std::experimental::propagate_const - cppreference.com (original) (raw)

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 requirement, but propagate_const is neither CopyConstructible nor CopyAssignable.

Contents

• 1 Requirements on pointer-like class types
• 2 Member types
• 3 Member functions
  • 3.1 Observers
• 4 Non-member functions
• 5 Helper classes
• 6 Example
• 7 Defect reports

[edit] 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:

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

[edit] Member functions

[edit] Non-member functions

[edit] Helper classes

[edit] Example

#include <experimental/propagate_const> #include #include   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()), m_autoPtrX(std::make_unique()) {}   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> m_propConstX; std::unique_ptr 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)

[edit] Defect reports

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