1.. title:: clang-tidy - performance-inefficient-vector-operation
2
3performance-inefficient-vector-operation
4========================================
5
6Finds possible inefficient ``std::vector`` operations (e.g. ``push_back``,
7``emplace_back``) that may cause unnecessary memory reallocations.
8
9It can also find calls that add element to protobuf repeated field in a loop
10without calling Reserve() before the loop. Calling Reserve() first can avoid
11unnecessary memory reallocations.
12
13Currently, the check only detects following kinds of loops with a single
14statement body:
15
16* Counter-based for loops start with 0:
17
18.. code-block:: c++
19
20  std::vector<int> v;
21  for (int i = 0; i < n; ++i) {
22    v.push_back(n);
23    // This will trigger the warning since the push_back may cause multiple
24    // memory reallocations in v. This can be avoid by inserting a 'reserve(n)'
25    // statement before the for statement.
26  }
27
28  SomeProto p;
29  for (int i = 0; i < n; ++i) {
30    p.add_xxx(n);
31    // This will trigger the warning since the add_xxx may cause multiple memory
32    // reallocations. This can be avoid by inserting a
33    // 'p.mutable_xxx().Reserve(n)' statement before the for statement.
34  }
35
36* For-range loops like ``for (range-declaration : range_expression)``, the type
37  of ``range_expression`` can be ``std::vector``, ``std::array``,
38  ``std::deque``, ``std::set``, ``std::unordered_set``, ``std::map``,
39  ``std::unordered_set``:
40
41.. code-block:: c++
42
43  std::vector<int> data;
44  std::vector<int> v;
45
46  for (auto element : data) {
47    v.push_back(element);
48    // This will trigger the warning since the 'push_back' may cause multiple
49    // memory reallocations in v. This can be avoid by inserting a
50    // 'reserve(data.size())' statement before the for statement.
51  }
52
53
54Options
55-------
56
57.. option:: VectorLikeClasses
58
59   Semicolon-separated list of names of vector-like classes. By default only
60   ``::std::vector`` is considered.
61
62.. option:: EnableProto
63
64   When `true`, the check will also warn on inefficient operations for proto
65   repeated fields. Otherwise, the check only warns on inefficient vector
66   operations. Default is `false`.
67