Document number:  

N3804
Date: 2013-10-09
Project: Programming Language C++
Reply-to: Beman Dawes <bdawes at acm dot org>
Kevlin Henney <kevlin at curbralan dot com>
Daniel Krügler <daniel dot kruegler at gmail dot com>

Any Library Proposal (Revision 3)

Introduction
Revision History
Motivation and Design
Design paths not taken
Examples
FAQ
Acknowledgements
Proposed Wording
    ValueType Requirements
    Header <experimental/any> synopsis
    Class bad_any_cast
    Class any
    Non-member functions
 

Introduction

This paper proposes a type-safe container for single values of value types. The C++ standards committee is planning to include the proposal in a library Technical Specification (TS).

The proposal is based on the Boost Any Library (see www.boost.org/libs/any). The Boost version of library has been in wide use for over a decade, and the library has been implemented at least twice in addition to the Boost implementation. The proposal is a pure addition to the standard library, requires no modifications to any C++14 standard library components, requires no compiler support, and will have no effect on existing namespace-disciplined code.

Revision History

N3804 - Revision 3

N3508 - Revision 2

N3390 - Revision 1

N1939 - Initial paper

Motivation and Design

Original Boost design

There are times when a generic (in the sense of general as opposed to template-based programming) type is needed: variables that are truly variable, accommodating values of many other more specific types rather than C++'s normal strict and static types. We can distinguish three basic kinds of generic type:

  1. Converting types that can hold one of a number of possible value types, e.g. int and string, and freely convert between them, for instance interpreting 5 as "5" or vice-versa. Such types are common in scripting and other interpreted languages. boost::lexical_cast supports such conversion functionality.
     
  2. Discriminated types that contain values of different types but do not attempt conversion between them, i.e. 5 is held strictly as an int and is not implicitly convertible either to "5" or to 5.0. Their indifference to interpretation but awareness of type effectively makes them safe, generic containers of single values, with no scope for surprises from ambiguous conversions.
     
  3. Indiscriminate types that can refer to anything but are oblivious to the actual underlying type, entrusting all forms of access and interpretation to the programmer. This niche is dominated by void *, which offers plenty of scope for surprising, undefined behavior.

The proposed any class (based on the class of the same name described in "Valued Conversions" by Kevlin Henney, C++ Report 12(7), July/August 2000) is a variant value type based on the second category. It supports copying of any value type and safe checked extraction of that value strictly against its type.

Design update for C++ 11/14

In several aspects, the design of std::any can be compared to std::function. This proposal differs from the specification of std::function type-erasure policy by emphasizing the worth of having guaranteed nothrow move-operations. While std::function imposes little restrictions upon the wrapped function object types where the small-object optimization can be applied, in this proposal std::any restricts these cases to those where the copy/move operations of the contained object cannot throw exceptions.

Design paths not taken

A similar design, offering more appropriate operators, could be used for a generalized function adaptor, a generalized iterator adaptor, and other object types that need uniform runtime treatment but support only compile-time template parameter conformance. Such components are not proposed here.

Examples

The following code demonstrates the syntax for using implicit conversions to and copying of any objects:

#include <list>
#include <experimental/any>

using std::experimental::any_cast;
using std::experimental::any;

typedef std::list<any> many;

void append_int(many& values, int value)
{
    any to_append = value;
    values.push_back(to_append);
}

void append_string(many& values, const std::string& value)
{
    values.push_back(value);
}

void append_char_ptr(many& values, const char* value)
{
    values.push_back(value);
}

void append_any(many& values, const any& value)
{
    values.push_back(value);
}

void append_nothing(many& values)
{
    values.push_back(any());
}

The following predicates follow from the previous definitions and demonstrate the use of queries on any objects:

bool is_empty(const any& operand)
{
    return operand.empty();
}

bool is_int(const any& operand)
{
    return operand.type() == typeid(int);
}

bool is_char_ptr(const any& operand)
{
    try
    {
        any_cast<const char *>(operand);
        return true;
    }
    catch(const std::tbd::bad_any_cast&)
    {
        return false;
    }
}

bool is_string(const any& operand)
{
    return any_cast<std::string*>(&operand);
}

void count_all(many& values, std::ostream& out)
{
    out << "#empty == "
        << std::count_if(values.begin(), values.end(), is_empty) << std::endl;
    out << "#int == "
        << std::count_if(values.begin(), values.end(), is_int) << std::endl;
    out << "#const char * == "
        << std::count_if(values.begin(), values.end(), is_char_ptr) << std::endl;
    out << "#string == "
        << std::count_if(values.begin(), values.end(), is_string) << std::endl;
}

The following type, patterned after the OMG's Property Service, defines name-value pairs for arbitrary value types:

struct property
{
    property();
    property(const std::string&, const any&);

    std::string name;
    any value;
};

typedef std::list<property> properties;

The following base class demonstrates one approach to runtime polymorphism based callbacks that also require arbitrary argument types. The absence of virtual member templates requires that different solutions have different trade-offs in terms of efficiency, safety, and generality. Using a checked variant type offers one approach:

class consumer
{
public:
    virtual void notify(const any&) = 0;
    ...
};

FAQ

What is the relationship between Boost.any and Boost.variant?

Boost::any is like a "typesafe void*", while Boost::variant is a "typesafe union".

Acknowledgements

Sean Parent and Daniel Krügler provided numerous comments, corrections, and suggestions, and were particularly helpful applying C++11 features to the library. Sean also provided a copy of his Adobe C++ implementation.  Daniel's expertise with library standardese markedly improved the proposed wording. Antony Polukhin, the maintainer of the Boost implementation, provided a helpful analysis of differences between the Boost implementation and the proposal.

Proposed Wording

Commentary that is not part of the proposed wording is shaded in gray.

Add the following section to the library TS working paper at a location to be determined by the project editor:

Any

This clause describes components that C++ programs may use to perform operations on objects of a discriminated type.

[Note: The discriminated type may contain values of different types but does not attempt conversion between them, i.e. 5 is held strictly as an int and is not implicitly convertible either to "5" or to 5.0. This indifference to interpretation but awareness of type effectively allows safe, generic containers of single values, with no scope for surprises from ambiguous conversions. -- end note.]

Header <experimental/any> synopsis

namespace std { namespace experimental { inline namespace any_v1 {
  class bad_any_cast : public bad_cast
  {
  public:
    virtual const char* what() const noexcept;
  };

  class any
  {
  public:
    // construct/destruct
    any() noexcept;

    any(const any& other);
    any(any&& x) noexcept;

    template <typename ValueType>
      any(ValueType&& value);

    template <class Allocator>
      any(allocator_arg_t, const Allocator& a) noexcept;
    template <class Allocator, typename ValueType>
      any(allocator_arg_t, const Allocator& a, ValueType&& value);
    template <class Allocator>
      any(allocator_arg_t, const Allocator& a, const any& other);
    template <class Allocator>
      any(allocator_arg_t, const Allocator& a, any&& other) noexcept;

   ~any();
  
    // assignments
    any& operator=(const any& rhs);
    any& operator=(any&& rhs) noexcept;

    template <typename ValueType>
      any& operator=(ValueType&& rhs);

    // modifiers
    void clear() noexcept;
    void swap(any& rhs) noexcept;

    // observers
    bool empty() const noexcept;
    const type_info& type() const noexcept;
  };

  void swap(any& x, any& y) noexcept;

  template<typename ValueType>
    ValueType any_cast(const any& operand);
  template<typename ValueType>
    ValueType any_cast(any& operand);
  template<typename ValueType>
    ValueType any_cast(any&& operand);

  template<typename ValueType>
    const ValueType* any_cast(const any* operand) noexcept;
  template<typename ValueType>
    ValueType* any_cast(any* operand) noexcept;
}}}

Class bad_any_cast

Objects of type bad_any_cast are thrown by a failed any_cast.

Class any

An object of class any stores an instance of any type that satisfies the constructor requirements or is empty, and this is referred to as the state of the class any object. The stored instance is called the contained object. Two states are equivalent if they are either both empty or if both are not empty and if the contained objects are equivalent.

The non-member any_cast functions provide type-safe access to the contained object.

Implementations should avoid the use of dynamically allocated memory for a small contained object. [Example: where the object constructed is holding only an int. -- end example] Such small-object optimization shall only be applied to nothrow copyable types.

The use of "should" in the above paragraph provides ISO normative encouragement. This is a deliberate design decision.

any construct/destruct

any() noexcept;

Postconditions: this->empty()

any(const any& other);

Effects: Constructs an object of type any with an equivalent state as other.

Throws: Any exceptions arising from the copy constructor of the contained object.

any(any&& other) noexcept;

Effects: Constructs an object of type any with a state equivalent to the original state of other.

Postconditions: other is left in a valid but otherwise unspecified state.

template<typename ValueType>
  any(ValueType&& value);

Let T be equal to decay<ValueType>::type.

Requires: T shall satisfy the CopyConstructible requirements. If is_copy_constructible<T>::value is false, the program is ill-formed.

Effects: Constructs an object of type any that contains an object of type T direct-initialized with std::forward<ValueType>(value).

Remarks: This constructor shall not participate in overload resolution if decay<ValueType>::type is the same type as std::any.

Throws: Any exception thrown by the selected constructor of T.

template <class Allocator>
  any(allocator_arg_t, const Allocator& a) noexcept;
template <class Allocator, typename ValueType>
  any(allocator_arg_t, const Allocator& a, ValueType&& value);
template <class Allocator>
  any(allocator_arg_t, const Allocator& a, const any& other);
template <class Allocator>
  any(allocator_arg_t, const Allocator& a, any&& other) noexcept;

Requires: Allocator shall meet the requirements for an Allocator ([allocator.requirements]).

Effects: Equivalent to the preceding constructors except that the contained object is constructed with uses-allocator construction ([allocator.uses.construction]) if memory allocation is performed.

~any();

Effects: clear().

any assignments

any& operator=(const any& rhs);

Effects: any(rhs).swap(*this), however, no effects if an exception is thrown.

Returns: *this

Throws: Any exceptions arising from the copy constructor of the contained object.

any& operator=(any&& rhs) noexcept;

Effects: any(std::move(rhs)).swap(*this).

Returns: *this

Postconditions: The state of *this is equivalent to the original state of rhs and rhs is left in a valid but otherwise unspecified state.

template<typename ValueType>
  any& operator=(ValueType&& rhs);

Let T be equal to decay<ValueType>::type.

Requires: T shall satisfy the CopyConstructible requirements. If is_copy_constructible<T>::value is false, the program is ill-formed.

Effects: Constructs an object tmp of type any that contains an object of type T direct-initialized with std::forward<ValueType>(rhs), and tmp.swap(*this); however, no effects if an exception is thrown.

Returns: *this

Remarks: This operator shall not participate in overload resolution if decay<ValueType>::type is the same type as std::any.

Throws: Any exception thrown by the selected constructor of T.

any modifiers

void clear() noexcept;

Effects: If not empty, destroys the contained object.

Postconditions: empty() == true.

void swap(any& rhs) noexcept;

Effects: Exchange the states of *this and rhs.

any observers

bool empty() const noexcept;

Returns: true if *this has no contained object, otherwise false.

const type_info& type() const noexcept;

Returns: If *this has a contained object of type T, typeid(T); otherwise typeid(void).

[Note: Useful for querying against types known either at compile time or only at runtime. --end note]

Non-member functions

void swap(any& x, any& y) noexcept;
Effects: x.swap(y).
The following specifications for any_cast have been revised to increase clarity. For the existing two signatures these changes were to exposition only. The actual behavior is unchanged from prior revisions.

The examples have been compiled and tested with GCC and VC++.

 
template<typename ValueType>
  ValueType any_cast(const any& operand);
template<typename ValueType>
  ValueType any_cast(any& operand);
template<typename ValueType>
  ValueType any_cast(any&& operand);

Requires: is_reference<ValueType>::value is true or is_copy_constructible<ValueType>::value is true. Otherwise the program is ill-formed.

Returns: For the first form, *any_cast<typename add_const<typename remove_reference<ValueType>::type>::type >(&operand). For the second and third forms, *any_cast<typename remove_reference<ValueType>::type>(&operand).

Throws: bad_any_cast if operand.type() != typeid(remove_reference<ValueType>::type).

[Example:

any x(5);                                   // x holds int
assert(any_cast<int>(x) == 5);              // cast to value
any_cast<int&>(x) = 10;                     // cast to reference
assert(any_cast<int>(x) == 10); 

x = "Meow";                                 // x holds const char*
assert(strcmp(any_cast<const char*>(x), "Meow") == 0);
any_cast<const char*&>(x) = "Harry";
assert(strcmp(any_cast<const char*>(x), "Harry") == 0);

x = string("Meow");                         // x holds string
string s, s2("Jane");
s = move(any_cast<string&>(x));             // move from any 
assert(s == "Meow");
any_cast<string&>(x) = move(s2);            // move to any
assert(any_cast<const string&>(x) == "Jane");

string cat("Meow");
const any y(cat);                           // const y holds string
assert(any_cast<const string&>(y) == cat);

any_cast<string&>(y);                       // error; cannot
                                            //  any_cast away const

--end example]

template<typename ValueType>
  const ValueType* any_cast(const any* operand) noexcept;
template<typename ValueType>
  ValueType* any_cast(any* operand) noexcept;

Returns: If operand != nullptr && operand->type() == typeid(ValueType), a pointer to the object contained by operand, otherwise nullptr.

[Example:

bool is_string(const any& operand)
{
  return any_cast<string>(&operand) != nullptr;
}

--end example]

© Copyright 2001, 2012, 2013 Kevlin Henney
© Copyright 2006, 2012, 2013 Beman Dawes
© Copyright 2013 Daniel Krügler