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      1 // Copyright 2003 Google Inc.
      2 // All rights reserved.
      3 //
      4 // Redistribution and use in source and binary forms, with or without
      5 // modification, are permitted provided that the following conditions are
      6 // met:
      7 //
      8 //     * Redistributions of source code must retain the above copyright
      9 // notice, this list of conditions and the following disclaimer.
     10 //     * Redistributions in binary form must reproduce the above
     11 // copyright notice, this list of conditions and the following disclaimer
     12 // in the documentation and/or other materials provided with the
     13 // distribution.
     14 //     * Neither the name of Google Inc. nor the names of its
     15 // contributors may be used to endorse or promote products derived from
     16 // this software without specific prior written permission.
     17 //
     18 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     19 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     20 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
     21 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
     22 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
     23 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
     24 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     25 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     26 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     27 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
     28 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     29 //
     30 // Authors: Dan Egnor (egnor (at) google.com)
     31 //
     32 // A "smart" pointer type with reference tracking.  Every pointer to a
     33 // particular object is kept on a circular linked list.  When the last pointer
     34 // to an object is destroyed or reassigned, the object is deleted.
     35 //
     36 // Used properly, this deletes the object when the last reference goes away.
     37 // There are several caveats:
     38 // - Like all reference counting schemes, cycles lead to leaks.
     39 // - Each smart pointer is actually two pointers (8 bytes instead of 4).
     40 // - Every time a pointer is assigned, the entire list of pointers to that
     41 //   object is traversed.  This class is therefore NOT SUITABLE when there
     42 //   will often be more than two or three pointers to a particular object.
     43 // - References are only tracked as long as linked_ptr<> objects are copied.
     44 //   If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS
     45 //   will happen (double deletion).
     46 //
     47 // A good use of this class is storing object references in STL containers.
     48 // You can safely put linked_ptr<> in a vector<>.
     49 // Other uses may not be as good.
     50 //
     51 // Note: If you use an incomplete type with linked_ptr<>, the class
     52 // *containing* linked_ptr<> must have a constructor and destructor (even
     53 // if they do nothing!).
     54 //
     55 // Bill Gibbons suggested we use something like this.
     56 //
     57 // Thread Safety:
     58 //   Unlike other linked_ptr implementations, in this implementation
     59 //   a linked_ptr object is thread-safe in the sense that:
     60 //     - it's safe to copy linked_ptr objects concurrently,
     61 //     - it's safe to copy *from* a linked_ptr and read its underlying
     62 //       raw pointer (e.g. via get()) concurrently, and
     63 //     - it's safe to write to two linked_ptrs that point to the same
     64 //       shared object concurrently.
     65 // TODO(wan (at) google.com): rename this to safe_linked_ptr to avoid
     66 // confusion with normal linked_ptr.
     67 
     68 #ifndef GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_
     69 #define GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_
     70 
     71 #include <stdlib.h>
     72 #include <assert.h>
     73 
     74 #include "gtest/internal/gtest-port.h"
     75 
     76 namespace testing {
     77 namespace internal {
     78 
     79 // Protects copying of all linked_ptr objects.
     80 GTEST_API_ GTEST_DECLARE_STATIC_MUTEX_(g_linked_ptr_mutex);
     81 
     82 // This is used internally by all instances of linked_ptr<>.  It needs to be
     83 // a non-template class because different types of linked_ptr<> can refer to
     84 // the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
     85 // So, it needs to be possible for different types of linked_ptr to participate
     86 // in the same circular linked list, so we need a single class type here.
     87 //
     88 // DO NOT USE THIS CLASS DIRECTLY YOURSELF.  Use linked_ptr<T>.
     89 class linked_ptr_internal {
     90  public:
     91   // Create a new circle that includes only this instance.
     92   void join_new() {
     93     next_ = this;
     94   }
     95 
     96   // Many linked_ptr operations may change p.link_ for some linked_ptr
     97   // variable p in the same circle as this object.  Therefore we need
     98   // to prevent two such operations from occurring concurrently.
     99   //
    100   // Note that different types of linked_ptr objects can coexist in a
    101   // circle (e.g. linked_ptr<Base>, linked_ptr<Derived1>, and
    102   // linked_ptr<Derived2>).  Therefore we must use a single mutex to
    103   // protect all linked_ptr objects.  This can create serious
    104   // contention in production code, but is acceptable in a testing
    105   // framework.
    106 
    107   // Join an existing circle.
    108   // L < g_linked_ptr_mutex
    109   void join(linked_ptr_internal const* ptr) {
    110     MutexLock lock(&g_linked_ptr_mutex);
    111 
    112     linked_ptr_internal const* p = ptr;
    113     while (p->next_ != ptr) p = p->next_;
    114     p->next_ = this;
    115     next_ = ptr;
    116   }
    117 
    118   // Leave whatever circle we're part of.  Returns true if we were the
    119   // last member of the circle.  Once this is done, you can join() another.
    120   // L < g_linked_ptr_mutex
    121   bool depart() {
    122     MutexLock lock(&g_linked_ptr_mutex);
    123 
    124     if (next_ == this) return true;
    125     linked_ptr_internal const* p = next_;
    126     while (p->next_ != this) p = p->next_;
    127     p->next_ = next_;
    128     return false;
    129   }
    130 
    131  private:
    132   mutable linked_ptr_internal const* next_;
    133 };
    134 
    135 template <typename T>
    136 class linked_ptr {
    137  public:
    138   typedef T element_type;
    139 
    140   // Take over ownership of a raw pointer.  This should happen as soon as
    141   // possible after the object is created.
    142   explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
    143   ~linked_ptr() { depart(); }
    144 
    145   // Copy an existing linked_ptr<>, adding ourselves to the list of references.
    146   template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }
    147   linked_ptr(linked_ptr const& ptr) {  // NOLINT
    148     assert(&ptr != this);
    149     copy(&ptr);
    150   }
    151 
    152   // Assignment releases the old value and acquires the new.
    153   template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {
    154     depart();
    155     copy(&ptr);
    156     return *this;
    157   }
    158 
    159   linked_ptr& operator=(linked_ptr const& ptr) {
    160     if (&ptr != this) {
    161       depart();
    162       copy(&ptr);
    163     }
    164     return *this;
    165   }
    166 
    167   // Smart pointer members.
    168   void reset(T* ptr = NULL) {
    169     depart();
    170     capture(ptr);
    171   }
    172   T* get() const { return value_; }
    173   T* operator->() const { return value_; }
    174   T& operator*() const { return *value_; }
    175 
    176   bool operator==(T* p) const { return value_ == p; }
    177   bool operator!=(T* p) const { return value_ != p; }
    178   template <typename U>
    179   bool operator==(linked_ptr<U> const& ptr) const {
    180     return value_ == ptr.get();
    181   }
    182   template <typename U>
    183   bool operator!=(linked_ptr<U> const& ptr) const {
    184     return value_ != ptr.get();
    185   }
    186 
    187  private:
    188   template <typename U>
    189   friend class linked_ptr;
    190 
    191   T* value_;
    192   linked_ptr_internal link_;
    193 
    194   void depart() {
    195     if (link_.depart()) delete value_;
    196   }
    197 
    198   void capture(T* ptr) {
    199     value_ = ptr;
    200     link_.join_new();
    201   }
    202 
    203   template <typename U> void copy(linked_ptr<U> const* ptr) {
    204     value_ = ptr->get();
    205     if (value_)
    206       link_.join(&ptr->link_);
    207     else
    208       link_.join_new();
    209   }
    210 };
    211 
    212 template<typename T> inline
    213 bool operator==(T* ptr, const linked_ptr<T>& x) {
    214   return ptr == x.get();
    215 }
    216 
    217 template<typename T> inline
    218 bool operator!=(T* ptr, const linked_ptr<T>& x) {
    219   return ptr != x.get();
    220 }
    221 
    222 // A function to convert T* into linked_ptr<T>
    223 // Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
    224 // for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
    225 template <typename T>
    226 linked_ptr<T> make_linked_ptr(T* ptr) {
    227   return linked_ptr<T>(ptr);
    228 }
    229 
    230 }  // namespace internal
    231 }  // namespace testing
    232 
    233 #endif  // GTEST_INCLUDE_GTEST_INTERNAL_GTEST_LINKED_PTR_H_
    234