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POcustom STL allocator causes first character to be dropped
 

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  1. POcustom STL allocator causes first character to be dropped
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    <p>Per suggestion from @BenVoigt in response to my <a href="https://stackoverflow.com/questions/8620383/std-compliant-stringstream-using-stack-allocated-storage/8648341#8648341">question regarding stack allocated stringstream storage</a>, I designed a stack_allocator (code follows below), and declared a basic_ostringstream type using it.</p> <p>I am experiencing a strange bug though. The first character I place into the stream is omitted when I print the resulting string!</p> <p>Here is an example:</p> <pre><code>template&lt;typename T, size_t capacity, size_t arr_size&gt; __thread bool stack_allocator&lt;T, capacity, arr_size&gt;::_used[arr_size] = {}; template&lt;typename T, size_t capacity, size_t arr_size&gt; __thread T stack_allocator&lt;T, capacity, arr_size&gt;::_buf[capacity][arr_size] = {}; typedef std::basic_ostringstream&lt;char, std::char_traits&lt;char&gt;, stack_allocator&lt;char, 1024, 5&gt; &gt; stack_ostringstream; int main() { stack_ostringstream _os; _os &lt;&lt; "hello world"; std::cout &lt;&lt; _os.str() &lt;&lt; std::endl; return 0; } </code></pre> <p>The resulting output is:</p> <blockquote> <p>ello world</p> </blockquote> <p>Can anyone elaborate on what is happening to the first character?</p> <p>The stack_allocator impl follows: It's pretty simplistic, and I'm sure has lots of room for improvement (not withstanding fixing the bug!)</p> <pre><code>#include &lt;cstddef&gt; #include &lt;limits&gt; #include &lt;bits/allocator.h&gt; template&lt;typename T, size_t capacity = 1024, size_t arr_size = 5&gt; class stack_allocator { public: typedef T value_type; typedef value_type* pointer; typedef const value_type* const_pointer; typedef value_type&amp; reference; typedef const value_type&amp; const_reference; typedef std::size_t size_type; typedef std::ptrdiff_t difference_type; inline explicit stack_allocator() { } template&lt;typename U&gt; inline explicit stack_allocator(const stack_allocator&lt;U, capacity, arr_size&gt;&amp; that) { } inline ~stack_allocator() {} template&lt;typename U&gt; struct rebind { typedef stack_allocator&lt;U, capacity, arr_size&gt; other; }; inline pointer allocate(size_type cnt, typename std::allocator&lt;void&gt;::const_pointer = 0) { if (cnt &gt; capacity) return reinterpret_cast&lt;pointer&gt;(::operator new(cnt * sizeof (T))); for (size_t i = 0; i &lt; arr_size; ++i) { if (!_used[i]) { _used[i] = true; return reinterpret_cast&lt;pointer&gt;(_buf[i]); } } } inline void deallocate(pointer p, size_type) { for (size_t i = 0; i &lt; arr_size; ++i) { if (p != _buf[i]) continue; _used[i] = false; return; } ::operator delete(p); } inline pointer address(reference r) { return &amp;r; } inline const_pointer address(const_reference r) { return &amp;r; } inline size_type max_size() const { return std::numeric_limits&lt;size_type&gt;::max() / sizeof(T); } inline void construct(pointer p, const T&amp; t) { new(p) T(t); } inline void destroy(pointer p) { p-&gt;~T(); } inline bool operator==(const stack_allocator&amp;) const { return true; } inline bool operator!=(const stack_allocator&amp; a) const { return !operator==(a); } private: static __thread bool _used[arr_size]; static __thread T _buf[capacity][arr_size]; }; </code></pre>
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