Would be nice if support check valid

[pvthuyet]

Additional context
Can support the valid checking for unique_resource?
Example:

auto file = make_unique_resource_checked(::fopen("nonexistentfile.txt", "r"), nullptr, fclose);
ASSERT_EQUAL((FILE*)NULL, file.get());
// reading the content 
// ...

I would expected:

auto file = make_unique_resource_checked(::fopen("nonexistentfile.txt", "r"), nullptr, fclose);
ASSERT_EQUAL((FILE*)NULL, file.get());
if (file.valid()) {
     // reading the content 
     // ...
}

[PeterSommerlad]

The current implementation does not invalidate the resource member, when it is in a moved-from or invalid state. To allow such a test, would mean to have the "empty" state as an additional template parameter. Microsoft has an implementation of a similar class that allows this (defining the empty state via a policy). With such, an empty/invalid state can be used.

I am thinking about resurrecting the unique_resource proposal with an implementation that keeps the resources in a std::optional. and have a moved-from state being an empty optional. This could then give a simple check for all such cases.

In the above case the query is the condition in the ASSERT_EQUAL

if( file.get() != nullptr )...