Merge pull request #65 from bemanproject/constexpr_example

Make sure constexpr functions are working under constexpr environment

Author: wusatosi

README.md

@@ -4,16 +4,14 @@ SPDX-License-Identifier: <SPDX License Expression>
 
 # beman.inplace\_vector: Dynamically-resizable vector with fixed capacity
 
-<!-- markdownlint-disable -->
-<img src="https://github.com/bemanproject/beman/blob/main/images/logos/beman_logo-beman_library_under_development.png" style="width:5%; height:auto;"> ![Continuous Integration Tests](https://github.com/bemanproject/inplace_vector/actions/workflows/ci_tests.yml/badge.svg)
+![Library Status](https://github.com/bemanproject/beman/blob/c6997986557ec6dda98acbdf502082cdf7335526/images/badges/beman_badge-beman_library_under_development.svg)
+![Continuous Integration Tests](https://github.com/bemanproject/inplace_vector/actions/workflows/ci_tests.yml/badge.svg)
 ![Code Format](https://github.com/bemanproject/inplace_vector/actions/workflows/pre-commit.yml/badge.svg)
-<!-- markdownlint-enable -->
 
 **Implements**: [`inplace_vector` (P0843R14)](https://wg21.link/P0843R14)
 
-<!-- markdownlint-disable -->
-**Status**: [Under development and not yet ready for production use.](https://github.com/bemanproject/beman/blob/main/docs/BEMAN_LIBRARY_MATURITY_MODEL.md#under-development-and-not-yet-ready-for-production-use)
-<!-- markdownlint-enable -->
+**Status**:
+[Under development and not yet ready for production use.](https://github.com/bemanproject/beman/blob/main/docs/BEMAN_LIBRARY_MATURITY_MODEL.md#under-development-and-not-yet-ready-for-production-use)
 
 ## Usage
 
@@ -30,13 +28,16 @@ which dynamic memory allocations are undesired.
 #### Note on implementation progress
 
 Current implementation implements all public interfaces defined in the paper.
-However constexpr related functionalities are not tested and maybe broken.
 
-There have been minor updates to the wording after the paper is accepted, notably [P3247](wg21.link/P3247).
+There have been minor updates to the wording after the paper is accepted,
+notably [P3247](wg21.link/P3247).
 Which changes the requirements for constexpr support.
 This will likely be preceded with [P3074](wg21.link/P3074).
 These has not been implemented yet.
 
+You can follow [this link](eel.is/c++draft/inplace.vector)
+to checkout the status of `inplace_vector` in the latest draft.
+
 Contributions are welcome.
 
 ### Code example
@@ -53,21 +54,46 @@ using namespace beman;
  * Generates fibonacci sequence using inplace_vector.
  * See: https://en.wikipedia.org/wiki/Fibonacci_sequence
  */
-template <int Capacity> inplace_vector<int, Capacity> fibonacci_to(int num) {
+template <int Capacity>
+constexpr inplace_vector<int, Capacity> fibonacci_to(int num) {
   assert(num < Capacity);
 
   inplace_vector<int, Capacity> vec;
 
-  constexpr static std::array<int, 2> first_two{0, 1};
+  constexpr std::array<int, 2> first_two{0, 1};
   for (auto i = 0; i <= num; ++i) {
     auto new_val = i < 2 ? first_two[i] : vec[i - 1] + vec[i - 2];
     vec.push_back(new_val);
   }
 
   return vec;
 }
+
+/*
+ * Check the result of the computation at compile time.
+ */
+constexpr bool check_5() {
+  auto got = fibonacci_to<10>(5);
+  constexpr inplace_vector<int, 10> correct{0, 1, 1, 2, 3, 5};
+  return got == correct;
+}
+
+static_assert(check_5());
+
 ```
 
+### Note on constexpr support
+
+Since `constexpr` requirements are actively changing,
+you can use `beman::has_constexpr_support` to detect if our implementation
+provide constexpr support for a specific specialization of `inplace_vector`.
+
+Note this is not part of the standard Library and should not be relied on once
+`constexpr` requirement stabilize.
+
+Example Usage:
+`static_assert(beman::has_constexpr_support<beman::inplace_vector<int, 5>>)`.
+
 ## How to Build
 
 ### Compiler support

examples/fibonacci.cpp

@@ -10,12 +10,13 @@ using namespace beman;
  * Generates fibonacci sequence using inplace_vector.
  * See: https://en.wikipedia.org/wiki/Fibonacci_sequence
  */
-template <int Capacity> inplace_vector<int, Capacity> fibonacci_to(int num) {
+template <int Capacity>
+constexpr inplace_vector<int, Capacity> fibonacci_to(int num) {
   assert(num < Capacity);
 
   inplace_vector<int, Capacity> vec;
 
-  constexpr static std::array<int, 2> first_two{0, 1};
+  constexpr std::array<int, 2> first_two{0, 1};
   for (auto i = 0; i <= num; ++i) {
     auto new_val = i < 2 ? first_two[i] : vec[i - 1] + vec[i - 2];
     vec.push_back(new_val);
@@ -24,6 +25,17 @@ template <int Capacity> inplace_vector<int, Capacity> fibonacci_to(int num) {
   return vec;
 }
 
+/*
+ * Check the result of the computation at compile time.
+ */
+constexpr bool check_5() {
+  auto got = fibonacci_to<10>(5);
+  constexpr inplace_vector<int, 10> correct{0, 1, 1, 2, 3, 5};
+  return got == correct;
+}
+
+static_assert(check_5());
+
 /**
  * Expected program output:
  *

include/beman/inplace_vector/inplace_vector.hpp

@@ -306,6 +306,9 @@ concept container_compatible_range =
     std::ranges::input_range<Rng> &&
     std::convertible_to<std::ranges::range_reference_t<Rng>, T>;
 
+template <typename T, std::size_t N>
+concept satify_constexpr = N == 0 || std::is_trivial_v<T>;
+
 } // namespace beman::details::inplace_vector
 
 // Types implementing the `inplace_vector`'s storage
@@ -425,13 +428,18 @@ template <class T, size_t N> struct non_trivial {
 // Selects the vector storage.
 template <class T, size_t N>
 using storage_for = std::conditional_t<
-    N == 0, zero_sized<T>,
-    std::conditional_t<std::is_trivial_v<T>, trivial<T, N>, non_trivial<T, N>>>;
+    !satify_constexpr<T, N>, non_trivial<T, N>,
+    std::conditional_t<N == 0, zero_sized<T>, trivial<T, N>>>;
 
 } // namespace beman::details::inplace_vector::storage
 
 namespace beman {
 
+template <typename IV>
+concept has_constexpr_support =
+    details::inplace_vector::satify_constexpr<typename IV::value_type,
+                                              IV::capacity()>;
+
 /// Dynamically-resizable fixed-N vector with inplace storage.
 template <class T, size_t N>
 struct inplace_vector

tests/beman/inplace_vector/CMakeLists.txt

@@ -40,3 +40,14 @@ add_gtest(triviality)
 add_gtest(constructors)
 add_gtest(size_n_data)
 add_gtest(erasure)
+
+# constexpr test
+add_executable(beman.inplace_vector.tests.constexpr constexpr.test.cpp)
+target_link_libraries(
+    beman.inplace_vector.tests.constexpr
+    PRIVATE beman.inplace_vector
+)
+add_test(
+    NAME beman.inplace_vector.tests.constexpr
+    COMMAND beman.inplace_vector.tests.constexpr
+)

tests/beman/inplace_vector/README.md

@@ -50,7 +50,12 @@ Not tested for now.
 > then no `inplace_vector<T, N>` member functions are usable in
 > constant expressions.
 
-Not tested for now.
+See [constexpr.test.cpp](constexpr.test.cpp),
+note that this test suite only ensure functions are usable in a constexpr
+environment.
+The validity of those functions are tested in the main test suite.
+This test also doesn't exhaustivly test constexpr functions' behavior
+when exception throwing is expected.
 
 #### 6.5 Bad alloc requirement
 
@@ -99,7 +104,6 @@ See [erasure.test.cpp](erasure.test.cpp)
 
 ## Known Issues/ Missed Tests
 
-- Constexpr related functionalities.
 - Emplacement minimal copy/ construction.
 - Exception safety on mutation.