Doc number: N4209
Revises: N4057, N3973
Date: 2014-10-9
Project: Programming Language C++, Library Evolution Working Group
Reply-to: Jonathan Coe <jbcoe@me.com>
Robert Mill <rob.mill.uk@gmail.com>

A Proposal to Add a Const-Propagating Wrapper to the Standard Library

I. Introduction

We propose the introduction of a propagate_const wrapper class that propagates const-ness to pointer-like member variables.

II. Motivation

The behaviour of const member functions on objects with pointer-like data members is seen to be surprising by many experienced C++ developers. A const member function can call non-const functions on pointer-like data members and will do so by default without use of const_cast.

Example:

  struct A
  {
    void bar() const 
    { 
      std::cout << "bar (const)" << std::endl; 
    }
    
    void bar() 
    { 
      std::cout << "bar (non-const)" << std::endl; 
    }
  };

  struct B
  {
    B() : m_ptrA(std::make_unique<A>()) {} 
    
    void foo() const 
    { 
      std::cout << "foo (const)" << std::endl;
      m_ptrA->bar(); 
    }           
    
    void foo() 
    { 
      std::cout << "foo (non-const)" << std::endl;
      m_ptrA->bar(); 
    }

    std::unique_ptr<A> m_ptrA;
  };

  int main()
  {    
    B b;
    b.foo();
    
    const B const_b;
    const_b.foo();
  }
  

Running this program gives the following output:

  foo (non-const)
  bar (non-const)
  foo (const)
  bar (non-const)

The behaviour above can be amended by re-writing void B::foo() const using const_cast to explicitly call the const member function of A. Such a change is unnatural and not common practice. We propose the introduction of a wrapper class which can be used on pointer-like member data to ensure propagation of const-ness.

Introducing propagate_const

The class propagate_const is designed to function as closely as possible to a traditional pointer or smart-pointer. Pointer-like member objects can be wrapped in a propagate_const object to ensure propagation of const-ness.

A const-propagating B would be written as

struct B
{
  B();              // unchanged
  
  void foo() const; // unchanged
  void foo();       // unchanged

  std::propagate_const<std::unique_ptr<A>> m_ptrA;
};

With an amended B, running the program from the earlier example will give the following output:

  foo (non-const)
  bar (non-const)
  foo (const)
  bar (const)

The pimpl idiom with propagate_const

The pimpl (pointer-to-implementation) idiom pushes implementation details of a class into a separate object, a pointer to which is stored in the original class [2].

class C
{
  void foo() const;
  void foo();
    
  std::unique_ptr<CImpl> m_pimpl;
};

void C::foo() const 
{ 
  m_pimpl->foo(); 
}

void C::foo() 
{ 
  m_pimpl->foo(); 
}

When using the pimpl idiom the compiler will not catch changes to member variables within const member functions. Member variables are kept in a separate object and the compiler only checks that the address of this object is unchanged. By introducing the pimpl idiom into a class to decouple interface and implementation, the author may have inadventantly lost compiler checks on const-correctness.

When the pimpl object is wrapped in propagate_const, const member functions will only be able to call const functions on the pimpl object and will be unable to modify (non-mutable) member variables of the pimpl object without explicit const_casts: const-correctness is restored. The class above would be modified as follows:

class C
{
  void foo() const;  // unchanged
  void foo();        // unchanged
  
  std::propagate_const<std::unique_ptr<CImpl>> m_pimpl;
};

Thread-safety and propagate_const

Herb Sutter introduced the appealing notion that const implies thread-safe [3]. Without propagate_const, changes outside a class with pointer-like members can render the const methods of that class non-thread-safe. This means that maintaining the rule const=>thread-safe requires a global review of the code base.

With only the const version of foo() the code below is thread-safe. Introduction of a non-const (and non-thread-safe) foo() into D renders E non-thread-safe.

struct D
{
  int foo() const { /* thread-safe */ }
  int foo() { /* non-thread-safe */ }
};

struct E
{
  E(D& pD) : m_pD{&pD} {}

  void operator() () const
  {
    m_pD->foo();
  }

  D* m_pD;
};

int main()
{
  D d;
  const E e1(d);
  const E e2(d);

  std::thread t1(e1);
  std::thread t2(e2);
  t1.join();
  t2.join();
}

One solution to the above is to forbid pointer-like member variables in classes if const=>thread-safe. This is undesirably restrictive. If instead all pointer-like member variables are decorated with propagate_const then the compiler will catch violations of const-ness that could render code non-thread-safe.

struct E
{
  E(D& pD);                 // unchanged

  void operator() () const; // unchanged

  std::propagate_const<D*> m_pD;
};

Introduction of propagate_const cannot automatically guarantee thread-safety but can allow const=>thread-safe to be locally verified during code review.

III. Impact On the Standard

This proposal is a pure library extension. It does not require changes to any standard classes, functions or headers.

IV. Design Decisions

Given absolute freedom we would propose changing the const keyword to propagate const-ness. That would be impractical, however, as it would break existing code and change behaviour in potentially undesirable ways. A second approach would be the introduction of a new keyword to modify const, for instance, deep const, which enforces const-propagation. Although this change would maintain backward-compatibility, it would require enhancements to the C++ compiler.

We suggest that the standard library supply a class that wraps member data where const-propagating behaviour is required. The propagate_const wrapper can be used much like the const keyword and will cause compilation failure wherever const-ness is violated. const-propagation can be introduced into existing code by decorating pointer-like members of a class with propagate_const.

The change required to introduce const-propagation to a class is simple and local enough to be enforced during code review and taught to C++ developers in the same way as smart-pointers are taught to ensure exception safety.

It is intended that propagate_const contain no member data besides the wrapped pointer. Inlining of function calls by the compiler will ensure that using propagate_const incurs no run-time cost.

Encapsulation vs inheritance

Inheritance from the wrapped pointer-like object (where it is a class type) was considered but ruled out. The purpose of this wrapper is to help the author ensure const-propagation; if propagate_const<T> were to inherit from T, then it would allow potentially non-const member functions of T to be called in a const context.

Construction and assignment

A propagate_const<T> should be constructable and assignable from a U or a propagate_const<U> where U is any type that T can be constructed or assigned from. There should be no additional cost of construction for a propagate_const<T> beyond that for construction of a T. The wrapped T should not be value-initialized as this would incur a cost for raw pointers. If value-initialization is desirable then it can be accomplished with another wrapper class like boost::value_initialized [4].

Pointer-like functions

operator* and operator-> are defined to preserve const-propagation. When a const propagate_const<T> is used only const member functions of T can be used without explicit casts.

get

The get function returns the address of the object pointed to by the wrapped pointer. get is intended to be used to ensure const-propagation is preserved when using interfaces which require raw C-style pointers

operator value*

When T is a raw pointer operator value* exists and allows implicit conversion to a raw pointer. This avoids using get to access the raw pointer in contexts where it was unnecesary before addition of the propagate_const wrapper.

Equality, inequality and comparison

Free-standing equality, inequality and comparison operators are provided so that a propagate_const<T> can be used in any equality, inequality or comparison where a T could be used. const-propagation should not alter the result of any equality, inequality or comparison operation.

swap

Neither the member swap function nor the free-standing swap function should add or remove const-ness but should not unduly restrict the types with which propagate_const<T> can be swapped. If T and U can be swapped then const-propagating T and U can be swapped.

get_underlying

get_underlying is a free-standing function which allows the underlying pointer to be accessed. The use of this function allows const-propagation to be dropped and is therefore discouraged. The function is named such that it will be easy to find in code review.

hash

The hash struct is specialized so that inclusion of propagate_const does not alter the result of hash evaluation.

V. Technical Specification

X.Y   Class template propagate_const [propagate_const]

X.Y.1  Class template propagate_const general [propagate_const.general]

The header <propagate_const> defines a template class propagate_const that imposes deep-const behaviour on pointer types.

X.Y.2  Class template propagate_const overview [propagate_const.overview]

propagate_const is a wrapper around a pointer type T which treats the wrapped pointer as a const pointer to const when the wrapper is accessed as a const object.

X.Y.3  Class template propagate_const requirements on T [propagate_const.requirements]

T is required to be a raw pointer to an object or a class type imitating a pointer to an object; anything else is ill-formed. decltype(*declval<T&>()) must be well-formed and must return an lvalue reference; anything else is ill-formed. If T is a pointer to const then propagate_const remains well-formed.

X.Y.4  Class template propagate_const noexcept status [propagate_const.noexcept]

When T is a raw pointer then all member functions of propagate_const are noexcept. When T is a class type imitating a pointer then member functions of propagate_const may be conditionally noexcept.

X.Y.5  Header <propagate_const> synopsis [propagate_const.synopsis]

namespace std {
  template <class T> class propagate_const { 
  public:                                                                                          
    typedef decltype(*declval<T&>()) element_type;
    typedef element_type* pointer_type;
       
    // Constructors
    constexpr propagate_const();
    template <class U> 
      propagate_const(U&& u); 
    template <class U> 
      propagate_const(const propagate_const<U>& pu);
    template <class U> 
      propagate_const(propagate_const<U>&& pu);
    
    // Destructor
    ~propagate_const() = default;
    
    // Assignment
    template <class U> 
      propagate_const operator=(U&& u); 
    template <class U> 
      propagate_const operator=(const propagate_const<U>& pu); 
    template <class U> 
      propagate_const operator=(propagate_const<U>&& pu); 
    
    // Observers
    explicit operator bool() const;
    const element_type* operator->() const;
    operator const element_type*() const; 
    const element_type& operator*() const;
    const element_type* get() const;
    
    // Accessors
    element_type* operator->();
    operator element_type*(); 
    element_type& operator*();
    element_type* get();

    // Modifiers
    void swap(propagate_const<T>& pt);
 
  private:
    T t_; //exposition only
  };
  
  // Relational operators
  template <class T, class U>
    bool operator==(const propagate_const<T>& pt, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator!=(const propagate_const<T>& pt, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator<(const propagate_const<T>& pt, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator>(const propagate_const<T>& pt, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator<=(const propagate_const<T>& pt, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator>=(const propagate_const<T>& pt, const propagate_const<U>& pu);

  template <class T, class U>
    bool operator==(const propagate_const<T>& pt, const U& u);
  template <class T, class U>
    bool operator!=(const propagate_const<T>& pt, const U& u);
  template <class T, class U>
    bool operator<(const propagate_const<T>& pt, const U& u);
  template <class T, class U>
    bool operator>(const propagate_const<T>& pt, const U& u);
  template <class T, class U>
    bool operator<=(const propagate_const<T>& pt, const U& u);
  template <class T, class U>
    bool operator>=(const propagate_const<T>& pt, const U& u);
  
  template <class T, class U>
    bool operator==(const T& t, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator!=(const T& t, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator<(const T& t, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator>(const T& t, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator<=(const T& t, const propagate_const<U>& pu);
  template <class T, class U>
    bool operator>=(const T& t, const propagate_const<U>& pu);
  
  // Specialized algorithms
  template <class T>
    void swap (propagate_const<T>& pt, propagate_const<T>& pt2);
  
  // Underlying pointer access
  template <class T>
    const T& get_underlying(const propagate_const<T>& pt);
  template <class T>
    T& get_underlying(propagate_const<T>& pt);
  
  // Hash support
  template <class T>
    struct hash<propagate_const<T>>;
}

X.Y.6  propagate_const constructors [propagate_const.ctor]

X.Y.7  propagate_const destructor [propagate_const.dtor]

X.Y.8  propagate_const assignment [propagate_const.assignment]

X.Y.9  propagate_const observers [propagate_const.observers]

X.Y.10  propagate_const accessors [propagate_const.accessors]

X.Y.11  propagate_const modifiers [propagate_const.modifiers]

X.Y.12  propagate_const relational operators [propagate_const.relational]

X.Y.13  propagate_const specialized algorithms [propagate_const.algorithms]

X.Y.14  propagate_const underlying pointer access [propagate_const.underlying]

Access to the underlying pointer type is through free functions rather than member functions. These functions are intended to resemble cast operations to encourage caution when using them.

X.Y.15  propagate_const hash support [propagate_const.hash]