C++ Fixed Containers

Header-only C++20 library that provides containers with the following properties:

• Fixed-capacity, declared at compile-time, no dynamic allocations
• constexpr - can be used at both compile-time and runtime (including mutation)
• containers retain the properties of T (e.g. if T is trivially copyable, then so is FixedVector)
• no pointers stored (data layout is purely self-referential and can be serialized directly)
• instances can be used as non-type template parameters

Features

• The following table shows the available fixed containers and their equivalent std-containers. The fixed-container types have identical APIs to their std:: equivalents, so you can refer to the traditional C++ docs for how to use them.

  fixed-container	std-container equivalent
  FixedVector	std::vector
  FixedDeque	std::deque
  FixedList	std::list
  FixedQueue	std::queue
  FixedStack	std::stack
  FixedCircularDeque	std::deque API with Circular Buffer semantics
  FixedCircularQueue	std::queue API with Circular Buffer semantics
  FixedString	std::string
  FixedMap	std::map
  FixedSet	std::set
  FixedUnorderedMap	std::unordered_map
  FixedUnorderedSet	std::unordered_set
  EnumMap	std::map for enum keys only
  EnumSet	std::set for enum keys only
  EnumArray	std::array but with typed accessors

• StringLiteral - Compile-time null-terminated literal string.

• Rich enums - enum & class hybrid.

Rich enum features

• Rich enums behave like an enum (compile-time known values, can be used in switch-statements, template parameters as well as EnumMap/EnumSet etc).
• Can have member functions and fields.
• Readily available count(), to_string().
• Conversion from string, ordinal.
• Implicit std::optional-like semantics.
• Avoid the need for error-prone sentinel values like UNKNOWN, UNINITIALIZED, COUNT etc.
• Avoid Undefined Behavior from uninitialized state. Default constructor can be disabled altogether.
• EnumAdapter<T> can adapt any enum-like class to the rich enum API.

static_assert(fixed_containers::rich_enums::is_rich_enum<Color>);  // Type-trait `concept`
inline constexpr const Color& COLOR = Color::RED();                // Note the parens
static_assert("RED" == COLOR.to_string());                         // auto-provided member
static_assert(COLOR.is_primary());                                 // Custom member
static_assert(COLOR == Color::value_of("RED").value());            // auto-provided
static_assert(4 == Color::count());                                // auto-provided

More examples can be found here.

Examples

• FixedVector

  constexpr auto v1 = []()
  {
      FixedVector<int, 11> v{};
      v.push_back(0);
      v.emplace_back(1);
      v.push_back(2);
      return v;
  }();
  static_assert(v1[0] == 0);
  static_assert(v1[1] == 1);
  static_assert(v1[2] == 2);
  static_assert(v1.size() == 3);
  static_assert(v1.capacity() == 11);

• FixedList

  constexpr auto v1 = []()
  {
      FixedList<int, 11> v{};
      v.push_back(0);
      v.emplace_back(1);
      v.push_front(2);
      return v;
  }();
  static_assert(*v1.begin() == 2);
  static_assert(v1.size() == 3);
  static_assert(v1.max_size() == 11);

• FixedDeque

  constexpr auto v1 = []()
  {
      FixedDeque<int, 11> v{};
      v.push_back(0);
      v.emplace_back(1);
      v.push_front(2);
      return v;
  }();
  static_assert(v1[0] == 2);
  static_assert(v1[1] == 0);
  static_assert(v1[2] == 1);
  static_assert(v1.size() == 3);
  static_assert(v1.max_size() == 11);

• FixedQueue

  constexpr auto s1 = []()
  {
      FixedQueue<int, 3> v1{};
      v1.push(77);
      v1.push(88);
      v1.push(99);
      return v1;
  }();
  
  static_assert(s1.front() == 77);
  static_assert(s1.back() == 99);
  static_assert(s1.size() == 3);

• FixedStack

  constexpr auto s1 = []()
  {
      FixedStack<int, 3> v1{};
      int my_int = 77;
      v1.push(my_int);
      v1.push(99);
      return v1;
  }();
  
  static_assert(s1.top() == 99);
  static_assert(s1.size() == 2);

• FixedCircularDeque

  constexpr auto v1 = []()
  {
      FixedCircularDeque<int, 3> v{};
      v.push_back(2);
      v.emplace_back(3);
      v.push_front(1);
      v.emplace_front(0);
      v.push_back(4);
      return v;
  }();
  
  static_assert(v1[0] == 1);
  static_assert(v1[1] == 2);
  static_assert(v1[2] == 4);
  static_assert(v1.size() == 3);

• FixedCircularQueue

  constexpr auto s1 = []()
  {
      FixedCircularQueue<int, 3> v1{};
      v1.push(55);
      v1.push(66);
      v1.push(77);
      v1.push(88);
      v1.push(99);
      return v1;
  }();
  
  static_assert(s1.front() == 77);
  static_assert(s1.back() == 99);
  static_assert(s1.size() == 3);

• FixedString

  constexpr auto v1 = []()
  {
      FixedString<11> v{"012"};
      v.at(1) = 'b';
  
      return v;
  }();
  
  static_assert(v1.at(0) == '0');
  static_assert(v1.at(1) == 'b');
  static_assert(v1.at(2) == '2');
  static_assert(v1.size() == 3);

• FixedMap

  constexpr auto m1 = []()
  {
      FixedMap<int, int, 11> m{};
      m.insert({2, 20});
      m[4] = 40;
      return m;
  }();
  static_assert(!m1.contains(1));
  static_assert(m1.contains(2));
  static_assert(m1.at(4) == 40);
  static_assert(m1.size() == 2);
  static_assert(m1.max_size() == 11);

• FixedSet

  constexpr auto s1 = []()
  {
      FixedSet<int, 11> s{};
      s.insert(2);
      s.insert(4);
      return s;
  }();
  static_assert(!s1.contains(1));
  static_assert(s1.contains(2));
  static_assert(s1.size() == 2);
  static_assert(s1.max_size() == 11);

• FixedUnorderedMap

  constexpr auto m1 = []()
  {
      FixedUnorderedMap<int, int, 11> m{};
      m.insert({2, 20});
      m[4] = 40;
      return m;
  }();
  static_assert(!m1.contains(1));
  static_assert(m1.contains(2));
  static_assert(m1.at(4) == 40);
  static_assert(m1.size() == 2);
  static_assert(m1.max_size() == 11);

• FixedUnorderedSet

  constexpr auto s1 = []()
  {
      FixedUnorderedSet<int, 11> s{};
      s.insert(2);
      s.insert(4);
      return s;
  }();
  static_assert(!s1.contains(1));
  static_assert(s1.contains(2));
  static_assert(s1.size() == 2);
  static_assert(s1.max_size() == 11);

• EnumMap

  enum class Color { RED, YELLOW, BLUE};
  
  constexpr auto m1 = []()
  {
      EnumMap<Color, int> m{};
      m.insert({Color::RED, 20});
      m[Color::YELLOW] = 40;
      return m;
  }();
  static_assert(!m1.contains(Color::BLUE));
  static_assert(m1.contains(Color::RED));
  static_assert(m1.at(Color::YELLOW) == 40);
  static_assert(m1.size() == 2);
  
  // Ensures all keys are specified, at compile-time
  constexpr auto m2 = EnumMap<Color, int>::create_with_all_entries({
      {Color::RED, 42},
      {Color::YELLOW, 7},
      {Color::BLUE, 6},
  });

• EnumSet

  enum class Color { RED, YELLOW, BLUE};
  
  constexpr auto s1 = []()
  {
      EnumSet<Color> s{};
      s.insert(Color::RED);
      s.insert(Color::YELLOW);
      return s;
  }();
  static_assert(!s1.contains(Color::BLUE));
  static_assert(s1.contains(Color::RED));
  static_assert(s1.size() == 2);
  
  constexpr auto s2 = EnumSet<Color>::all(); // full set
  constexpr auto s3 = EnumSet<Color>::none(); // empty set
  constexpr auto s4 = EnumSet<Color>::complement_of(s2); // empty set

• EnumArray

  constexpr EnumArray<TestEnum1, int> s1{{TestEnum1::ONE, 10}, {TestEnum1::FOUR, 40}};
  static_assert(4 == s1.max_size());
  static_assert(s1.at(TestEnum1::ONE) == 10);
  static_assert(s1.at(TestEnum1::TWO) == 0);
  static_assert(s1.at(TestEnum1::THREE) == 0);
  static_assert(s1.at(TestEnum1::FOUR) == 40);

• StringLiteral

  static constexpr const char* s = "blah"; // strlen==4, sizeof==8
  static constexpr const char s[5] = "blah";  // strlen==4, sizeof==5 (null terminator)
  static constexpr StringLiteral s = "blah";  // constexpr .size()==4

• Using instances as non-type template parameters

  // Similarly to simple types like ints/enums and std::array,
  // fixed_container instances can be used as template parameters
  template <FixedVector<int, 5> /*MY_VEC*/>
  constexpr void fixed_vector_instance_can_be_used_as_a_template_parameter()
  {
  }
  
  void test()
  {
      static constexpr FixedVector<int, 5> VEC1{};
      fixed_vector_instance_can_be_used_as_a_template_parameter<VEC1>();
  }

Integration

cmake

find_package(fixed_containers CONFIG REQUIRED)
target_link_libraries(<your_binary> fixed_containers::fixed_containers)

bazel

If you are managing dependencies with the newer bzlmod system, use the following in your MODULE.bazel file:

bazel_dep(name = "fixed_containers")
archive_override(
    module_name = "fixed_containers",
    strip_prefix = "fixed-containers-<commit>",
    urls = ["https://github.com/teslamotors/fixed-containers/archive/<commit>.tar.gz"],
)

If you are managing dependencies with the older WORKSPACE system, use the following in your WORKSPACE file:

http_archive(
    name = "fixed_containers",
    urls = ["https://github.com/teslamotors/fixed-containers/archive/<commit>.tar.gz"],
    strip_prefix = "fixed-containers-<commit>",
)

load("@fixed_containers//:fixed_containers_deps.bzl", "fixed_containers_deps")
fixed_containers_deps()

Then use in your targets like this:

cc_test(
    name = "test",
    srcs = ["test.cpp"],
    deps = [
        "@fixed_containers//:fixed_vector",
        "@fixed_containers//:enum_map",
        "@fixed_containers//:enum_set",
    ],
    copts = ["-std=c++20"],
)

Alternative

Since this is a header-only library, you can also:

• Add the include/ folder to your includes
• Get the dependencies. For example, with vcpkg:

vcpkg install magic-enum

Running the tests

cmake

1. Build with the vcpkg toolchain file

mkdir build && cd build
cmake .. -DCMAKE_C_COMPILER=/bin/clang-13 -DCMAKE_CXX_COMPILER=/bin/clang++-13 -DCMAKE_TOOLCHAIN_FILE=/path/to/vcpkg/scripts/buildsystems/vcpkg.cmake
cmake --build .

2. Run tests

ctest -C Debug

bazel

clang

1. Build separately (optional)

CC=clang++-13 bazel build --config=clang ...

2. Run tests

CC=clang++-13 bazel test --config=clang :all_tests

gcc

1. Build separately (optional)

CC=g++-11 bazel build --config=gcc ...

2. Run tests

CC=g++-11 bazel test --config=gcc :all_tests

Tested Compilers

• Clang 13
• GCC 11
• MSVC++ 14.29 / Visual Studio 2019

Licensed under the MIT License