std::experimental::propagate_const

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.

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:

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 functions

Non-member functions

Helper classes

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";
        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();
}

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