.4.4), ranges::size(E) is expression-equivalent to decay-copyauto(t.size()).
• Otherwise, if T is a class or enumeration type, disable_sized_range<remove_cv_t<T>> is false and decay-copyauto(size(t)) is a valid expression of integer-like type with overload resolution performed in a context in which unqualified lookup for size finds only the declarations
    void size(auto&) = delete;
  void size(const auto&) = delete;
  then ranges::size(E) is expression-equivalent to decay-copyauto(size(t)) with overload resolution performed in the above context.
• […]

Modify 24.3.12 [range.access.data]/2 as indicated:

Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E. Then:

• If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges::data(E) is ill-formed.
• Otherwise, if T is an array type (6.8.2) and remove_all_extents_t<T> is an incomplete type, ranges::data(E) is ill-formed with no diagnostic required.
• Otherwise, if decay-copyauto(t.data()) is a valid expression of pointer to object type, ranges::data(E) is expression-equivalent to decay-copyauto(t.data()).
• […]

Modify 24.7.3 [range.all]/2 as indicated:

The name views::all denotes a range adaptor object (24.7.1). Given a subexpression E, the expression views::all(E) is expression-equivalent to:

• decay-copyauto(E) if the decayed type of Ethe type of that expression models view.
• Otherwise, ref_view{E} if that expression is well-formed.
• Otherwise, subrange{E}.

Modify 24.7.6.1 [range.take.overview]/2 as indicated:

The name views::take denotes a range adaptor object (24.7.1). Let E and F be expressions, let T be remove_cvref_t<decltype((E))>, and let D be range_difference_t<decltype((E))>. If decltype((F)) does not model convertible_to<D>, views::take(E, F) is ill-formed. Otherwise, the expression views::take(E, F) is expression-equivalent to:

• If T is a specialization of ranges::empty_view (24.6.1.2), then ((void) F, decay-copyauto(E)).
• […]

Modify 24.7.8.1 [range.drop.overview]/2 as indicated:

The name views::drop denotes a range adaptor object (24.7.1). Let E and F be expressions, let T be remove_cvref_t<decltype((E))>, and let D be range_difference_t<decltype((E))>. If decltype((F)) does not model convertible_to<D>, views::drop(E, F) is ill-formed. Otherwise, the expression views::drop(E, F) is expression-equivalent to:

• If T is a specialization of ranges::empty_view (24.6.1.2), then ((void) F, decay-copyauto(E)).
• […]

Modify 32.4.2.2 [thread.thread.constr]/6 as indicated:

Effects: The new thread of execution executes

  invoke(decay-copyauto(std::forward<F>(f)),
   decay-copyauto(std::forward<Args>>(args))…)

with the calls to decay-copy being evaluatedvalues produced by auto being materialized in the constructing thread. Any return value from this invocation is ignored. […]

Modify 32.4.3.1 [thread.jthread.cons]/6 as indicated:

Effects: Initializes ssource. The new thread of execution executes

  invoke(decay-copyauto(std::forward<F>(f)), get_stop_token(),
   decay-copyauto(std::forward<Args>>(args))…)

if that expression is well-formed, otherwise

  invoke(decay-copyauto(std::forward<F>(f)),
   decay-copyauto(std::forward<Args>>(args))…)

with the calls to decay-copy being evaluatedvalues produced by auto being materialized in the constructing thread. Any return value from this invocation is ignored. […]

Modify 32.9.9 [futures.async]/4 as indicated:

Effects: The first function behaves the same as a call to the second function with a policy argument of launch::async | launch::deferred […]:

• If launch::async is set in policy, calls invoke(decay-copyauto(std::forward<F>(f)), decay-copyauto(std::forward<Args>>(args))…) (20.14.3, 32.4.2.2) as if in a new thread of execution represented by a thread object with the calls to decay-copy being evaluatedvalues produced by auto being materialized in the thread that called async. Any return value is stored as the result in the shared state. Any exception propagated from the execution of invoke(decay-copyauto(std::forward<F>(f)), decay-copyauto(std::forward<Args>>(args))…) is stored as the exceptional result in the shared state. The thread object is stored in the shared state and affects the behavior of any asynchronous return objects that reference that state.
• If launch::deferred is set in policy, stores decay-copyauto(std::forward<F>(f)) and decay-copyauto(std::forward<Args>(args))... in the shared state. These copies of f and args constitute a deferred function. Invocation of the deferred function evaluates invoke(std::move(g), std::move(xyz)) where g is the stored value of decay-copyauto(std::forward<F>(f)) and xyz is the stored copy of decay-copyauto(std::forward<Args>(args)).... Any return value is stored as the result in the shared state. Any exception propagated from the execution of the deferred function is stored as the exceptional result in the shared state. […]

Acknowledgments

Thank Alisdair Meredith, Arthur O’Dwyer, and Billy O’Neal for providing examples and feedback for this paper. Thank James Touton for presenting the paper and bringing it forward.

References