////
Copyright (c) 2022 Dmitry Arkhipov (grisumbras@yandex.ru)

Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

Official repository: https://github.com/boostorg/json
////

= Custom Conversions
Boost.JSON uses two mechanisms to customize conversion between <<ref_value>>
and user types. One mechanism involves specializing type traits. The other one
is more powerful and requires defining overloads of `tag_invoke`. Both
mechanisms will be further explained in this section.

== Conversion Traits
Previously a number of conversion type traits, like <<ref_is_tuple_like>> or
<<ref_is_sequence_like>>,  were introduced. The library tries the traits one
after another and uses the implementation that corresponds to the first
matching trait. In some cases, though, a type would match a trait with a higher
priority, but the user intends for it to belong to a lower priority category.
If this happens the user can specialize the trait that's not supposed to match
for that type to be an equivalent of `std::false_type`.

Consider this type:

[source]
----
include::../../../test/doc_types.hpp[tag=snippet_conv_spec_trait1,indent=0]
----

It exposes both a sequence API and a tuple API. But converting from
<<ref_value>> to `user_ns::ip_address` would not be able to use implementation
for sequences, since those are constructed empty and then populated one element
at a time, while `ip_address` has a fixed size of 4. The tuple conversion would
fit, though. The only problem is that <<ref_is_tuple_like>> has a lower
priority than <<ref_is_sequence_like>>. In order to circumvent this, the user
only needs to specialize <<ref_is_sequence_like>> to not match `ip_address`.

[source]
----
include::../../../test/snippets.cpp[tag=snippet_conv_spec_trait2,indent=0]
----

== `tag_invoke` Overloads
The second, more powerful approach, is to provide the conversion implementation
yourself. With Boost.JSON this is done by defining an overload of `tag_invoke`
function (the benefits of this mechanism are outlined in
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf[C++
proposal P1895]. In essence, `tag_invoke` provides a uniform interface for
defining customization points by using argument-dependent lookup to find
a viable overload from the point at which it is called. As the name suggests,
a tag type is passed as an argument in order to:

* discard candidates that are unrelated to that particular
customization point, and

* embed the user-defined type into the arguments list (e.g. by using a tag type
template such as `value_to_tag<T>`) so that its
http://eel.is/c++draft/basic.lookup.argdep#2[associated namespaces and
entities] are examined when name lookup is performed.

This has the effect of finding user-provided `tag_invoke` overloads, even if
they are declared (lexically) after the definition of the calling function.

Overloads of `tag_invoke` called by <<ref_value_from>> take the form:

```
void tag_invoke( const value_from_tag&, value&, T );
```

While overloads of `tag_invoke` called by <<ref_value_to>> take the form:

```
T tag_invoke( const value_to_tag< T >&, const value& );
```

If we implemented conversion for `user_ns::ip_address` manually with this
approach, it would look like this:

[source]
----
include::../../../test/snippets.cpp[tag=snippet_tag_invoke_1,indent=0]
----

Since the type being converted is embedded into the function's signature,
user-provided overloads are visible to argument-dependent lookup and will be
candidates when a conversion is performed:

[source]
----
include::../../../test/snippets.cpp[tag=snippet_tag_invoke_2,indent=0]
----

Users can freely combine types with custom conversions with types with
library-provided conversions. The library handles them correctly:

[source]
----
include::../../../test/snippets.cpp[tag=snippet_tag_invoke_3,indent=0]
----
