value_to

## `value_to`

Convert a `value` to an object of type `T`.

### Synopsis

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

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

    template<
        class T>
    T
    value_to(
        const value& jv); (2)

    template<
        class T,
        class U>
    T
    value_to(
        U const& jv) = delete; (3)

### Description

This function attempts to convert a `value` to `T` using

- one of `value`'s accessors, or

- a library-provided generic conversion, or

- a user-provided overload of `tag_invoke`.

Out of the box the function supports default constructible types satisfying SequenceContainer, arrays, arithmetic types, `bool`, `std::tuple`, `std::pair`, `std::optional`, `std::variant`, `std::nullptr_t`, and structs and enums described using Boost.Describe.

Conversion of other types is done by calling an overload of `tag_invoke` found by argument-dependent lookup. Its signature should be similar to:

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

or

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

or

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

The overloads are checked for existence in that order and the first that matches will be selected.

The object returned by the function call is returned by `value_to` as the result of the conversion.

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.

Overload (3) is deleted and participates in overload resolution only when `U` is not `value`. The overload exists to prevent unintented creation of temporary `value` instances, e.g.

    auto flag = value_to<bool>(true);

### Constraints

    ! std::is_reference< T >::value

### Exception Safety

Strong guarantee.

### Template Parameters

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

### Return Value

`jv` converted to `result<T>`.

### Parameters

Name | Description
--- | ---
`jv` | The `value` to convert.
`ctx` | Context passed to the conversion function.

### See Also

`value_to_tag`, `value_from`, tag_invoke: A general pattern for supporting customisable functions

Convenience header <boost/json.hpp>

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