value_ptr.h

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#ifndef FWCore_Utilities_value_ptr_h
#define FWCore_Utilities_value_ptr_h

// ----------------------------------------------------------------------
//
// value_ptr.h - Smart pointer permitting copying of pointed-to object.
//
// Purpose: value_ptr provides a smart pointer template that provides
// sole ownership of the pointed-to object. When a value_ptr object is
// copied, the new value_ptr object is given a copy of the object pointed
// to by the original value_ptr object.
//
// The value_ptr_traits template is provided to allow specialization
// of the copying behavior. See the notes below.
//
// Use value_ptr only when deep-copying of the pointed-to object
// is desireable. Use std::shared_ptr when sharing of the
// pointed-to  object is desirable. Use std::unique_ptr
// when no copying is desirable.
//
// The design of value_ptr is taken from Herb Sutter's More
// Exceptional C++, with modifications by Marc Paterno and further
// modifications by Walter Brown. This version is based on the
// ValuePtr found in the Fermilab ZOOM library.
//
//
// Supports the following syntax
//   value_ptr<T> ptr(...);
//   if (ptr) { ...
// Where the conditional will evaluate as true if and only if the
// pointer the value_ptr contains is not null.
//
// ----------------------------------------------------------------------

#include <algorithm>  // for std::swap()
#include <memory>
#include "FWCore/Utilities/interface/get_underlying_safe.h"
#include "FWCore/Utilities/interface/propagate_const.h"

namespace edm {

  // --------------------------------------------------------------------
  //
  //  Auxiliary traits class template providing default clone()
  //  Users should specialize this template for types that have their
  //  own self-copy operations; failure to do so may lead to slicing!
  //
  // --------------------------------------------------------------------

  template <typename T>
  struct value_ptr_traits {
    static T* clone(T const* p) { return new T(*p); }
    static void destroy(T* p) { delete p; }
  };

  // --------------------------------------------------------------------
  //
  // Copyable smart pointer class template
  //
  // --------------------------------------------------------------------

  template <typename T>
  class value_ptr {
  public:
    // --------------------------------------------------
    // Default constructor/destructor:
    // --------------------------------------------------

    value_ptr() : myP(nullptr) {}
    explicit value_ptr(T* p) : myP(p) {}
    ~value_ptr() { value_ptr_traits<T>::destroy(myP.get()); }

    // --------------------------------------------------
    // Copy constructor/copy assignment:
    // --------------------------------------------------

    value_ptr(value_ptr const& orig) : myP(createFrom(get_underlying_safe(orig.myP))) {}

    value_ptr& operator=(value_ptr const& orig) {
      value_ptr<T> temp(orig);
      swap(temp);
      return *this;
    }

    // --------------------------------------------------
    // Move constructor/move assignment:
    // --------------------------------------------------

    value_ptr(value_ptr&& orig) : myP(orig.myP) { orig.myP = nullptr; }

    value_ptr& operator=(value_ptr&& orig) {
      if (myP != orig.myP) {
        delete myP.get();
        myP = orig.myP;
        orig.myP = nullptr;
      }
      return *this;
    }

    // --------------------------------------------------
    // Access mechanisms:
    // --------------------------------------------------

    T const& operator*() const { return *myP; }
    T& operator*() { return *myP; }
    T const* operator->() const { return get_underlying_safe(myP); }
    T* operator->() { return get_underlying_safe(myP); }

    // --------------------------------------------------
    // Manipulation:
    // --------------------------------------------------

    void swap(value_ptr& orig) { std::swap(myP, orig.myP); }

    // --------------------------------------------------
    // Copy-like construct/assign from compatible value_ptr<>:
    // --------------------------------------------------

    template <typename U>
    value_ptr(value_ptr<U> const& orig) : myP(createFrom(orig.operator->())) {}

    template <typename U>
    value_ptr& operator=(value_ptr<U> const& orig) {
      value_ptr<T> temp(orig);
      swap(temp);
      return *this;
    }

    // --------------------------------------------------
    // Move-like construct/assign from unique_ptr<>:
    // --------------------------------------------------

    value_ptr(std::unique_ptr<T> orig) : myP(orig.release()) { orig = nullptr; }

    value_ptr& operator=(std::unique_ptr<T> orig) {
      value_ptr<T> temp(std::move(orig));
      swap(temp);
      return *this;
    }

    // The following typedef, function, and operator definition
    // support the following syntax:
    //   value_ptr<T> ptr(..);
    //   if (ptr) { ...
    // Where the conditional will evaluate as true if and only if the
    // pointer value_ptr contains is not null.
  private:
    typedef void (value_ptr::*bool_type)() const;
    void this_type_does_not_support_comparisons() const {}

  public:
    operator bool_type() const {
      return myP != nullptr ? &value_ptr<T>::this_type_does_not_support_comparisons : nullptr;
    }

  private:
    // --------------------------------------------------
    // Implementation aid:
    // --------------------------------------------------

    template <typename U>
    static T* createFrom(U const* p) {
      return p ? value_ptr_traits<U>::clone(p) : nullptr;
    }

    // --------------------------------------------------
    // Member data:
    // --------------------------------------------------

    edm::propagate_const<T*> myP;

  };  // value_ptr

  // --------------------------------------------------------------------
  //
  // Free-standing swap()
  //
  // --------------------------------------------------------------------

  template <typename T>
  inline void swap(value_ptr<T>& vp1, value_ptr<T>& vp2) {
    vp1.swap(vp2);
  }

  // Do not allow nonsensical comparisons that the bool_type
  // conversion operator definition above would otherwise allow.
  // The function call inside the next 4 operator definitions is
  // private, so compilation will fail if there is an attempt to
  // instantiate these 4 operators.
  template <typename T, typename U>
  inline bool operator==(value_ptr<T> const& lhs, U const& rhs) {
    lhs.this_type_does_not_support_comparisons();
    return false;
  }

  template <typename T, typename U>
  inline bool operator!=(value_ptr<T> const& lhs, U const& rhs) {
    lhs.this_type_does_not_support_comparisons();
    return false;
  }

  template <typename T, typename U>
  inline bool operator==(U const& lhs, value_ptr<T> const& rhs) {
    rhs.this_type_does_not_support_comparisons();
    return false;
  }

  template <typename T, typename U>
  inline bool operator!=(U const& lhs, value_ptr<T> const& rhs) {
    rhs.this_type_does_not_support_comparisons();
    return false;
  }
}  // namespace edm

#endif  // FWCoreUtilities_value_ptr_h