result_from

`result_from_errno`

Create boost::system::result storing a portable error code.

Synopsis

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

template<
    class T>
result_for< T, value >::type
result_from_errno(
    int e,
    boost::source_location const* loc) noexcept;

Description

This function constructs a boost::system::result<T> that stores boost::system::error_code with value() equal to e and category() equal to boost::system::generic_category().

The main use for this function is in implementation of functions returning boost::system::result, without including boost/json/system_error.hpp or even <system_error>. In particular, it may be useful for customizations of try_value_to without creating a physical dependency on Boost.JSON. For example:

#include <cerrno>
#include <boost/assert/source_location.hpp>

namespace boost
{
namespace json
{

class value;

template<class T>
struct try_value_to_tag;

template<class T1, class T2>
struct result_for;

template <class T>
typename result_for<T, value>::type
result_from_errno(int e, boost::source_location const* loc) noexcept

}
}

namespace mine
{

class my_class;
...
template<class JsonValue>
boost::json::result_for<my_class, JsonValue>
tag_invoke(boost::json::try_value_to_tag<my_class>, const JsonValue& jv)
{
    BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
    if( !jv.is_null() )
        return boost::json::result_from_errno<my_class>(EINVAL, &loc);
    return my_class();
}

}

Exception Safety

Does not throw exceptions.

Template Parameters

Type	Description
`T`	The value type of returned `result`.

Return Value

boost::system::error_code with value() equal to e and category() equal to boost::system::generic_category().

Parameters

Name Description

Name	Description
`e`	The error value.
`loc`	The error location.

e

The error value.

loc

The error location.

result_from_errno