Extra Clang Tools 3.8 documentation

Use-Auto Transform

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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 -use-auto option of clang-modernize. For example:

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:

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.


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.

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:
// 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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