value_from

`value_from`

Convert an object of type T to value.

Synopsis

Defined in header <boost/json/value_from.hpp>.

template<
    class T,
    class Context>
void
value_from(
    T&& t,
    Context const& ctx,
    value& jv); (1)

template<
    class T,
    class Context>
value
value_from(
    T&& t,
    Context const& ctx,
    storage_ptr sp = {}); (2)

template<
    class T>
void
value_from(
    T&& t,
    value& jv); (3)

template<
    class T>
value
value_from(
    T&& t,
    storage_ptr sp = {}); (4)

Description

This function attempts to convert an object of type T to value using

one of value's constructors,
a library-provided generic conversion, or
a user-provided overload of tag_invoke.

In order to perform the conversion the function selects an appropriate implementation based on the types T and Context (if provided).

If Context is available and is not std::tuple<C...>
1. check if use_category<T, Context>::value is not conversion_category::unknown; otherwise
2. check if a tag_invoke overload from the list below that takes a Context const& exists.
Otherwise, if Context is available, and is std::tuple<C...> repeat steps 1 and 2 recursively for every C until either step 1.a or 1.b succeeds for some C.
Failing that,
1. check if use_category<T>::value is not conversion_category::unknown; otherwise
2. check if a tag_invoke overload from the list below that takes only 2 parameters exists; otherwise
3. check if T is one of value, array, object, or string; otherwise
4. check if T matches one of the categories of types described in the table "Conversion categories" in Value Conversion section.

These steps determine both the appropriate category of conversion for T, and, if necessary, the effective context C that will be used for conversion. If the category is selected on steps 1.a, 3.a, 3.c, or 3.d, the library provides a suitable conversion implementation. If the category is selected on steps 2.b or 3.b, then a user-provided tag_invoke overload is used.

The overloads of tag_invoke that will be considered by this function are in the following list. Overloads that appear higher in the list have higher priority.

template< class FullContext >
void tag_invoke( value_from_tag, value&, T, const Context&, const FullContext& );

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

void tag_invoke( value_from_tag, value&, T );

The ctx argument can be used either as a tag type to provide conversions for third-party types, or to pass extra data to the conversion function.

Overloads (2) and (4) construct their return value using the storage_ptr sp, which ensures that the memory resource is correctly propagated.

Exception Safety

Strong guarantee.

Template Parameters

Type Description

Type	Description
`T`	The type of the object to convert.
`Context`	The type of context passed to the conversion function.

T

The type of the object to convert.

Context

The type of context passed to the conversion function.

Return Value

Overloads (2) and (4) return t converted to value. Overloads (1) and 3 return void instead and pass their result via the out parameter jv.

Parameters

Name Description

Name	Description
`t`	The object to convert.
`ctx`	Context passed to the conversion function.
`jv`	`value` out parameter.
`sp`	A storage pointer referring to the memory resource to use for the returned `value`.

t

The object to convert.

ctx

Context passed to the conversion function.

jv

value out parameter.

sp

A storage pointer referring to the memory resource to use for the returned value.