C++ std::optional optional as return value
Example
std::optional<float> divide(float a, float b) {
if (b!=0.f) return a/b;
return {};
}
Here we return either the fraction a/b, but if it is not defined (would be infinity) we instead return the empty optional.
A more complex case:
template<class Range, class Pred>
auto find_if( Range&& r, Pred&& p ) {
using std::begin; using std::end;
auto b = begin(r), e = end(r);
auto r = std::find_if(b, e , p );
using iterator = decltype(r);
if (r==e)
return std::optional<iterator>();
return std::optional<iterator>(r);
}
template<class Range, class T>
auto find( Range&& r, T const& t ) {
return find_if( std::forward<Range>(r), [&t](auto&& x){return x==t;} );
}
find( some_range, 7 ) searches the container or range some_range for something equal to the number 7. find_if does it with a predicate.
It returns either an empty optional if it was not found, or an optional containing an iterator tothe element if it was.
This allows you to do:
if (find( vec, 7 )) {
// code
}
or even
if (auto oit = find( vec, 7 )) {
vec.erase(*oit);
}
without having to mess around with begin/end iterators and tests.
