value::try_as

`value::try_as_null`

Return engaged result if the value is null.

boost::system::result< std::nullptr_t >
try_as_null() const noexcept;

If is_null() is true, the result is engaged, otherwise it contains an error_code.

The return value can be used in both a boolean context and to assign a variable:

if( auto r = jv.try_as_null() )
    return *r;

But can also be used to throw an exception on error:

return jv.try_as_null().value();

Constant.

No-throw guarantee.