.. title:: clang-tidy - modernize-use-auto
modernize-use-auto
==================
This check is responsible for using the ``auto`` type specifier for variable
declarations to *improve code readability and maintainability*. 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 check will only replace iterator type-specifiers when all of the following
conditions are satisfied:
* The iterator is for one of the standard containers 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``.
New expressions
---------------
Frequently, when a pointer is declared and initialized with ``new``, the
pointee type is written twice: in the declaration type and in the
``new`` expression. In this case, the declaration type can be replaced with
``auto`` improving readability and maintainability.
.. code-block:: c++
TypeName *my_pointer = new TypeName(my_param);
// becomes
auto *my_pointer = new TypeName(my_param);
The check will also replace the declaration type in multiple declarations, if
the following conditions are satisfied:
* All declared variables have the same type (i.e. all of them are pointers to
the same type).
* All declared variables are initialized with a ``new`` expression.
* The types of all the new expressions are the same than the pointee of the
declaration type.
.. code-block:: c++
TypeName *my_first_pointer = new TypeName, *my_second_pointer = new TypeName;
// becomes
auto *my_first_pointer = new TypeName, *my_second_pointer = new TypeName;
Cast expressions
----------------
Frequently, when a variable is declared and initialized with a cast, the
variable type is written twice: in the declaration type and in the
cast expression. In this case, the declaration type can be replaced with
``auto`` improving readability and maintainability.
.. code-block:: c++
TypeName *my_pointer = static_cast<TypeName>(my_param);
// becomes
auto *my_pointer = static_cast<TypeName>(my_param);
The check handles ``static_cast``, ``dynamic_cast``, ``const_cast``,
``reinterpret_cast``, functional casts, C-style casts and function templates
that behave as casts, such as ``llvm::dyn_cast``, ``boost::lexical_cast`` and
``gsl::narrow_cast``. Calls to function templates are considered to behave as
casts if the first template argument is explicit and is a type, and the function
returns that type, or a pointer or reference to it.
Known Limitations
-----------------
* If the initializer is an explicit conversion constructor, the check will not
replace the type specifier even though it would be safe to do so.
* User-defined iterators are not handled at this time.
Options
-------
.. option:: MinTypeNameLength
If the option is set to non-zero (default `5`), the check will ignore type
names having a length less than the option value. The option affects
expressions only, not iterators.
Spaces between multi-lexeme type names (``long int``) are considered as one.
If the :option:`RemoveStars` option (see below) is set to `true`, then ``*s``
in the type are also counted as a part of the type name.
.. code-block:: c++
// MinTypeNameLength = 0, RemoveStars=0
int a = static_cast<int>(foo()); // ---> auto a = ...
// length(bool *) = 4
bool *b = new bool; // ---> auto *b = ...
unsigned c = static_cast<unsigned>(foo()); // ---> auto c = ...
// MinTypeNameLength = 5, RemoveStars=0
int a = static_cast<int>(foo()); // ---> int a = ...
bool b = static_cast<bool>(foo()); // ---> bool b = ...
bool *pb = static_cast<bool*>(foo()); // ---> bool *pb = ...
unsigned c = static_cast<unsigned>(foo()); // ---> auto c = ...
// length(long <on-or-more-spaces> int) = 8
long int d = static_cast<long int>(foo()); // ---> auto d = ...
// MinTypeNameLength = 5, RemoveStars=1
int a = static_cast<int>(foo()); // ---> int a = ...
// length(int * * ) = 5
int **pa = static_cast<int**>(foo()); // ---> auto pa = ...
bool b = static_cast<bool>(foo()); // ---> bool b = ...
bool *pb = static_cast<bool*>(foo()); // ---> auto pb = ...
unsigned c = static_cast<unsigned>(foo()); // ---> auto c = ...
long int d = static_cast<long int>(foo()); // ---> auto d = ...
.. option:: RemoveStars
If the option is set to `true` (default is `false`), the check will remove
stars from the non-typedef pointer types when replacing type names with
``auto``. Otherwise, the check will leave stars. For example:
.. code-block:: c++
TypeName *my_first_pointer = new TypeName, *my_second_pointer = new TypeName;
// RemoveStars = 0
auto *my_first_pointer = new TypeName, *my_second_pointer = new TypeName;
// RemoveStars = 1
auto my_first_pointer = new TypeName, my_second_pointer = new TypeName;
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