auto(x): decay-copy in the language

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 value as if passing x as a function argument by value. The functionality is realized as the decay-copy function in the standard for exposition only.

The paper also proposes decltype(auto)(x) and decltype(auto){x} for forwarding x.

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 a 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();

Its primary purpose is to declare a variable while being a copy is a property of the declaration. 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 the role of auto(x):

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()) creates a copy of x.front() even if x.front() is already a prvalue (thus, already a copy).

There is a less obvious issue which 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, but in a more general setting, where a type A has access to a set of type T's private or protected copy/move constructors, decay-copy an object of T fails inside A's class scope, but auto(x) continues to work.

Constrained auto wants to forward without T

On CppCon 2019, the author observed that multiple speakers are using std::forward<decltype(arg)>(arg) in their slides. The context is that in generic functions that use the constrained auto syntax, there is no access to the type T that “declared” arg:

auto f(Copyable auto&& arg) {
    /* ... */ std::forward<??>(arg);
}

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.

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 (given 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:

decltype(auto){x} means the right thing

One problem in teaching std::forward is that we may have to explain why and how forwarding rvalues as lvalues are forbidden. Such a gap implies that the approach we took diverges from the meaning we want to express.

“Forwarding” means restoring an expression’s value category given its type. decltype(auto){x} does precisely that. decltype(auto) is the expression’s type, so decltype(auto){x} casts the expression into a different one whose value category matches the type.

decltype(auto){x} is not only a shorter way^[2] to forward arguments, but also means a right way to forward arguments.

Implementation

Try it out (not including decltype(auto)(x)): Godbolt

Wording

The wording is relative to N4830.

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

A simple-type-specifier (9.1.8.2) or typename-specifier (13.7) 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 (12.3.1.8) for the remainder of this section. Otherwise, if the type is auto or decltype(auto), it is replaced by the type deduced for the variable x in the invented declaration (9.1.8.5):

auto x init;

or

decltype(auto) x init;

, respectively, where init is the initializer.

Modify 9.1.8.5 [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 (13.1). The auto and decltype(auto) type-specifier can also be used as the simple-type-specifier in an explicit type conversion (functional notation) (7.6.1.3).

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); }

Search and replace “calls to decay-copy being evaluated in” (the thread/the current thread) with “where the values produced by auto are materialized in”.

Search and replace “decay-copy” with “auto”.

Acknowledgments

Thank Alisdair Meredith, Arthur O’Dwyer, and Billy O’Neal for providing examples and feedback for this paper.

References