std::experimental::propagate_const

 

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experimental::propagate_const

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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 requirement, but propagate_const is neither CopyConstructible nor CopyAssignable.

Type requirements

- T must be cv-unqualified pointer-to-object type or a cv-unqualified pointer-like class type, as specified below.

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, an lvalue of type const T that denotes the same object as t (equivalent to std::as_const(t) since C++17),
• 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 a 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 <experimental/propagate_const>
#include <iostream>
#include <memory>
 
struct X
{
    void g() const { std::cout << "X::g (const)\n"; }
    void g() { std::cout << "X::g (non-const)\n"; }
};
 
struct Y
{
    Y() : m_propConstX(std::make_unique<X>()), m_autoPtrX(std::make_unique<X>()) {}
 
    void f() const
    {
        std::cout << "Y::f (const)\n";
        m_propConstX->g();
        m_autoPtrX->g();
    }
 
    void f()
    {
        std::cout << "Y::f (non-const)\n";
        m_propConstX->g();
        m_autoPtrX->g();
    }
 
    std::experimental::propagate_const<std::unique_ptr<X>> m_propConstX;
    std::unique_ptr<X> m_autoPtrX;
};
 
int main()
{
    Y y;
    y.f();
 
    const Y cy;
    cy.f();
}

Output:

Y::f (non-const)
X::g (non-const)
X::g (non-const)
Y::f (const)
X::g (const)
X::g (non-const)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3136	LFTSv2	meaningless T like int* const, void*, or const PtrLike were allowed	disallowed