C++ Term Traits:
I would say traits are type properties
struct oracle;
struct postgres;
template<typename T>
void print_database(T t /* value only for type deduction -- avoiding
explicit <>*/)
{
/*Type resolution: database is unknown at this stage, database is a
dependend type, but type dependend */
if constexpr (is_same<typename T::database,oracle>::value) { cout
<< "oracle" << endl; return; }
else if constexpr (is_same<typename T::database,postgres>::value) {
cout << "postgres" << endl; return;}
else { static_assert("Unsupported type"); }
}
struct abc
{
};
struct def
{
};
template<typename T>
struct db_traits;
template<>
struct db_traits<abc>
{
using database = oracle;
};
template<>
struct db_traits<def>
{
using database = postgres;
};
template<typename T>
struct something
{
using database = typename T::database;
};
int main()
{
something<abc> s;
print_database(s);
return 0;
}
Typetraits are lightweight information because the implementations of
the types/structs/classes
are not of interest. Types are only names in this context.
Remark: Generic programming is like any abstraction if only one example
is available.
It makes things more complicated. But looking on universal properties
it's clear it needs
a bunch of examples to get a benefit. So many developers can live
without it. But everyone
will try it - the "feature" effect.
C++ term concepts:
It's relatively new, available since C++-20. It an improvement in the
area specification of
templated constructions. Typically, in the C++ world it's forbidden to
use a mathematical
vocabulary. Make better specifications seems to be the goal. I have no
personal experiences.
Remark2: C++ concepts is an additional feature but not a concept. The
world of C++ scientists
is restricted by C++. It's more a (distinct) feeling: replace concepts
by ats-proofing.
Be sure, other c++ developers will tell you other things.
Am 03.07.20 um 20:10 schrieb gmhwxi:
Traits:
Based on my understanding, traits in C++ can be implemented in a more
or less
straightforward manner. To me, traits are an template-based approach
for generating
ifdef-guards used in C.
However, I would prefer to take a deeper look at the template
resolution algorithm. Traits are
a bit like band-aid used at surface. One can expect to achieve a lot
more if there is a logical way
to affect the manner in which template code is replaced with
non-template code.
Concepts:
It is a feature mainly for statically debugging templates. I am
looking into it. Debugging templates
statically is a big thing; I prefer something with clearer semantics.
Let's wait and see.
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