std::experimental::propagate_const

 

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Technical Specifications

 

Technical specifications

Filesystem library (filesystem TS)

Library fundamentals (library fundamentals TS)

Library fundamentals 2 (library fundamentals 2 TS)

Extensions for parallelism (parallelism TS)

Concepts (concepts TS)

Extensions for concurrency (concurrency TS)

 

Library fundamentals v2

experimental::propagate_const

experimental::not_fn

experimental::observer_ptr

experimental::make_array

experimental::ostream_joiner

experimental::gcd

experimental::lcm

experimental::source_location

detection idiom

uniform container erasure

 

std::experimental::propagate_const

Member functions

propagate_const::propagate_const

propagate_const::operator=

propagate_const::swap

Observers

propagate_const::get

propagate_const::operator bool

propagate_const::operator*propagate_const::operator->

propagate_const::operator element_type*propagate_const::operator const element_type*

Non-member functions

operator==operator!=operator<operator>operator<=operator>operator>=

swap

get_underlying

Helper classes

std::hash

std::equal_tostd::not_equal_tostd::lessstd::greaterstd::less_equalstd::greater_equal

 

Defined in header <experimental/propagate_const>
template<class T> class propagate_const;		(library fundamentals TS v2)

std::experimental::propagate_const is a const-propagating wrapper for pointers and pointer-like objects. It treats the wrapped pointer as a pointer to const when accessed through a const access path, hence the name.

The class satisfies the requirements of MoveConstructible and MoveAssignable if the underlying pointer-like type satisfies the corresponding concept, but propagate_const is neither CopyConstructible nor CopyAssignable.

Type requirements

- T must be an object pointer type or a pointer-like class type, as specified below. The program is ill-formed if T is an array type, reference type, pointer to function type, pointer to (possibly cv-qualified) void, or if decltype(*std::declval<T&>()) is not an lvalue reference type.

Requirements on pointer-like class types

If T is a class type, it must satisfy the requirements in this subsection.

Given

• t, a modifiable lvalue expression of type T
• ct, a const T& bound to T
• element_type, an object type

The following expressions must be valid and have their specified effects:

Expression	Return type	Pre-conditions	Operational semantics
t.get()	element_type*
ct.get()	element_type* or const element_type*		t.get() == ct.get()
*t	element_type&	t.get() != nullptr	*t refers to the same object as *(t.get())
*ct	element_type& or const element_type&	ct.get() != nullptr	*ct refers to the same object as *(ct.get())
t.operator->()	element_type*	t.get() != nullptr	t.operator->() == t.get()
ct.operator->()	element_type* or const element_type*	ct.get() != nullptr	ct.operator->() == ct.get()
(bool)t	bool		(bool)t is equivalent to t.get() != nullptr
(bool)ct	bool		(bool)ct is equivalent to ct.get() != nullptr

Further, T and const T shall be contextually convertible to bool.

In addition, if T is implicitly convertible to element_type*, then (element_type*)t shall be equal to t.get(). Similarly, if const T is implicitly convertible to const element_type*, then (const element_type*)ct shall be equal to ct.get().

Member types

Member type	Definition
element_type	std::remove_reference_t<decltype(*std::declval<T&>())>, the type of the object pointed to by T

Member functions

(constructor)	constructs a new propagate_const (public member function)
(destructor) (implicitly declared)	destructs an propagate_const, destroying the contained pointer (public member function)
operator=	assigns the propagate_const object (public member function)
swap	swaps the wrapped pointer (public member function)
Observers
get	returns a pointer to the object pointed to by the wrapped pointer (public member function)
operator bool	checks if the wrapped pointer is null (public member function)
operator*operator->	dereferences the wrapped pointer (public member function)
operator element_type*operator const element_type*	implicit conversion function to pointer (public member function)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=	compares to another propagate_const, another pointer, or with nullptr (function template)
std::experimental::swap(std::experimental::propagate_const)	specializes the swap algorithm (function template)
get_underlying	Retrieves a reference to the wrapped pointer-like object (function template)

Helper classes

std::hash<std::experimental::propagate_const>	hash support for propagate_const (class template specialization)
std::equal_to<std::experimental::propagate_const>std::not_equal_to<std::experimental::propagate_const>std::less<std::experimental::propagate_const>std::greater<std::experimental::propagate_const>std::less_equal<std::experimental::propagate_const>std::greater_equal<std::experimental::propagate_const>	Specializations of the standard comparison function objects for propagate_const (class template specialization)

Example

Run this code

#include <iostream>
#include <memory>
#include <experimental/propagate_const>
 
struct X
{
    void g() const { std::cout << "g (const)\n"; }
    void g() { std::cout << "g (non-const)\n"; }
};
 
struct Y
{
    Y() : m_ptrX(std::make_unique<X>()) { }
 
    void f() const
    {
        std::cout << "f (const)\n";
        m_ptrX->g();
    }
 
    void f()
    {
        std::cout << "f (non-const)\n";
        m_ptrX->g();
    }
 
    std::experimental::propagate_const<std::unique_ptr<X>> m_ptrX;
};
 
int main()
{
    Y y;
    y.f();
 
    const Y cy;
    cy.f();
}

Output:

f (non-const)
g (non-const)
f (const)
g (const)