From 0b383b72841e4af2890d082e4caed3119484fe95 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Tue, 7 Feb 2017 08:08:43 +0200
Subject: Add const_ptr, const-propagating smart pointer

---
 butl/buildfile |  1 +
 butl/const-ptr | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+)
 create mode 100644 butl/const-ptr

diff --git a/butl/buildfile b/butl/buildfile
index 734195e..090f9ed 100644
--- a/butl/buildfile
+++ b/butl/buildfile
@@ -5,6 +5,7 @@
 lib{butl}:                               \
    {hxx         cxx}{ base64              } \
    {hxx         cxx}{ char-scanner        } \
+   {hxx            }{ const-ptr           } \
    {hxx         cxx}{ diagnostics         } \
    {hxx            }{ export              } \
    {hxx ixx     cxx}{ fdstream            } \
diff --git a/butl/const-ptr b/butl/const-ptr
new file mode 100644
index 0000000..23686bc
--- /dev/null
+++ b/butl/const-ptr
@@ -0,0 +1,77 @@
+// 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
-- 
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