Section

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.

# Declarations

template< class T >
class propagate_const;

(library fundamentals TS v2)

# Example

#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();
}

# Defect reports

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

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