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+// file : butl/const-ptr -*- C++ -*-
+// copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
+// license : MIT; see accompanying LICENSE file
+
+#ifndef BUTL_CONST_PTR
+#define BUTL_CONST_PTR
+
+#include <cstddef> // nullptr_t
+
+namespace butl
+{
+ // Const-propagating pointer.
+ //
+ // It has the semantics of a raw pointer except that it passes on its own
+ // const-ness to the pointed-to object. In other words, if you have a const
+ // instance of this pointer, then you can only obtain a const raw pointer to
+ // the underlying object. It is normally used as a data member, for example:
+ //
+ // struct tree
+ // {
+ // const_ptr<tree> left;
+ // const_ptr<tree> right;
+ //
+ // void modify ();
+ // };
+ //
+ // tree* x = ...;
+ // const tree* y = ...;
+ //
+ // x.left->modify (); // Ok.
+ // y.left->modify (); // Error.
+ //
+ // Note that due to this semantics, copy construction/assignment requires
+ // a non-const instance of const_ptr.
+ //
+ // Known drawbacks/issues:
+ //
+ // 1. Cannot do static_cast<mytree*> (x.left).
+ //
+ template <typename T>
+ class const_ptr
+ {
+ public:
+ const_ptr () = default;
+ explicit const_ptr (T* p): p_ (p) {}
+ const_ptr (std::nullptr_t): p_ (nullptr) {}
+
+ const_ptr& operator= (T* p) {p_ = p; return *this;}
+ const_ptr& operator= (std::nullptr_t) {p_ = nullptr; return *this;}
+
+ template <class T1> explicit const_ptr (T1* p): p_ (p) {}
+ template <class T1> const_ptr (const_ptr<T1>& p): p_ (p.p_) {}
+
+ template <class T1> const_ptr& operator= (T1* p) {p_ = p; return *this;}
+ template <class T1> const_ptr& operator= (const_ptr<T1>& p) {
+ p_ = p.p_; return *this;}
+
+ T* operator-> () {return p_;}
+ const T* operator-> () const {return p_;}
+
+ T& operator* () {return *p_;}
+ const T& operator* () const {return *p_;}
+
+ operator T* () {return p_;}
+ operator const T* () const {return p_;}
+
+ explicit operator bool () const {return p_ != nullptr;}
+
+ T* get () {return p_;}
+ const T* get () const {return p_;}
+
+ private:
+ T* p_;
+ };
+};
+
+#endif // BUTL_CONST_PTR