Why does std::optional operator* not have debug mode assertion for has_value()?

Question

I perfectly understand that because of performance reasons the operator* in std::optional does not make any checks for the actual existence of a contained value. However, in debug mode performance considerations should not matter and it would make a lot of sense to me that some kind of assertion should be made while in debug mode.

Visual studio doesn't seem to have such an assertion (though I am not sure of other compilers).

My question is: Is there any fundamental reason why the compiler would NOT make such an assertion on debug mode or is it just a missing feature?

Pompey

The more your debug build diverges from the release, the less useful it becomes. – Edgebone

Answer 1

Is there any fundamental reason why the compiler would NOT make such an assertion on debug mode or is it just a missing feature?

ODR violations. std::optional is a class template and hence implemented in a header. Different behavior of code inside a header for different preprocessor symbols is as dangerous as it gets. Consider this example (not tested, you'll get the point):

clang++ -DNDEBUG usesOptionalOfInt.cpp -shared -c -o myLib.so
clang++ alsoUsesOptionalOfInt.cpp main.cpp -lmyLib -o ./ub-please

There you go with undefined behavior. Note that it's very unlikely that the difference in std::optional::operator* would actually cause any harm in this example, but still - you want to avoid these situations.

Blayze

But if one is to use STL over static libraries, you have to have them built with the same compiler options, anyway, otherwise nothing will work! std::vector is full of assertions in Visual Studio already (see _STL_VERIFY() under various _ITERATOR_DEBUG_LEVELs ). What is the difference between those assertions and the one that @FranciscoMartinez proposes ? – Puissant

Those are reserved names - client code is not allowed to mess with those. From cppreference: "the identifiers that begin with an underscore followed by an uppercase letter are reserved". This is the main difference to e.g. the NDEBUG macro. – Blayze