Technical specifications
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals 2 TS)
Extensions for parallelism (parallelism TS)
Extensions for concurrency (concurrency TS)
Concepts (concepts TS)
Ranges (ranges TS)
Special mathematical functions (special math TR)
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.


[edit] Requirements on pointer-like class types

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


  • 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().

[edit] Member types

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

[edit] Member functions

constructs a new propagate_const
(public member function) [edit]
(implicitly declared)
destructs an propagate_const, destroying the contained pointer
(public member function) [edit]
assigns the propagate_const object
(public member function) [edit]
swaps the wrapped pointer
(public member function) [edit]
returns a pointer to the object pointed to by the wrapped pointer
(public member function) [edit]
checks if the wrapped pointer is null
(public member function) [edit]
dereferences the wrapped pointer
(public member function) [edit]
implicit conversion function to pointer
(public member function) [edit]

[edit] Non-member functions

compares to another propagate_const, another pointer, or with nullptr
(function template) [edit]
specializes the swap algorithm
(function template) [edit]
Retrieves a reference to the wrapped pointer-like object
(function template) [edit]

[edit] Helper classes

hash support for propagate_const
(class template specialization) [edit]
Specializations of the standard comparison function objects for propagate_const
(class template specialization) [edit]

[edit] Example

#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";
    void f()
        std::cout << "f (non-const)\n";
    std::experimental::propagate_const<std::unique_ptr<X>> m_ptrX;
int main()
    Y y;
    const Y cy;


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