57 lines
2.2 KiB
Plaintext
57 lines
2.2 KiB
Plaintext
|
[/
|
||
|
|
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/cppalliance/json
|
||
|
|
]
|
||
|
|
|
||
|
|
[/-----------------------------------------------------------------------------]
|
||
|
|
|
||
|
|
[section Non-throwing conversions]
|
||
|
|
|
||
|
|
For the case where throwing exceptions is undesirable, Boost.JSON also provides
|
||
|
|
a non-throwing version of __value_to__, the function template
|
||
|
|
__try_value_to__. It returns __result__, a specialised variant that either
|
||
|
|
holds a value or an __error_code__.
|
||
|
|
|
||
|
|
[note There's no non-throwing equivalent for __value_from__. This is simply
|
||
|
|
because we haven't yet encountered a situation where __value_from__ needed to
|
||
|
|
communicate an error to the caller.]
|
||
|
|
|
||
|
|
The library provides non-throwing conversions for all the categories of types
|
||
|
|
it supports with __value_to__. If a user wants to use it with custom types,
|
||
|
|
an overload of `tag_invoke` of this form needs to be provided:
|
||
|
|
|
||
|
|
```
|
||
|
|
result_for<T, value>::type
|
||
|
|
tag_invoke( const try_value_to_tag< T >&, const value& );
|
||
|
|
```
|
||
|
|
|
||
|
|
For the class `ip_address` from the section
|
||
|
|
[link json.conversion.custom_conversions Custom conversions] this overload may
|
||
|
|
look like this:
|
||
|
|
|
||
|
|
[snippet_nothrow_1]
|
||
|
|
|
||
|
|
The overload lets us use `ip_address` with __try_value_to__.
|
||
|
|
|
||
|
|
[snippet_nothrow_2]
|
||
|
|
|
||
|
|
If __try_value_to__ is used with a type, for which there's no `tag_invoke`
|
||
|
|
overload of the form described in this section, but there is one of the form
|
||
|
|
intended for __value_to__, then the library still tries to perform the
|
||
|
|
conversion. It uses the throwing overload, and attempts to convert any thrown
|
||
|
|
exception into an __error_code__. Note, though, that such approach will likely
|
||
|
|
be slower then a dedicated overload.
|
||
|
|
|
||
|
|
The opposite is also true: if there's a `tag_invoke` overload intended for
|
||
|
|
__try_value_to__, but not for __value_to__, then calling `__value_to__` will
|
||
|
|
invoke the non-throwing overload, then construct a __system_error__ from the
|
||
|
|
__error_code__ and throw it. Due to these fallbacks, it is recommended that
|
||
|
|
users provide the overload from this section, rather then the other one, if
|
||
|
|
they ever intend to use __try_value_to__.
|
||
|
|
|
||
|
|
[endsect]
|