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Support moving from rvalues in an std::initializer_list

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This commit works around an issue in std::initializer_list design.
By using a detail::json_ref proxy with a mutable value inside,
rvalue-ness of an input to list initializer is remembered and
used later to move from the proxy instead of copying.
This commit is contained in:
Nikita Ofitserov 2017-07-23 23:57:17 +03:00
parent cf3ca3b78c
commit 09cda57309
1 changed files with 95 additions and 34 deletions
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129
src/json.hpp
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@ -7081,6 +7081,60 @@ class serializer
/// the indentation string
string_t indent_string;
};
template<typename BasicJsonType>
struct json_ref
{
typedef BasicJsonType value_type;
json_ref(value_type&& value)
: value_(std::move(value))
, is_rvalue_(true)
{}
json_ref(const value_type& value)
: value_(value)
, is_rvalue_(false)
{}
json_ref(std::initializer_list<json_ref> init)
: value_(init)
, is_rvalue_(true)
{}
template <class... Args>
json_ref(Args... args)
: value_(std::forward<Args>(args)...)
, is_rvalue_(true)
{}
value_type moved() const
{
if (is_rvalue_)
{
return std::move(value_);
}
else
{
return value_;
}
}
value_type const& operator*() const
{
return value_;
}
value_type const* operator->() const
{
return &value_;
}
private:
mutable value_type value_;
bool is_rvalue_;
};
} // namespace detail
/// namespace to hold default `to_json` / `from_json` functions
@ -7583,6 +7637,7 @@ class basic_json
template<typename T, typename SFINAE>
using json_serializer = JSONSerializer<T, SFINAE>;
using initializer_list_t = std::initializer_list<detail::json_ref<basic_json>>;
////////////////
// exceptions //
@ -8614,10 +8669,10 @@ class basic_json
With the rules described above, the following JSON values cannot be
expressed by an initializer list:
- the empty array (`[]`): use @ref array(std::initializer_list<basic_json>)
- the empty array (`[]`): use @ref array(initializer_list_t)
with an empty initializer list in this case
- arrays whose elements satisfy rule 2: use @ref
array(std::initializer_list<basic_json>) with the same initializer list
array(initializer_list_t) with the same initializer list
in this case
@note When used without parentheses around an empty initializer list, @ref
@ -8629,8 +8684,8 @@ class basic_json
@param[in] type_deduction internal parameter; when set to `true`, the type
of the JSON value is deducted from the initializer list @a init; when set
to `false`, the type provided via @a manual_type is forced. This mode is
used by the functions @ref array(std::initializer_list<basic_json>) and
@ref object(std::initializer_list<basic_json>).
used by the functions @ref array(initializer_list_t) and
@ref object(initializer_list_t).
@param[in] manual_type internal parameter; when @a type_deduction is set
to `false`, the created JSON value will use the provided type (only @ref
@ -8641,7 +8696,7 @@ class basic_json
`value_t::object`, but @a init contains an element which is not a pair
whose first element is a string. In this case, the constructor could not
create an object. If @a type_deduction would have be `true`, an array
would have been created. See @ref object(std::initializer_list<basic_json>)
would have been created. See @ref object(initializer_list_t)
for an example.
@complexity Linear in the size of the initializer list @a init.
@ -8649,23 +8704,23 @@ class basic_json
@liveexample{The example below shows how JSON values are created from
initializer lists.,basic_json__list_init_t}
@sa @ref array(std::initializer_list<basic_json>) -- create a JSON array
@sa @ref array(initializer_list_t) -- create a JSON array
value from an initializer list
@sa @ref object(std::initializer_list<basic_json>) -- create a JSON object
@sa @ref object(initializer_list_t) -- create a JSON object
value from an initializer list
@since version 1.0.0
*/
basic_json(std::initializer_list<basic_json> init,
basic_json(initializer_list_t init,
bool type_deduction = true,
value_t manual_type = value_t::array)
{
// check if each element is an array with two elements whose first
// element is a string
bool is_an_object = std::all_of(init.begin(), init.end(),
[](const basic_json & element)
[](const detail::json_ref<basic_json>& element_ref)
{
return element.is_array() and element.size() == 2 and element[0].is_string();
return element_ref->is_array() and element_ref->size() == 2 and (*element_ref)[0].is_string();
});
// adjust type if type deduction is not wanted
@ -8690,16 +8745,19 @@ class basic_json
m_type = value_t::object;
m_value = value_t::object;
std::for_each(init.begin(), init.end(), [this](const basic_json & element)
std::for_each(init.begin(), init.end(), [this](const detail::json_ref<basic_json>& element_ref)
{
m_value.object->emplace(*(element[0].m_value.string), element[1]);
basic_json element = element_ref.moved();
m_value.object->emplace(
std::move(*((*element.m_value.array)[0].m_value.string)),
std::move((*element.m_value.array)[1]));
});
}
else
{
// the initializer list describes an array -> create array
m_type = value_t::array;
m_value.array = create<array_t>(init);
m_value.array = create<array_t>(init.begin(), init.end());
}
assert_invariant();
@ -8714,7 +8772,7 @@ class basic_json
@note This function is only needed to express two edge cases that cannot
be realized with the initializer list constructor (@ref
basic_json(std::initializer_list<basic_json>, bool, value_t)). These cases
basic_json(initializer_list_t, bool, value_t)). These cases
are:
1. creating an array whose elements are all pairs whose first element is a
string -- in this case, the initializer list constructor would create an
@ -8732,15 +8790,14 @@ class basic_json
@liveexample{The following code shows an example for the `array`
function.,array}
@sa @ref basic_json(std::initializer_list<basic_json>, bool, value_t) --
@sa @ref basic_json(initializer_list_t, bool, value_t) --
create a JSON value from an initializer list
@sa @ref object(std::initializer_list<basic_json>) -- create a JSON object
@sa @ref object(initializer_list_t) -- create a JSON object
value from an initializer list
@since version 1.0.0
*/
static basic_json array(std::initializer_list<basic_json> init =
std::initializer_list<basic_json>())
static basic_json array(initializer_list_t init = {})
{
return basic_json(init, false, value_t::array);
}
@ -8753,10 +8810,10 @@ class basic_json
the initializer list is empty, the empty object `{}` is created.
@note This function is only added for symmetry reasons. In contrast to the
related function @ref array(std::initializer_list<basic_json>), there are
related function @ref array(initializer_list_t), there are
no cases which can only be expressed by this function. That is, any
initializer list @a init can also be passed to the initializer list
constructor @ref basic_json(std::initializer_list<basic_json>, bool, value_t).
constructor @ref basic_json(initializer_list_t, bool, value_t).
@param[in] init initializer list to create an object from (optional)
@ -8764,7 +8821,7 @@ class basic_json
@throw type_error.301 if @a init is not a list of pairs whose first
elements are strings. In this case, no object can be created. When such a
value is passed to @ref basic_json(std::initializer_list<basic_json>, bool, value_t),
value is passed to @ref basic_json(initializer_list_t, bool, value_t),
an array would have been created from the passed initializer list @a init.
See example below.
@ -8773,15 +8830,14 @@ class basic_json
@liveexample{The following code shows an example for the `object`
function.,object}
@sa @ref basic_json(std::initializer_list<basic_json>, bool, value_t) --
@sa @ref basic_json(initializer_list_t, bool, value_t) --
create a JSON value from an initializer list
@sa @ref array(std::initializer_list<basic_json>) -- create a JSON array
@sa @ref array(initializer_list_t) -- create a JSON array
value from an initializer list
@since version 1.0.0
*/
static basic_json object(std::initializer_list<basic_json> init =
std::initializer_list<basic_json>())
static basic_json object(initializer_list_t init = {})
{
return basic_json(init, false, value_t::object);
}
@ -8953,6 +9009,11 @@ class basic_json
// other constructors and destructor //
///////////////////////////////////////
basic_json(const detail::json_ref<basic_json>& ref)
: basic_json(ref.moved())
{
}
/*!
@brief copy constructor
@ -11755,7 +11816,7 @@ class basic_json
@brief add an object to an array
@copydoc push_back(basic_json&&)
*/
reference operator+=(basic_json&& val)
reference operator+=(basic_json && val)
{
push_back(std::move(val));
return *this;
@ -11870,12 +11931,12 @@ class basic_json
@liveexample{The example shows how initializer lists are treated as
objects when possible.,push_back__initializer_list}
*/
void push_back(std::initializer_list<basic_json> init)
void push_back(initializer_list_t init)
{
if (is_object() and init.size() == 2 and init.begin()->is_string())
if (is_object() and init.size() == 2 and (*init.begin())->is_string())
{
const string_t key = *init.begin();
push_back(typename object_t::value_type(key, *(init.begin() + 1)));
basic_json&& key = init.begin()->moved();
push_back(typename object_t::value_type(std::move(key.get_ref<string_t&>()), (init.begin() + 1)->moved()));
}
else
{
@ -11885,9 +11946,9 @@ class basic_json
/*!
@brief add an object to an object
@copydoc push_back(std::initializer_list<basic_json>)
@copydoc push_back(initializer_list_t)
*/
reference operator+=(std::initializer_list<basic_json> init)
reference operator+=(initializer_list_t init)
{
push_back(init);
return *this;
@ -12172,7 +12233,7 @@ class basic_json
@since version 1.0.0
*/
iterator insert(const_iterator pos, std::initializer_list<basic_json> ilist)
iterator insert(const_iterator pos, initializer_list_t ilist)
{
// insert only works for arrays
if (not is_array())
@ -12188,7 +12249,7 @@ class basic_json
// insert to array and return iterator
iterator result(this);
result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, ilist);
result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, ilist.begin(), ilist.end());
return result;
}