.. title:: clang-tidy - modernize-use-emplace
modernize-use-emplace
=====================
The check flags insertions to an STL-style container done by calling the
``push_back``, ``push``, or ``push_front`` methods with an
explicitly-constructed temporary of the container element type. In this case,
the corresponding ``emplace`` equivalent methods result in less verbose and
potentially more efficient code. Right now the check doesn't support
``insert``. It also doesn't support ``insert`` functions for associative
containers because replacing ``insert`` with ``emplace`` may result in
`speed regression <https://htmlpreview.github.io/?https://github.com/HowardHinnant/papers/blob/master/insert_vs_emplace.html>`_, but it might get support with some addition flag in the future.
The :option:`ContainersWithPushBack`, :option:`ContainersWithPush`, and
:option:`ContainersWithPushFront` options are used to specify the container
types that support the ``push_back``, ``push``, and ``push_front`` operations
respectively. The default values for these options are as follows:
* :option:`ContainersWithPushBack`: ``std::vector``, ``std::deque``, and ``std::list``.
* :option:`ContainersWithPush`: ``std::stack``, ``std::queue``, and ``std::priority_queue``.
* :option:`ContainersWithPushFront`: ``std::forward_list``, ``std::list``, and ``std::deque``.
This check also reports when an ``emplace``-like method is improperly used,
for example using ``emplace_back`` while also calling a constructor. This
creates a temporary that requires at best a move and at worst a copy. Almost all
``emplace``-like functions in the STL are covered by this, with ``try_emplace``
on ``std::map`` and ``std::unordered_map`` being the exception as it behaves
slightly differently than all the others. More containers can be added with the
:option:`EmplacyFunctions` option, so long as the container defines a
``value_type`` type, and the ``emplace``-like functions construct a
``value_type`` object.
Before:
.. code-block:: c++
std::vector<MyClass> v;
v.push_back(MyClass(21, 37));
v.emplace_back(MyClass(21, 37));
std::vector<std::pair<int, int>> w;
w.push_back(std::pair<int, int>(21, 37));
w.push_back(std::make_pair(21L, 37L));
w.emplace_back(std::make_pair(21L, 37L));
After:
.. code-block:: c++
std::vector<MyClass> v;
v.emplace_back(21, 37);
v.emplace_back(21, 37);
std::vector<std::pair<int, int>> w;
w.emplace_back(21, 37);
w.emplace_back(21L, 37L);
w.emplace_back(21L, 37L);
By default, the check is able to remove unnecessary ``std::make_pair`` and
``std::make_tuple`` calls from ``push_back`` calls on containers of
``std::pair`` and ``std::tuple``. Custom tuple-like types can be modified by
the :option:`TupleTypes` option; custom make functions can be modified by the
:option:`TupleMakeFunctions` option.
The other situation is when we pass arguments that will be converted to a type
inside a container.
Before:
.. code-block:: c++
std::vector<boost::optional<std::string> > v;
v.push_back("abc");
After:
.. code-block:: c++
std::vector<boost::optional<std::string> > v;
v.emplace_back("abc");
In some cases the transformation would be valid, but the code wouldn't be
exception safe. In this case the calls of ``push_back`` won't be replaced.
.. code-block:: c++
std::vector<std::unique_ptr<int>> v;
v.push_back(std::unique_ptr<int>(new int(0)));
auto *ptr = new int(1);
v.push_back(std::unique_ptr<int>(ptr));
This is because replacing it with ``emplace_back`` could cause a leak of this
pointer if ``emplace_back`` would throw exception before emplacement (e.g. not
enough memory to add a new element).
For more info read item 42 - "Consider emplacement instead of insertion." of
Scott Meyers "Effective Modern C++".
The default smart pointers that are considered are ``std::unique_ptr``,
``std::shared_ptr``, ``std::auto_ptr``. To specify other smart pointers or
other classes use the :option:`SmartPointers` option.
Check also doesn't fire if any argument of the constructor call would be:
- a bit-field (bit-fields can't bind to rvalue/universal reference)
- a ``new`` expression (to avoid leak)
- if the argument would be converted via derived-to-base cast.
This check requires C++11 or higher to run.
Options
-------
.. option:: ContainersWithPushBack
Semicolon-separated list of class names of custom containers that support
``push_back``.
.. option:: ContainersWithPush
Semicolon-separated list of class names of custom containers that support
``push``.
.. option:: ContainersWithPushFront
Semicolon-separated list of class names of custom containers that support
``push_front``.
.. option:: IgnoreImplicitConstructors
When `true`, the check will ignore implicitly constructed arguments of
``push_back``, e.g.
.. code-block:: c++
std::vector<std::string> v;
v.push_back("a"); // Ignored when IgnoreImplicitConstructors is `true`.
Default is `false`.
.. option:: SmartPointers
Semicolon-separated list of class names of custom smart pointers.
.. option:: TupleTypes
Semicolon-separated list of ``std::tuple``-like class names.
.. option:: TupleMakeFunctions
Semicolon-separated list of ``std::make_tuple``-like function names. Those
function calls will be removed from ``push_back`` calls and turned into
``emplace_back``.
.. option:: EmplacyFunctions
Semicolon-separated list of containers without their template parameters
and some ``emplace``-like method of the container. Example:
``vector::emplace_back``. Those methods will be checked for improper use and
the check will report when a temporary is unnecessarily created.
Example
^^^^^^^
.. code-block:: c++
std::vector<MyTuple<int, bool, char>> x;
x.push_back(MakeMyTuple(1, false, 'x'));
x.emplace_back(MakeMyTuple(1, false, 'x'));
transforms to:
.. code-block:: c++
std::vector<MyTuple<int, bool, char>> x;
x.emplace_back(1, false, 'x');
x.emplace_back(1, false, 'x');
when :option:`TupleTypes` is set to ``MyTuple``, :option:`TupleMakeFunctions`
is set to ``MakeMyTuple``, and :option:`EmplacyFunctions` is set to
``vector::emplace_back``.
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