Ichthyostega
e205e1e1a0
start a systematic research about the coexistence of std::hash and boost::hash. The goal is to build an automatic bridge function -- but this is hampered by the unfortunate standard implementation of std::hash Since meanwhile even the GCC people seem to have realized this wasn't a good idea, I am geared towards using a hack to work around this problem, which can be expected to go away with GCC 4.8.x A possible idea how to construct such a workaround is http://stackoverflow.com/questions/12753997/check-if-type-is-hashable I start this investigation by defining two custom types, each with his own extension point for hashing. The goal would then be to use both in a standard hashtable container.
118 lines
3.4 KiB
C++
118 lines
3.4 KiB
C++
/* try.cpp - for trying out some language features....
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* scons will create the binary bin/try
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*
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*/
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// 8/07 - how to control NOBUG??
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// execute with NOBUG_LOG='ttt:TRACE' bin/try
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// 1/08 - working out a static initialisation problem for Visitor (Tag creation)
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// 1/08 - check 64bit longs
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// 4/08 - comparison operators on shared_ptr<Asset>
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// 4/08 - conversions on the value_type used for boost::any
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// 5/08 - how to guard a downcasting access, so it is compiled in only if the involved types are convertible
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// 7/08 - combining partial specialisation and subclasses
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// 10/8 - abusing the STL containers to hold noncopyable values
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// 6/09 - investigating how to build a mixin template providing an operator bool()
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// 12/9 - tracking down a strange "warning: type qualifiers ignored on function return type"
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// 1/10 - can we determine at compile time the presence of a certain function (for duck-typing)?
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// 4/10 - pretty printing STL containers with python enabled GDB?
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// 1/11 - exploring numeric limits
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// 1/11 - integer floor and wrap operation(s)
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// 1/11 - how to fetch the path of the own executable -- at least under Linux?
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// 10/11 - simple demo using a pointer and a struct
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// 11/11 - using the boost random number generator(s)
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// 12/11 - how to detect if string conversion is possible?
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// 1/12 - is partial application of member functions possible?
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// 5/14 - c++11 transition: detect empty function object
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// 7/14 - c++11 transition: std hash function vs. boost hash
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/** @file try.cpp
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** Investigation: how to supply a hash function for custom types.
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** This example defines two custom types, each of which provides a way
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** to calculate hash values. But in one case, we use the new \c std::hash
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** as a framework, in the other case we use the extension mechanism for
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** \c boost::hash. The latter has the benefit of being simpler and less verbose
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** to write, since a simple ADL function is sufficient as extension point
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**
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*/
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//#include <type_traits>
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#include <iostream>
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#include <string>
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#include <vector>
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#include <boost/functional/hash.hpp>
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#include <functional>
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using std::vector;
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using std::string;
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using std::cout;
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using std::endl;
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class S
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{
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string s_;
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friend std::hash<S>;
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public:
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S(string ss ="")
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: s_(ss)
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{ }
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};
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namespace std {
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template<>
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struct hash<S>
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{
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size_t
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operator() (S const& val) const noexcept
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{
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hash<string> string_hasher;
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return string_hasher(val.s_);
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}
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};
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}
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class V
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{
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vector<string> v_;
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public:
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V(string ss ="")
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{
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v_.push_back(ss);
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}
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friend size_t
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hash_value (V const& v)
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{
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return boost::hash_value(v.v_);
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}
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};
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int
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main (int, char**)
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{
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string p("Путин"), pp(p);
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S s(p), ss(pp);
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V v(p), vv(pp);
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std::hash<string> std_stringHasher;
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boost::hash<string> boo_stringHasher;
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std::hash<S> std_customHasher;
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boost::hash<V> boo_customHasher;
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cout << "raw hash(std) = " << std_stringHasher(p) <<"|"<< std_stringHasher(pp)
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<< " (boost) = " << boo_stringHasher(p) <<"|"<< boo_stringHasher(pp)
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<< "\n custom hash (std) " << std_customHasher(s) <<"|"<< std_customHasher(ss)
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<< "\n custom hash (boost) " << boo_customHasher(v) <<"|"<< boo_customHasher(vv)
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;
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cout << "\n.gulp.\n";
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return 0;
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}
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