Boost C++ Libraries: Ticket #10777: unordered_map treats operator== on stateful allocators as stateless

Boost C++ Libraries: Ticket #10777: unordered_map treats operator== on stateful allocators as stateless https://svn.boost.org/trac10/ticket/10777 <p> Hi, </p> <p> here is my problem: I have a stateful allocator that has a bunch of stuff inside of it. It can be move-assigned. The problem is that boost::unordered_map does this when you have propagate_on_container_move_assign typedeffed to true_type: </p> <pre class="wiki">my_alloc a; my_alloc b; a = std::move(b); ASSERT(a == b); </pre><p> That assert can obviously never be true if my_alloc is stateful, but this sequence of events currently happens in boost::unordered_map. </p> <p> Here is an allocator that shows the problem: </p> <pre class="wiki">#pragma once #include <memory> #include <vector> template<typename T> struct plalloc { typedef T value_type; plalloc() = default; template<typename U> plalloc(const plalloc<U> &) {} plalloc(const plalloc &) {} plalloc & operator=(const plalloc &) { return *this; } plalloc(plalloc &&) = default; plalloc & operator=(plalloc &&) = default; typedef std::true_type propagate_on_container_copy_assignment; typedef std::true_type propagate_on_container_move_assignment; typedef std::true_type propagate_on_container_swap; bool operator==(const plalloc & other) const { return this == &other; } bool operator!=(const plalloc & other) const { return !(*this == other); } T * allocate(size_t num_to_allocate) { if (num_to_allocate != 1) { return static_cast<T *>(::operator new(sizeof(T) * num_to_allocate)); } else if (available.empty()) { // first allocate 8, then double whenever // we run out of memory size_t to_allocate = 8 << memory.size(); available.reserve(to_allocate); std::unique_ptr<value_holder[]> allocated(new value_holder[to_allocate]); value_holder * first_new = allocated.get(); memory.emplace_back(std::move(allocated)); size_t to_return = to_allocate - 1; for (size_t i = 0; i < to_return; ++i) { available.push_back(std::addressof(first_new[i].value)); } return std::addressof(first_new[to_return].value); } else { T * result = available.back(); available.pop_back(); return result; } } void deallocate(T * ptr, size_t num_to_free) { if (num_to_free == 1) { available.push_back(ptr); } else { ::operator delete(ptr); } } // boilerplate that shouldn't be needed, except // libstdc++ doesn't use allocator_traits yet template<typename U> struct rebind { typedef plalloc<U> other; }; typedef T * pointer; typedef const T * const_pointer; typedef T & reference; typedef const T & const_reference; template<typename U, typename... Args> void construct(U * object, Args &&... args) { new (object) U(std::forward<Args>(args)...); } template<typename U, typename... Args> void construct(const U * object, Args &&... args) = delete; template<typename U> void destroy(U * object) { object->~U(); } private: union value_holder { value_holder() {} ~value_holder() {} T value; }; std::vector<std::unique_ptr<value_holder[]>> memory; std::vector<T *> available; }; </pre><p> And here is a sequence of events with which you can get it: </p> <pre class="wiki">boost::unordered_map<int, int, std::hash<int>, std::equal_to<int>, plalloc<int>> a = { { 1, 2 } }; boost::unordered_map<int, int, std::hash<int>, std::equal_to<int>, plalloc<int>> b = { { 3, 4 } }; boost::unordered_map<int, int, std::hash<int>, std::equal_to<int>, plalloc<int>> c = { { 5, 6 } }; a = std::move(b); a = c; </pre> en-us Boost C++ Libraries /htdocs/site/boost.png https://svn.boost.org/trac10/ticket/10777 Trac 1.4.3 Daniel James Sun, 09 Nov 2014 23:49:31 GMT status, description changed https://svn.boost.org/trac10/ticket/10777#comment:1 https://svn.boost.org/trac10/ticket/10777#comment:1 <ul> <li><strong>status</strong> <span class="trac-field-old">new</span> → <span class="trac-field-new">assigned</span> </li> <li><strong>description</strong> modified (<a href="/trac10/ticket/10777?action=diff&version=1">diff</a>) </li> </ul> <p> I've removed the assertion in develop. </p> <p> <a class="ext-link" href="https://github.com/boostorg/unordered/commit/31211a607f1c294f973eed10e54e461a8ef920ba"><span class="icon"></span>https://github.com/boostorg/unordered/commit/31211a607f1c294f973eed10e54e461a8ef920ba</a> </p> Ticket Daniel James Tue, 12 May 2015 17:57:01 GMT status changed; resolution set https://svn.boost.org/trac10/ticket/10777#comment:2 https://svn.boost.org/trac10/ticket/10777#comment:2 <ul> <li><strong>status</strong> <span class="trac-field-old">assigned</span> → <span class="trac-field-new">closed</span> </li> <li><strong>resolution</strong> → <span class="trac-field-new">fixed</span> </li> </ul> Ticket