feat(cpp14): add 01-relaxed-constexpr exercises and solutions

Author: lczllx

book/en/src/cpp14/01-relaxed-constexpr.md

@@ -118,7 +118,51 @@ static_assert(fib(10) == 55, "");
 static_assert(fib(0) == 0, "");
 ```
 
-## II. Notes
+## II. Real-World Case — Compile-Time Index Sequences in the STL
+
+> C++14 introduced `std::integer_sequence` and `std::make_index_sequence`, which rely on relaxed constexpr for compile-time integer generation. The examples below cite the vendored [MSVC STL](https://github.com/mcpp-community/d2mcpp/tree/main/msvc-stl) (source: [`msvc-stl/stl/inc/tuple`](https://github.com/mcpp-community/d2mcpp/blob/main/msvc-stl/stl/inc/tuple#L323-L343)); `_Tag` / `_Tpl` are internal tags and types and can be ignored while reading
+
+### std::tuple Compile-Time Construction — index_sequence Unfolds Parameter Packs
+
+`std::tuple` must extract elements one by one from a tuple-like object and construct its own members — this process must happen at compile time. MSVC STL uses `make_index_sequence` to generate compile-time indices and expand parameter packs:
+
+```cpp
+// MSVC STL · msvc-stl/stl/inc/tuple (abridged)
+template <size_t... _Indices, class _Tpl>
+constexpr tuple(_Tag, _Tpl&& _Right, index_sequence<_Indices...>)
+    : _Mybase(static_cast<_Tpl&&>(_Right)._Get_rest()) {
+    // _Indices... is a compile-time-generated sequence 0, 1, 2, ... N-1
+    // Elements are extracted via get<_Indices>(_Right) one by one
+}
+
+// Public constructor — uses make_index_sequence to auto-generate indices
+explicit(false) constexpr tuple(_Exact_args_t, _Tpl&& _Right)
+    : tuple(_Tag{}, _STD forward<_Tpl>(_Right),
+        make_index_sequence<tuple_size_v<remove_reference_t<_Tpl>>>{}) {}
+```
+
+`make_index_sequence<N>` generates `index_sequence<0, 1, ..., N-1>` at compile time, enabling the tuple constructor to extract elements via `get<0>` / `get<1>` / ... sequentially. This is the canonical post-C++14 constexpr pattern — iteration is not a runtime concept but achieved through compile-time integer sequences and pack expansion
+
+### std::integer_sequence — the C++14 Compile-Time Integer Carrier
+
+```cpp
+// MSVC STL · msvc-stl/stl/inc/utility (abridged)
+template <class _Ty, _Ty... _Vals>
+struct integer_sequence {
+    static_assert(is_integral_v<_Ty>,
+        "integer_sequence<T, I...> requires T to be an integral type.");
+};
+
+template <size_t... _Vals>
+using index_sequence = integer_sequence<size_t, _Vals...>;
+
+template <size_t _Size>
+using make_index_sequence = __make_integer_seq<integer_sequence, size_t, _Size>;
+```
+
+> Summary: Both `std::make_index_sequence` and `std::tuple`'s compile-time construction depend on the relaxed constexpr environment introduced in C++14. Without constexpr that supports loops and branches, the standard library would rely entirely on compiler internals for integer sequence generation rather than on the expressive power of the C++ language itself
+
+## III. Notes
 
 ### Operations Still Banned in C++14 constexpr
 
@@ -157,7 +201,7 @@ int main() {
 
 A constexpr specifier does not change the ODR linkage in C++14 (C++17 later made constexpr functions implicitly inline), nor does it mean every function that can be constexpr should be. If a function is almost always called at runtime, adding constexpr increases the interface constraint with little practical benefit
 
-## III. Exercise Code
+## IV. Exercise Code
 
 ### Exercise Code Topics
 
@@ -170,7 +214,7 @@ A constexpr specifier does not change the ODR linkage in C++14 (C++17 later made
 d2x checker relaxed-constexpr
 ```
 
-## IV. Other
+## V. Other
 
 - [Discussion Forum](https://forum.d2learn.org/category/20)
 - [d2mcpp Tutorial Repository](https://github.com/mcpp-community/d2mcpp)

book/src/cpp14/01-relaxed-constexpr.md

@@ -118,7 +118,51 @@ static_assert(fib(10) == 55, "");
 static_assert(fib(0) == 0, "");
 ```
 
-## 二、注意事项
+## 二、真实案例 - STL 中的 constexpr 编译期索引序列
+
+> C++14 同步引入了 `std::integer_sequence` 和 `std::make_index_sequence`, 它们正是依赖放宽后的 constexpr 才能实现的编译期工具。下面以仓库内置的 [MSVC STL](https://github.com/mcpp-community/d2mcpp/tree/main/msvc-stl) 为例 (源码: [`msvc-stl/stl/inc/tuple`](https://github.com/mcpp-community/d2mcpp/blob/main/msvc-stl/stl/inc/tuple#L323-L343)), `_Tag` / `_Tpl` 是库内部标记和类型, 阅读时可忽略
+
+### std::tuple 的编译期构造 — index_sequence 展开参数包
+
+`std::tuple` 需要从一个"类元组"对象中逐元素提取并构造自己的成员, 这个过程必须在编译期完成。MSVC STL 使用 `make_index_sequence` 生成编译期索引, 配合参数包展开实现:
+
+```cpp
+// MSVC STL · msvc-stl/stl/inc/tuple (有删节)
+template <size_t... _Indices, class _Tpl>
+constexpr tuple(_Tag, _Tpl&& _Right, index_sequence<_Indices...>)
+    : _Mybase(static_cast<_Tpl&&>(_Right)._Get_rest()) {
+    // _Indices... 是编译期生成的 0, 1, 2, ... N-1 索引序列
+    // 通过 get<_Indices>(_Right) 逐元素提取并构造
+}
+
+// 公开构造函数 — 用 make_index_sequence 自动生成索引
+explicit(false) constexpr tuple(_Exact_args_t, _Tpl&& _Right)
+    : tuple(_Tag{}, _STD forward<_Tpl>(_Right),
+        make_index_sequence<tuple_size_v<remove_reference_t<_Tpl>>>{}) {}
+```
+
+`make_index_sequence<N>` 在编译期生成 `index_sequence<0, 1, ..., N-1>`, 让 tuple 构造可以用 `get<0>` / `get<1>` / ... 依次提取元素。这正是 C++14 放宽 constexpr 后最经典的应用 — 循环不再是运行期概念, 而是通过编译期整数序列 + 参数包展开来完成
+
+### std::integer_sequence — C++14 引入的编译期整数载体
+
+```cpp
+// MSVC STL · msvc-stl/stl/inc/utility (有删节)
+template <class _Ty, _Ty... _Vals>
+struct integer_sequence {
+    static_assert(is_integral_v<_Ty>,
+        "integer_sequence<T, I...> requires T to be an integral type.");
+};
+
+template <size_t... _Vals>
+using index_sequence = integer_sequence<size_t, _Vals...>;
+
+template <size_t _Size>
+using make_index_sequence = __make_integer_seq<integer_sequence, size_t, _Size>;
+```
+
+> 小结: `std::make_index_sequence` 和 `std::tuple` 的编译期构造都依赖 C++14 放宽后的 constexpr 环境。没有 loop + branch 的 constexpr, 标准库就只能在编译器内部用黑魔法生成整数序列, 而不能用 C++ 语言自身的表达能力来实现
+
+## 三、注意事项
 
 ### C++14 constexpr 仍不支持的操作
 
@@ -157,7 +201,7 @@ int main() {
 
 把函数标记为 constexpr 并不改变其 ODR 链接属性 (C++17 起 constexpr 函数才隐式 inline), 也不代表所有能 constexpr 的都该 constexpr。如果一个函数几乎只在运行期调用, 加 constexpr 只增加了接口约束, 实际收益很小
 
-## 三、练习代码
+## 四、练习代码
 
 ### 练习代码主题
 
@@ -170,7 +214,7 @@ int main() {
 d2x checker relaxed-constexpr
 ```
 
-## 四、其他
+## 五、其他
 
 - [交流讨论](https://forum.d2learn.org/category/20)
 - [d2mcpp教程仓库](https://github.com/mcpp-community/d2mcpp)

dslings/cpp14/01-relaxed-constexpr-0.cpp

@@ -0,0 +1,68 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// file: dslings/cpp14/01-relaxed-constexpr-0.cpp
+//
+// Exercise/练习: cpp14 | 01 - relaxed constexpr | constexpr 循环
+//
+// Tips/提示:
+// - C++14 constexpr 函数可以使用 for / while 循环
+// - 循环中可以有局部变量和复合语句
+//
+// Docs/文档:
+//   - https://en.cppreference.com/w/cpp/language/constexpr
+//   - https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp14/01-relaxed-constexpr.md
+//
+// 练习交流讨论: http://forum.d2learn.org/category/20
+//
+// Auto-Checker/自动检测命令:
+//
+//   d2x checker relaxed-constexpr
+//
+
+#include <d2x/cpp/common.hpp>
+
+// constexpr 阶乘 — C++14 可以用循环
+constexpr int factorial(int n) {
+    D2X_YOUR_ANSWER result = 1;
+    for (int i = 1; D2X_YOUR_ANSWER; ++i) {
+        result *= i;
+    }
+    return result;
+}
+
+// constexpr 平方幂 — while 循环版本
+constexpr int power(int base, int exp) {
+    int result = 1;
+    int i = 0;
+    while (D2X_YOUR_ANSWER) {
+        result *= base;
+        ++i;
+    }
+    return result;
+}
+
+int main() {
+
+    // 0. for 循环 — 编译期阶乘
+    constexpr int f5 = factorial(5);
+    static_assert(f5 == 120, "factorial(5) should be 120");
+    d2x_assert_eq(f5, 120);
+
+    constexpr int f0 = factorial(0);
+    static_assert(f0 == 1, "factorial(0) should be 1");
+
+    constexpr int f10 = factorial(10);
+    d2x_assert_eq(f10, 3628800);
+
+    // 1. while 循环 — 编译期幂运算
+    constexpr int p2 = power(2, 3);
+    static_assert(p2 == 8, "power(2,3) should be 8");
+    d2x_assert_eq(p2, D2X_YOUR_ANSWER);
+
+    constexpr int p5 = power(5, 2);
+    d2x_assert_eq(p5, 25);
+
+    D2X_WAIT
+
+    return 0;
+}

dslings/cpp14/01-relaxed-constexpr-1.cpp

@@ -0,0 +1,86 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// file: dslings/cpp14/01-relaxed-constexpr-1.cpp
+//
+// Exercise/练习: cpp14 | 01 - relaxed constexpr | constexpr 分支与局部变量
+//
+// Tips/提示:
+// - C++14 constexpr 函数可以使用 if / switch 分支
+// - 可以在 constexpr 中声明和使用多个局部变量
+//
+// Docs/文档:
+//   - https://en.cppreference.com/w/cpp/language/constexpr
+//   - https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp14/01-relaxed-constexpr.md
+//
+// 练习交流讨论: http://forum.d2learn.org/category/20
+//
+// Auto-Checker/自动检测命令:
+//
+//   d2x checker relaxed-constexpr
+//
+
+#include <d2x/cpp/common.hpp>
+
+// constexpr 绝对值 — if 分支
+constexpr int my_abs(int x) {
+    if (D2X_YOUR_ANSWER) {
+        return -x;
+    }
+    return x;
+}
+
+// constexpr 月份天数 — switch 分支
+constexpr int days_in_month(D2X_YOUR_ANSWER month) {
+    switch (month) {
+        case 1: case 3: case 5: case 7: case 8: case 10: case 12:
+            return 31;
+        case 4: case 6: case 9: case 11:
+            return 30;
+        case 2:
+            return 28;
+        D2X_YOUR_ANSWER:
+            return 0;
+    }
+}
+
+// constexpr 斐波那契 — 多局部变量 + 循环
+constexpr int fib(int n) {
+    int a = 0, b = D2X_YOUR_ANSWER;
+    for (int i = 0; i < n; ++i) {
+        int tmp = a + b;
+        a = b;
+        b = tmp;
+    }
+    return D2X_YOUR_ANSWER;
+}
+
+int main() {
+
+    // 0. if 分支
+    constexpr int a1 = my_abs(-10);
+    static_assert(a1 == 10, "");
+    d2x_assert_eq(a1, 10);
+
+    constexpr int a2 = my_abs(0);
+    d2x_assert_eq(a2, 0);
+
+    // 1. switch 分支
+    constexpr int d7 = days_in_month(7);
+    static_assert(d7 == 31, "July has 31 days");
+    d2x_assert_eq(d7, 31);
+
+    constexpr int d2 = days_in_month(2);
+    d2x_assert_eq(d2, D2X_YOUR_ANSWER);
+
+    // 2. 多局部变量 + 循环 — fib
+    constexpr int f10 = fib(10);
+    static_assert(f10 == 55, "fib(10) should be 55");
+    d2x_assert_eq(f10, 55);
+
+    constexpr int f0 = fib(0);
+    d2x_assert_eq(f0, 0);
+
+    D2X_WAIT
+
+    return 0;
+}

dslings/en/cpp14/01-relaxed-constexpr-0.cpp

@@ -0,0 +1,68 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// file: dslings/en/cpp14/01-relaxed-constexpr-0.cpp
+//
+// Exercise: cpp14 | 01 - relaxed constexpr | constexpr with loops
+//
+// Tips:
+// - C++14 constexpr functions can use for / while loops
+// - Loops may contain local variables and compound statements
+//
+// Docs:
+//   - https://en.cppreference.com/w/cpp/language/constexpr
+//   - https://github.com/mcpp-community/d2mcpp/blob/main/book/en/src/cpp14/01-relaxed-constexpr.md
+//
+// Discussion Forum: http://forum.d2learn.org/category/20
+//
+// Auto-Checker:
+//
+//   d2x checker relaxed-constexpr
+//
+
+#include <d2x/cpp/common.hpp>
+
+// constexpr factorial — C++14 allows a for loop
+constexpr int factorial(int n) {
+    D2X_YOUR_ANSWER result = 1;
+    for (int i = 1; D2X_YOUR_ANSWER; ++i) {
+        result *= i;
+    }
+    return result;
+}
+
+// constexpr power — while loop version
+constexpr int power(int base, int exp) {
+    int result = 1;
+    int i = 0;
+    while (D2X_YOUR_ANSWER) {
+        result *= base;
+        ++i;
+    }
+    return result;
+}
+
+int main() {
+
+    // 0. for loop — compile-time factorial
+    constexpr int f5 = factorial(5);
+    static_assert(f5 == 120, "factorial(5) should be 120");
+    d2x_assert_eq(f5, 120);
+
+    constexpr int f0 = factorial(0);
+    static_assert(f0 == 1, "factorial(0) should be 1");
+
+    constexpr int f10 = factorial(10);
+    d2x_assert_eq(f10, 3628800);
+
+    // 1. while loop — compile-time power
+    constexpr int p2 = power(2, 3);
+    static_assert(p2 == 8, "power(2,3) should be 8");
+    d2x_assert_eq(p2, D2X_YOUR_ANSWER);
+
+    constexpr int p5 = power(5, 2);
+    d2x_assert_eq(p5, 25);
+
+    D2X_WAIT
+
+    return 0;
+}