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POHow to implement copy_if in terms of std::remove_copy_if?
 

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  1. POHow to implement copy_if in terms of std::remove_copy_if?
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    <p>I have worked out a solution that works most of the time:</p> <pre><code>#include &lt;algorithm&gt; #include &lt;functional&gt; #include &lt;iostream&gt; #include &lt;vector&gt; // Overload that takes a function pointer template&lt;class ForwardIterator, class OutputIterator, class ArgType&gt; void copy_if(ForwardIterator begin, ForwardIterator end, OutputIterator out, bool (*inPredicate)(ArgType)) { typedef std::pointer_to_unary_function&lt;ArgType, bool&gt; Adapter; std::remove_copy_if(begin, end, out, std::unary_negate&lt;Adapter&gt;(Adapter(inPredicate))); } // Overload that takes a function object template&lt;class ForwardIterator, class OutputIterator, class Functor&gt; void copy_if(ForwardIterator begin, ForwardIterator end, OutputIterator out, Functor inFunctor) { std::remove_copy_if(begin, end, out, std::unary_negate&lt;Functor&gt;(inFunctor)); } bool is_odd(int inValue) { return inValue % 2 == 1; } bool is_odd_const_ref(const int &amp; inValue) { return inValue % 2 == 1; } struct is_odd_functor : public std::unary_function&lt;int, bool&gt; { bool operator() (const int &amp; inValue) const { return inValue % 2 == 1; } }; int main() { std::vector&lt;int&gt; numbers; numbers.push_back(0); numbers.push_back(1); numbers.push_back(2); numbers.push_back(3); std::vector&lt;int&gt; copy; // Functor: Ok copy_if(numbers.begin(), numbers.end(), std::back_inserter(copy), is_odd_functor()); // Function pointer: Ok copy_if(numbers.begin(), numbers.end(), std::back_inserter(copy), is_odd); // Function pointer that takes const ref: Compiler error copy_if(numbers.begin(), numbers.end(), std::back_inserter(copy), is_odd_const_ref); return 0; } </code></pre> <p>The only situation where it doesn't work is when the function pointer takes a const ref argument. This results in the following compiler error:</p> <pre><code>/usr/include/c++/4.2.1/bits/stl_function.h: In instantiation of ‘std::unary_negate&lt;std::pointer_to_unary_function&lt;const int&amp;, bool&gt; &gt;’: main.cpp:11: instantiated from ‘void copy_if(ForwardIterator, ForwardIterator, OutputIterator, bool (*)(ArgType)) [with ForwardIterator = __gnu_cxx::__normal_iterator&lt;int*, std::vector&lt;int, std::allocator&lt;int&gt; &gt; &gt;, OutputIterator = std::back_insert_iterator&lt;std::vector&lt;int, std::allocator&lt;int&gt; &gt; &gt;, ArgType = const int&amp;]’ main.cpp:53: instantiated from here /usr/include/c++/4.2.1/bits/stl_function.h:322: error: forming reference to reference type ‘const int&amp;’ /usr/include/c++/4.2.1/bits/stl_algo.h: In function ‘_OutputIterator std::remove_copy_if(_InputIterator, _InputIterator, _OutputIterator, _Predicate) [with _InputIterator = __gnu_cxx::__normal_iterator&lt;int*, std::vector&lt;int, std::allocator&lt;int&gt; &gt; &gt;, _OutputIterator = std::back_insert_iterator&lt;std::vector&lt;int, std::allocator&lt;int&gt; &gt; &gt;, _Predicate = std::unary_negate&lt;std::pointer_to_unary_function&lt;const int&amp;, bool&gt; &gt;]’: main.cpp:11: instantiated from ‘void copy_if(ForwardIterator, ForwardIterator, OutputIterator, bool (*)(ArgType)) [with ForwardIterator = __gnu_cxx::__normal_iterator&lt;int*, std::vector&lt;int, std::allocator&lt;int&gt; &gt; &gt;, OutputIterator = std::back_insert_iterator&lt;std::vector&lt;int, std::allocator&lt;int&gt; &gt; &gt;, ArgType = const int&amp;]’ main.cpp:53: instantiated from here /usr/include/c++/4.2.1/bits/stl_algo.h:1227: error: no match for call to ‘(std::unary_negate&lt;std::pointer_to_unary_function&lt;const int&amp;, bool&gt; &gt;) (int&amp;)’ </code></pre> <p>Apparently a reference to a reference is attempted to be made here.</p> <p>My question is: How can I implement a <code>copy_if</code> that meets the following conditions:</p> <ol> <li>implementation in terms of std::remove_copy_if</li> <li>must work with function objects and function pointers</li> <li>without defining helper classes (outside the function scope)</li> <li>using pure C++ and STL (no C++0x or boost)</li> </ol> <h3>Update</h3> <p>I assume that this is possible because it works with <code>boost::bind</code> :</p> <pre><code>template&lt;class ForwardIterator, class OutputIterator, class Functor&gt; void copy_if(ForwardIterator begin, ForwardIterator end, OutputIterator out, Functor inFunctor) { std::remove_copy_if(begin, end, out, !boost::bind(inFunctor, _1)); } </code></pre>
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    1. COhttp://stackoverflow.com/questions/1448817/why-there-is-no-stdcopy-if-algorithm
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    2. CO@sbi That post doesn't provide a solution that implements copy_if in terms of remove_copy_if and that works with both function pointers and function objects.
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    3. COIs there a particular reason you can't just write out the loop in the body of copy_if? Why do you *have* to use remove_copy_if?
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