As a lazy developer, I like to use this trick to specify a default function:
template <class Type, unsigned int Size, class Function = std::less<Type> >
void arrange(std::array<Type, Size> &x, Function&& f = Function())
{
std::sort(std::begin(x), std::end(x), f);
}
But I have a problem in a very particular case, which is the following:
template <class Type, unsigned int Size, class Function = /*SOMETHING 1*/>
void index(std::array<Type, Size> &x, Function&& f = /*SOMETHING 2*/)
{
for (unsigned int i = 0; i < Size; ++i) {
x[i] = f(i);
}
}
In this case, I would like the default function to be the equivalent of: [](const unsigned int i){return i;}
(a function that just returns the passed value).
In order to do that, what do I have to write instead of /*SOMETHING 1*/
and /*SOMETHING 2*/
?
There is no standard functor that does this, but it is easy enough to write (though the exact form is up for some dispute):
This can be used as follows:
You can just build your own identity functor:
boost::phoenix offers a complete functional toolbox, here 'arg1' is the ident to identity ;-)
This is called the
identity
function. Unfortunately, it is not part of the C++ standard, but you can easily build one yourself.If you happen to use g++, you can activate its extensions with
-std=gnu++11
and thenMaybe it will be available in C++20, see
std::identity
. Until then you may look at boost's version at boost::compute::identity.A way to deal with this is to have two different functions. I find quite sane not to use default parameters.