Type predicates and introspection

The core enabler of Avendish is the ability to filter struct definitions according to predicates.

That is, given:

struct Foo {
  int16_t x;
  float y;
  int32_t z;
  std::string txt;        
};

we want to filter for instance all integral types, and get:

tuple<int16_t, int32_t> filtered = filter_integral_types(Foo{});

or run code on the members and other similar things.

Fields introspection

The template fields_introspection<T> allows to introspect the fields of a struct in a generic way.

Doing something for each member of a struct without an instance

fields_instrospection<T>::for_all(
    [] <std::size_t Index, typename T> (avnd::field_reflection<Index, T>) {
      // Index is the index of the struct member, e.g. 1 for y in the above example
      // T is the type of the struct member
});

Doing something for each member of a struct with an instance

Foo foo;
fields_instrospection<T>::for_all(
    foo,
    [] <typename T> (T& t) {
      // t will be x, y, z, txt.
});

Doing something for the Nth member of a struct without an instance

This function lifts a run-time index into a compile-time value. This is useful for instance for mapping a run-time parameter id coming from a DAW, into a field, such as when a parameter changes.

fields_instrospection<T>::for_nth(
    index,
    [] <std::size_t Index, typename T> (avnd::field_reflection<Index, T>) {
      // Index is the index of the struct member, e.g. 1 for y in the above example
      // T is the type of the struct member
});

Doing something for each member of a struct with an instance

Same as above but with an actual instance.

Foo foo;
fields_instrospection<T>::for_nth(
    foo,
    index,
    [] <typename T> (T& t) {
      // t will be x, y, z, txt.
});

Getting the index of a member pointer

Foo foo;
avnd::index_in_struct(foo, &Foo::z) == 2;

Predicate introspection

The type predicate_introspection<T, Pred> allows similar operations on a filtered subset of the struct members.

For instance, given the predicate:

template<typename Field>
using is_int<Field> = std::integral_constant<bool, std::is_integral_v<Field>>;

then

using ints = predicate_introspection<Foo, is_int>;

will allow to query all the int16_t and int32_t members of the struct. This example is assumed for all the cases below.

predicate_introspection<Foo, is_int> will work within this referential: indices will refer to these types. That is, the element 0 will be int16_t and element 1 will be int32_t. Multiple methods are provided to go from and to indices in the complete struct, to indices in the filtered version:


member	x	y	z	txt
field index	0	1	2	3
filtered index	0	-	1	-

Doing something for each member of a struct without an instance

predicate_instrospection<T, P>::for_all(
    [] <std::size_t Index, typename T> (avnd::field_reflection<Index, T>) {
        // Called for x, z
        // Index is 0 for x, 1 for z
});

Doing something for each member of a struct with an instance

Foo foo;
predicate_instrospection<T, P>::for_all(
    foo,
    [] <typename T> (T& t) {
      // Called for x, z
});

// This version also passes the compile-time index
Foo foo;
predicate_instrospection<T, P>::for_all_n(
    foo,
    [] <std::size_t Index, typename T> (T& t, avnd::predicate_index<Index>) {
      // x: Index == 0
      // y: Index == 1
});

// This version passes both the field index and the filtered index
Foo foo;
predicate_instrospection<T, P>::for_all_n(
    foo,
    [] <std::size_t LocalIndex, std::size_t FieldIndex, typename T> (T& t, avnd::predicate_index<LocalIndex>, avnd::field_index<FieldIndex>) {
      // x: LocalIndex == 0 ; FieldIndex == 0
      // y: LocalIndex == 1 ; FieldIndex == 2
});

// This version will return early if the passed lambda returns false
Foo foo;
bool ok = predicate_instrospection<T, P>::for_all_unless(
    foo,
    [] <typename T> (T& t) -> bool {
      return some_condition(t);
});

Doing something for the Nth member of a struct without an instance

Two cases are possible depending on whether one has an index in the struct, or an index in the filtered part of it:

predicate_instrospection<T, P>::for_nth_raw(
    field_index,
    [] <std::size_t Index, typename T> (avnd::field_reflection<Index, T>) {
       // field_index == 0: x
       // field_index == 1: nothing
       // field_index == 2: z
       // field_index == 3: nothing
});

predicate_instrospection<T, P>::for_nth_mapped(
    filtered_index,
    [] <std::size_t Index, typename T> (avnd::field_reflection<Index, T>) {
       // filtered_index == 0: x
       // filtered_index == 1: z
});

Doing something for each member of a struct with an instance

Same as above but with an actual instance.

Foo foo;
predicate_instrospection<T, P>::for_nth_raw(
    foo,
    field_index,
    [] <std::size_t Index, typename T> (T& t) {
       // field_index == 0: x
       // field_index == 1: nothing
       // field_index == 2: z
       // field_index == 3: nothing
});

Foo foo;
predicate_instrospection<T, P>::for_nth_mapped(
    foo,
    filtered_index,
    [] <std::size_t Index, typename T> (T& t) {
       // filtered_index == 0: x
       // filtered_index == 1: z
});

Getting the type of the Nth element

// int16_t
using A = typename predicate_instrospection<T, P>::nth_element<0>;
// int32_t
using B = typename predicate_instrospection<T, P>::nth_element<1>;

Getting the Nth element

Foo foo;

int16_t& a = predicate_instrospection<T, P>::get<0>(foo);
int32_t& b = predicate_instrospection<T, P>::get<1>(foo);

Getting a tuple of the elements

Foo foo;

// Get references:
std::tuple<int16_t&, int32_t&> tpl = predicate_instrospection<T, P>::tie(foo);

// Get copies:
std::tuple<int16_t, int32_t> tpl = predicate_instrospection<T, P>::make_tuple(foo);

// Apply a function to each and return the tuple of that
std::tuple<std::vector<int16_t>, std::vector<int32_t>> tpl = 
  predicate_instrospection<T, P>::filter_tuple(
    foo, 
    [] <typename T>(T& member) { return std::vector<T>{member}; });

Getting the index of the filtered element

// Go from index in the filtered members to index in the struct
predicate_instrospection<T, P>::map<0>() == 0;
predicate_instrospection<T, P>::map<1>() == 2;
predicate_instrospection<T, P>::map<...>() == compile-error;

// Go from index in the host struct to index in the filtered members
predicate_instrospection<T, P>::unmap<0>() == 0;
predicate_instrospection<T, P>::unmap<1>() == compile-error;
predicate_instrospection<T, P>::unmap<2>() == 1;
predicate_instrospection<T, P>::unmap<...>() == compile-error;

Avendish documentation