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POCompile problems: building a Composite_Key variadic template class using tuples
 

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  1. POCompile problems: building a Composite_Key variadic template class using tuples
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    <p>I have an <code>ID</code> class template that takes a parameter <code>T</code>. </p> <ul> <li>If <code>T</code> has a <code>key_type</code> defined, <code>ID</code> calls <code>get_key()</code> on an object of type <code>T</code> to get the identifier for storage. </li> <li>If <code>T</code> does not have <code>key_type</code> defined, <code>ID</code> will use the address of the object as the identifier.</li> </ul> <p>The code works fine up to this point.</p> <p>Now, I would like to define a new variadic class template <code>Composite_Key</code> which takes variadic template parameters as a <code>std::tuple</code>. I am trying to get this new code to work with <code>ID</code>, but I am getting a wall of compilation errors that I am having a hard time understanding.</p> <p>The errors seem to indicate a missing <code>operator&lt;()</code>, which is odd because it's defined in <code>ID</code>; I'm at a loss as to what I'm doing wrong.</p> <p>The code below, including test code, works fine up until that last line (commented).</p> <p><strong>CODE</strong></p> <pre><code>#include &lt;string&gt; #include &lt;tuple&gt; #include &lt;set&gt; #include &lt;cassert&gt; template&lt;typename T&gt; struct void_ { using type = void; }; // ----------------------------------------------------------------------------- template&lt;typename T, typename = void&gt; struct ptr_or_key_type { using type = T const*; // our default key_type : a ptr static type get_key( T const&amp; t ) { return &amp;t; } }; template&lt;typename T&gt; struct ptr_or_key_type&lt;T, typename void_&lt;typename T::key_type&gt;::type&gt; { using type = typename T::key_type; // the specialised key_type static type get_key( T const&amp; t ) { return t.get_key(); } }; // ----------------------------------------------------------------------------- template&lt;typename T&gt; class ID { private: typename ptr_or_key_type&lt;T&gt;::type m_id; public: ID( T const&amp; t ) : m_id( ptr_or_key_type&lt;T&gt;::get_key( t )) { } ID( ID const&amp; rhs ) : m_id( rhs.m_id ) { } ~ID() { } ID&amp; operator=( ID const&amp; rhs ) { if ( &amp;rhs!=this ) m_id = rhs.m_id; return *this; } public: bool operator==( ID const&amp; rhs ) const { return m_id==rhs.m_id; } bool operator!=( ID const&amp; rhs ) const { return !(*this==rhs); } bool operator&lt;( ID const&amp; rhs ) const { return m_id&lt;rhs.m_id; } bool operator&lt;=( ID const&amp; rhs ) const { return m_id&lt;=rhs.m_id; } bool operator&gt;( ID const&amp; rhs ) const { return m_id&gt;rhs.m_id; } bool operator&gt;=( ID const&amp; rhs ) const { return m_id&gt;=rhs.m_id; } }; // ----------------------------------------------------------------------------- struct Plain_Class { }; struct String_Key { using key_type = std::string; std::string m_key; String_Key( std::string const&amp; key ) : m_key( key ) { } std::string const&amp; get_key() const { return m_key; } }; struct Char_Key { using key_type = char; char m_key; Char_Key( char key ) : m_key( key ) { } char get_key() const { return m_key; } }; struct Int_Key { using key_type = int; int m_key; Int_Key( int key ) : m_key( key ) { } int get_key() const { return m_key; } }; template&lt;typename... Args&gt; struct Composite_Key { using key_type = std::tuple&lt;Args...&gt;; key_type m_key; Composite_Key( key_type const&amp; key ) : m_key( key ) { } key_type const&amp; get_key() const { return m_key; } }; // ----------------------------------------------------------------------------- int main( int argc, char* argv[] ) { // Plain_Class will use address of object as key Plain_Class f,g; ID&lt;Plain_Class&gt; id_f( f ), id_g( g ); assert( id_f!=id_g ); std::set&lt;ID&lt;Plain_Class&gt;&gt; s; s.insert( f ); s.insert( g ); assert( s.size()==2u ); // two unique addresses, so two in the set // String_Key will use std::string as the key String_Key h( "abc" ), i( "abc" ); std::set&lt;ID&lt;String_Key&gt;&gt; s2; s2.insert( h ); s2.insert( i ); assert( s2.size()==1u ); // since sets must have unique values // attempt a composite key type using My_Composite = Composite_Key&lt;String_Key,Int_Key,Char_Key&gt;; My_Composite j( std::make_tuple( String_Key{ "foo" }, Int_Key{ 1 }, Char_Key{ 'c' } )), k( std::make_tuple( String_Key{ "foo" }, Int_Key{ 1 }, Char_Key{ 'c' } )) ; std::set&lt;ID&lt;My_Composite&gt;&gt; s3; s3.insert( j ); // FAILURE: everything above this line compiles fine #if 0 s3.insert( k ); assert( s3.size()==1u ); // since sets must have unique values #endif } </code></pre> <p><strong>WALL OF ERRORS</strong></p> <pre><code>In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple: In instantiation of ‘static bool std::__tuple_compare&lt;0ul, __i, __j, _Tp, _Up&gt;::__less(const _Tp&amp;, const _Up&amp;) [with long unsigned int __i = 0ul; long unsigned int __j = 3ul; _Tp = std::tuple&lt;String_Key, Int_Key, Char_Key&gt;; _Up = std::tuple&lt;String_Key, Int_Key, Char_Key&gt;]’: /usr/include/c++/4.7/tuple:814:62: required from ‘bool std::operator&lt;(const std::tuple&lt;_TElements ...&gt;&amp;, const std::tuple&lt;_Elements ...&gt;&amp;) [with _TElements = {String_Key, Int_Key, Char_Key}; _UElements = {String_Key, Int_Key, Char_Key}]’ sandbox.cpp:50:59: required from ‘bool ID&lt;T&gt;::operator&lt;(const ID&lt;T&gt;&amp;) const [with T = Composite_Key&lt;String_Key, Int_Key, Char_Key&gt;; ID&lt;T&gt; = ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt;]’ /usr/include/c++/4.7/bits/stl_function.h:237:22: required from ‘bool std::less&lt;_Tp&gt;::operator()(const _Tp&amp;, const _Tp&amp;) const [with _Tp = ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt;]’ /usr/include/c++/4.7/bits/stl_tree.h:1285:4: required from ‘std::pair&lt;std::_Rb_tree_iterator&lt;_Val&gt;, bool&gt; std::_Rb_tree&lt;_Key, _Val, _KeyOfValue, _Compare, _Alloc&gt;::_M_insert_unique(_Arg&amp;&amp;) [with _Arg = ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt;; _Key = ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt;; _Val = ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt;; _KeyOfValue = std::_Identity&lt;ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt; &gt;; _Compare = std::less&lt;ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt; &gt;; _Alloc = std::allocator&lt;ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt; &gt;]’ /usr/include/c++/4.7/bits/stl_set.h:424:40: required from ‘std::pair&lt;typename std::_Rb_tree&lt;_Key, _Key, std::_Identity&lt;_Key&gt;, _Compare, typename _Alloc::rebind&lt;_Key&gt;::other&gt;::const_iterator, bool&gt; std::set&lt;_Key, _Compare, _Alloc&gt;::insert(std::set&lt;_Key, _Compare, _Alloc&gt;::value_type&amp;&amp;) [with _Key = ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt;; _Compare = std::less&lt;ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt; &gt;; _Alloc = std::allocator&lt;ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt; &gt;; typename std::_Rb_tree&lt;_Key, _Key, std::_Identity&lt;_Key&gt;, _Compare, typename _Alloc::rebind&lt;_Key&gt;::other&gt;::const_iterator = std::_Rb_tree_const_iterator&lt;ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt; &gt;; std::set&lt;_Key, _Compare, _Alloc&gt;::value_type = ID&lt;Composite_Key&lt;String_Key, Int_Key, Char_Key&gt; &gt;]’ sandbox.cpp:121:15: required from here /usr/include/c++/4.7/tuple:781:63: error: no match for ‘operator&lt;’ in ‘std::get&lt;0ul, {String_Key, Int_Key, Char_Key}&gt;((* &amp; __u)) &lt; std::get&lt;0ul, {String_Key, Int_Key, Char_Key}&gt;((* &amp; __t))’ /usr/include/c++/4.7/tuple:781:63: note: candidates are: In file included from /usr/include/c++/4.7/bits/stl_algobase.h:65:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template&lt;class _T1, class _T2&gt; constexpr bool std::operator&lt;(const std::pair&lt;_T1, _T2&gt;&amp;, const std::pair&lt;_T1, _T2&gt;&amp;) /usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::pair&lt;_T1, _T2&gt;’ In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template&lt;class _Iterator&gt; bool std::operator&lt;(const std::reverse_iterator&lt;_Iterator&gt;&amp;, const std::reverse_iterator&lt;_Iterator&gt;&amp;) /usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::reverse_iterator&lt;_Iterator&gt;’ In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template&lt;class _IteratorL, class _IteratorR&gt; bool std::operator&lt;(const std::reverse_iterator&lt;_IteratorL&gt;&amp;, const std::reverse_iterator&lt;_IteratorR&gt;&amp;) /usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::reverse_iterator&lt;_IteratorL&gt;’ In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template&lt;class _IteratorL, class _IteratorR&gt; bool std::operator&lt;(const std::move_iterator&lt;_IteratorL&gt;&amp;, const std::move_iterator&lt;_IteratorR&gt;&amp;) /usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::move_iterator&lt;_IteratorL&gt;’ In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_iterator.h:1063:5: note: template&lt;class _Iterator&gt; bool std::operator&lt;(const std::move_iterator&lt;_Iterator&gt;&amp;, const std::move_iterator&lt;_Iterator&gt;&amp;) /usr/include/c++/4.7/bits/stl_iterator.h:1063:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::move_iterator&lt;_Iterator&gt;’ In file included from /usr/include/c++/4.7/string:54:0, from sandbox.cpp:1: /usr/include/c++/4.7/bits/basic_string.h:2566:5: note: template&lt;class _CharT, class _Traits, class _Alloc&gt; bool std::operator&lt;(const std::basic_string&lt;_CharT, _Traits, _Alloc&gt;&amp;, const std::basic_string&lt;_CharT, _Traits, _Alloc&gt;&amp;) /usr/include/c++/4.7/bits/basic_string.h:2566:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::basic_string&lt;_CharT, _Traits, _Alloc&gt;’ In file included from /usr/include/c++/4.7/string:54:0, from sandbox.cpp:1: /usr/include/c++/4.7/bits/basic_string.h:2578:5: note: template&lt;class _CharT, class _Traits, class _Alloc&gt; bool std::operator&lt;(const std::basic_string&lt;_CharT, _Traits, _Alloc&gt;&amp;, const _CharT*) /usr/include/c++/4.7/bits/basic_string.h:2578:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::basic_string&lt;_CharT, _Traits, _Alloc&gt;’ In file included from /usr/include/c++/4.7/string:54:0, from sandbox.cpp:1: /usr/include/c++/4.7/bits/basic_string.h:2590:5: note: template&lt;class _CharT, class _Traits, class _Alloc&gt; bool std::operator&lt;(const _CharT*, const std::basic_string&lt;_CharT, _Traits, _Alloc&gt;&amp;) /usr/include/c++/4.7/bits/basic_string.h:2590:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: mismatched types ‘const _CharT*’ and ‘String_Key’ /usr/include/c++/4.7/tuple:808:5: note: template&lt;class ... _TElements, class ... _UElements&gt; bool std::operator&lt;(const std::tuple&lt;_TElements ...&gt;&amp;, const std::tuple&lt;_Elements ...&gt;&amp;) /usr/include/c++/4.7/tuple:808:5: note: template argument deduction/substitution failed: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::tuple&lt;_TElements ...&gt;’ In file included from /usr/include/c++/4.7/set:60:0, from sandbox.cpp:3: /usr/include/c++/4.7/bits/stl_tree.h:873:5: note: template&lt;class _Key, class _Val, class _KeyOfValue, class _Compare, class _Alloc&gt; bool std::operator&lt;(const std::_Rb_tree&lt;_Key, _Val, _KeyOfValue, _Compare, _Alloc&gt;&amp;, const std::_Rb_tree&lt;_Key, _Val, _KeyOfValue, _Compare, _Alloc&gt;&amp;) /usr/include/c++/4.7/bits/stl_tree.h:873:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::_Rb_tree&lt;_Key, _Val, _KeyOfValue, _Compare, _Alloc&gt;’ In file included from /usr/include/c++/4.7/set:61:0, from sandbox.cpp:3: /usr/include/c++/4.7/bits/stl_set.h:721:5: note: template&lt;class _Key, class _Compare, class _Alloc&gt; bool std::operator&lt;(const std::set&lt;_Key, _Compare, _Alloc&gt;&amp;, const std::set&lt;_Key, _Compare, _Alloc&gt;&amp;) /usr/include/c++/4.7/bits/stl_set.h:721:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::set&lt;_Key, _Compare, _Alloc&gt;’ In file included from /usr/include/c++/4.7/set:62:0, from sandbox.cpp:3: /usr/include/c++/4.7/bits/stl_multiset.h:702:5: note: template&lt;class _Key, class _Compare, class _Alloc&gt; bool std::operator&lt;(const std::multiset&lt;_Key, _Compare, _Alloc&gt;&amp;, const std::multiset&lt;_Key, _Compare, _Alloc&gt;&amp;) /usr/include/c++/4.7/bits/stl_multiset.h:702:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::multiset&lt;_Key, _Compare, _Alloc&gt;’ /usr/include/c++/4.7/tuple:781:63: error: no match for ‘operator&lt;’ in ‘std::get&lt;0ul, {String_Key, Int_Key, Char_Key}&gt;((* &amp; __t)) &lt; std::get&lt;0ul, {String_Key, Int_Key, Char_Key}&gt;((* &amp; __u))’ /usr/include/c++/4.7/tuple:781:63: note: candidates are: In file included from /usr/include/c++/4.7/bits/stl_algobase.h:65:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template&lt;class _T1, class _T2&gt; constexpr bool std::operator&lt;(const std::pair&lt;_T1, _T2&gt;&amp;, const std::pair&lt;_T1, _T2&gt;&amp;) /usr/include/c++/4.7/bits/stl_pair.h:212:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::pair&lt;_T1, _T2&gt;’ In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template&lt;class _Iterator&gt; bool std::operator&lt;(const std::reverse_iterator&lt;_Iterator&gt;&amp;, const std::reverse_iterator&lt;_Iterator&gt;&amp;) /usr/include/c++/4.7/bits/stl_iterator.h:299:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::reverse_iterator&lt;_Iterator&gt;’ In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template&lt;class _IteratorL, class _IteratorR&gt; bool std::operator&lt;(const std::reverse_iterator&lt;_IteratorL&gt;&amp;, const std::reverse_iterator&lt;_IteratorR&gt;&amp;) /usr/include/c++/4.7/bits/stl_iterator.h:349:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::reverse_iterator&lt;_IteratorL&gt;’ In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template&lt;class _IteratorL, class _IteratorR&gt; bool std::operator&lt;(const std::move_iterator&lt;_IteratorL&gt;&amp;, const std::move_iterator&lt;_IteratorR&gt;&amp;) /usr/include/c++/4.7/bits/stl_iterator.h:1057:5: note: template argument deduction/substitution failed: In file included from sandbox.cpp:2:0: /usr/include/c++/4.7/tuple:781:63: note: ‘const String_Key’ is not derived from ‘const std::move_iterator&lt;_IteratorL&gt;’ In file included from /usr/include/c++/4.7/bits/stl_algobase.h:68:0, from /usr/include/c++/4.7/bits/char_traits.h:41, from /usr/include/c++/4.7/string:42, from sandbox.cpp:1: /usr/include/c++/4.7/bits/stl_iterator.h:1063:5: note: template&lt;class _Iterator&gt; bool </code></pre> <p><strong>ANSWER</strong></p> <p>As indicated by <a href="https://stackoverflow.com/a/14860850/975129">jmetcalf below</a>, all the Key classes must also implement <code>operator&lt;()</code>. This makes sense because the <code>operator&lt;()</code> in <code>ID</code> actually depends on <code>std::tuple</code>'s <code>operator&lt;()</code>, which depends on all the individual types' <code>operator&lt;()</code>.</p>
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