* or including +<boost/functional-hash>+ and +using boost::hash+
The boost version additionally provides pre defined hash functors for STL containers holding
custom types -- and it provides an easy to use extension mechanism for writing hash functions
for custom types. Effectively this means that, assuming usage of the boost-include, the actual
implementation and the way it is picked up is _different but compatible_ to the standard way.
Boost: hashing custom types
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The extension mechanism used by the boost version is best explained by looking
at the code
.boost/functional/hash/extensions.hpp
[source,C]
----
template <class T> struct hash
: std::unary_function<T, std::size_t>
{
std::size_t operator()(T const& val) const
{
return hash_value(val);
}
}
----
So this templated standard implementation just _invokes an unqualified function_
with the name +hash_value(val)+ -- when instantiating this template for your custom
class or type, the compiler will search this function not only in the current scope,
but also in the namespace defining your custom type +T+ (this mechanism is known as
``**A**rgument **D**ependant **L**ookup''). Meaning that all we'd need to do is to define a
free function or friend function named +hash_value+ alongside with our custom data types (classes).
To further facilitate providing custom hash functions, boost defines a function
+boost::hash_combine(size_t seed, size_t hashValue)+, allowing to _chain up_ the
calculated hash values of the parts forming a composite data structure.
- see Lumiera's link:http://git.lumiera.org/gitweb?p=LUMIERA;a=blob;f=src/proc/asset/category.hpp;h=b7c8df2f2ce69b0ccf89439954de8346fe8d9276;hb=master#l104[asset::Category]
