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.. index:: Use-Auto Transform
==================
Use-Auto Transform
==================
The Use-Auto Transform is responsible for using the ``auto`` type specifier for
variable declarations to *improve code readability and maintainability*. The
transform is enabled with the :option:`-use-auto` option of
:program:`clang-modernize`. For example:
.. code-block:: c++
std::vector<int>::iterator I = my_container.begin();
// transforms to:
auto I = my_container.begin();
The ``auto`` type specifier will only be introduced in situations where the
variable type matches the type of the initializer expression. In other words
``auto`` should deduce the same type that was originally spelled in the source.
However, not every situation should be transformed:
.. code-block:: c++
int val = 42;
InfoStruct &I = SomeObject.getInfo();
// Should not become:
auto val = 42;
auto &I = SomeObject.getInfo();
In this example using ``auto`` for builtins doesn't improve readability. In
other situations it makes the code less self-documenting impairing readability
and maintainability. As a result, ``auto`` is used only introduced in specific
situations described below.
Iterators
=========
Iterator type specifiers tend to be long and used frequently, especially in
loop constructs. Since the functions generating iterators have a common format,
the type specifier can be replaced without obscuring the meaning of code while
improving readability and maintainability.
.. code-block:: c++
for (std::vector<int>::iterator I = my_container.begin(),
E = my_container.end();
I != E; ++I) {
}
// becomes
for (auto I = my_container.begin(), E = my_container.end(); I != E; ++I) {
}
The transform will only replace iterator type-specifiers when all of the
following conditions are satisfied:
* The iterator is for one of the standard container in ``std`` namespace:
* ``array``
* ``deque``
* ``forward_list``
* ``list``
* ``vector``
* ``map``
* ``multimap``
* ``set``
* ``multiset``
* ``unordered_map``
* ``unordered_multimap``
* ``unordered_set``
* ``unordered_multiset``
* ``queue``
* ``priority_queue``
* ``stack``
* The iterator is one of the possible iterator types for standard containers:
* ``iterator``
* ``reverse_iterator``
* ``const_iterator``
* ``const_reverse_iterator``
* In addition to using iterator types directly, typedefs or other ways of
referring to those types are also allowed. However, implementation-specific
types for which a type like ``std::vector<int>::iterator`` is itself a
typedef will not be transformed. Consider the following examples:
.. code-block:: c++
// The following direct uses of iterator types will be transformed.
std::vector<int>::iterator I = MyVec.begin();
{
using namespace std;
list<int>::iterator I = MyList.begin();
}
// The type specifier for J would transform to auto since it's a typedef
// to a standard iterator type.
typedef std::map<int, std::string>::const_iterator map_iterator;
map_iterator J = MyMap.begin();
// The following implementation-specific iterator type for which
// std::vector<int>::iterator could be a typedef would not be transformed.
__gnu_cxx::__normal_iterator<int*, std::vector> K = MyVec.begin();
* The initializer for the variable being declared is not a braced initializer
list. Otherwise, use of ``auto`` would cause the type of the variable to be
deduced as``std::initializer_list``.
Known Limitations
=================
* If the initializer is an explicit conversion constructor, the transform will
not replace the type specifier even though it would be safe to do so.
* User-defined iterators are not handled at this time.
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