Metafunctions to classify types, in order to be able to restrict template parameters, and some other helpers for type computation.
Metafunctions to classify types, in order to be able to restrict template parameters, and some other helpers for type computation.
These functions are executed by the compiler, not at runtime, and can be used to restrict the typse that can be passed to template functions or discriminate between them during execution via if constexpr. The functions introduced here can later be employed to implement c++20 concepts that serve the same purpose, but with less syntactic overhead and cleaner error messages. Note that we call metafunctions implemented here concepts for bervity as well, despite the ambiguity with the C++20 concepts feature. Note also that while some of them are named like C++ NamedRequirements (e.g. is_container), they do not check if a type actually fullfills the respective NamedRequirement. The metafunctions in type_traits.hh implement a type hierarchy:
- an iterable is a type that has a subtype
iterator. - a string is either std::basic_string<T>, std::basic_string_view<T> or char* or const char*
- a container is an iterable that is not a string. Note that C++ defines std::basic_string as a container as well, but HDF5 does treat arrays and strings differently and hence this distinction is necessary here in order to get the DataIO backend to work correctly.
- an associative container is a container T that defines T::key_type and T::key_compare
- an unordered associative container is a container that defines T::key_type, T::value_type and T::hasher.
- a linear container is a container that is not an associative container or unordered associative container
- a random access container is a linear container the iterator of which has std::random_access_iterator tag or another tag that can be converted to std::random_access_iterator_tag.
- a type T is array-like if it is a container and has compile time constant size given by an overload of the
get_sizemetafunction. - a type T is tuple-like if it has compile time constant size given by an overload of the
get_sizemetafunction. - a type is a graph if it has both vertex-descriptor and edge-descriptor subtypes.
Aside from these type-restriction metafunctions, many more metafunctions are implemented therein that help modify types in a useful way, like recursively removing type extensions (const volative T****& --> T) or similar, or for checking if a type is callable or not.
