49 lines
1.6 KiB
Plaintext
49 lines
1.6 KiB
Plaintext
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Copyright (c) 2022 Dmitry Arkhipov (grisumbras@yandex.ru)
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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Official repository: https://github.com/cppalliance/json
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]
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[/-----------------------------------------------------------------------------]
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[section Allocation control]
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As __value_from__ creates a __value__ object, users may want to control the way
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memory is allocated for it. For this reason the function has an optional
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__storage_ptr__ parameter, that is used to set the __memory_resource__ for the
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result.
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[note __value_to__ does not have a similar parameter,
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as __value__ is not created.]
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As the conversion result is set via an output parameter of type `value&`, the
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intended __storage_ptr__ is correctly propagated. But users still should take
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care to not create temporaries using the default __memory_resource__
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by accident.
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For example, consider this alternative implementation of `tag_invoke` for
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`ip_address` from the section [link json.conversion.custom_conversions Custom
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conversions].
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```
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void
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tag_invoke( const value_from_tag&, value& jv, ip_address const& addr )
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{
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jv = array{ b[0], b[1], b[2], b[3] };
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}
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```
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This implementation explicitly creates an __array__ rather than relying on
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assignment from an initializer list. But the array uses default
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__memory_resource__, not the one used by `jv`.
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To avoid creating such temporaries with an incorrect __memory_resource__,
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using `value`'s member functions `emplace_array`, `emplace_object`, and
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`emplace_string` can be helpful.
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[endsect]
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