feat(cpp14): add 00-generic-lambdas exercises and solutions

Exercise progression:
  0. Basic generic lambda — auto parameter identity/comparison/type-size
     exercises. Learner fills auto param type, return expression, and
     sizeof argument. 4 D2X_YOUR_ANSWER.
  1. Generic lambda with STL algorithms — same lambda reused across
     vector<int> and vector<double> for sort/find_if, plus a factory
     pattern returning a lambda. 5 D2X_YOUR_ANSWER.

Design rationale:
  Generic lambdas have three layers worth teaching independently:
  (1) syntax — auto in lambda parameters generates a templated operator();
  (2) reuse — a single lambda serves multiple container types, the
  primary motivation for the feature;
  (3) interaction — generic lambda returns + captures work naturally
  without extra syntax.

  ex 0 isolates layer (1). identity forces the learner to write auto
  in a parameter position. greater verifies the return expression is
  type-checked per instantiation. get_type_size confirms that sizeof
  on a generic parameter works correctly, reinforcing the "compiler
  stamps out per-type copies" mental model. Everything is self-contained
  and runnable without headers beyond <string>.

  ex 1 layers (2) and (3) on top. The desc lambda applied to both
  vector<int> and vector<double> makes the payoff tangible — contrast
  with the C++11 approach of duplicating the comparator. The find_if
  exercise adds a capture without complicating the generic parameter.
  The make_multiplier factory previews generic-lambda-returning-lambda,
  which is a pattern that only becomes practical with C++14 return type
  deduction.

  We deliberately limited this chapter to 2 exercises. A third exercise
  covering perfect forwarding + auto&& + decltype(auto) is better placed
  in the decltype(auto) chapter where the forwarding semantics are the
  primary topic, not a side note.

Files: dslings/cpp14/00-generic-lambdas-{0,1}.cpp (zh),
       dslings/en/cpp14/00-generic-lambdas-{0,1}.cpp (en),
       dslings/cpp14/xmake.lua, dslings/en/cpp14/xmake.lua,
       solutions/cpp14/00-generic-lambdas-{0,1}.cpp,
       solutions/cpp14/xmake.lua

Author: lczllx

dslings/cpp14/00-generic-lambdas-0.cpp

@@ -0,0 +1,56 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// file: dslings/cpp14/00-generic-lambdas-0.cpp
+//
+// Exercise/练习: cpp14 | 00 - generic lambdas | 泛型 lambda
+//
+// Tips/提示:
+// - lambda 参数使用 auto, 编译器为 operator() 生成隐式模板
+// - 同一个泛型 lambda 可以接受不同类型的参数
+//
+// Docs/文档:
+//   - https://en.cppreference.com/w/cpp/language/lambda
+//   - https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp14/00-generic-lambdas.md
+//
+// 练习交流讨论: http://forum.d2learn.org/category/20
+//
+// Auto-Checker/自动检测命令:
+//
+//   d2x checker generic-lambdas
+//
+
+#include <d2x/cpp/common.hpp>
+#include <string>
+
+int main() {
+
+    // 0. 简单泛型 lambda — identity
+    auto identity = [](D2X_YOUR_ANSWER x) {
+        return x;
+    };
+
+    d2x_assert_eq(identity(42), 42);
+    d2x_assert(identity(std::string("hello")) == "hello");
+    d2x_assert_eq(identity(3.14), D2X_YOUR_ANSWER);
+
+    // 1. 泛型 lambda 做比较
+    auto greater = [](auto a, auto b) {
+        return D2X_YOUR_ANSWER;
+    };
+
+    d2x_assert(greater(5, 3));
+    d2x_assert(greater(2.5, 1.2));
+    d2x_assert(greater(std::string("z"), std::string("a")));
+
+    // 2. 推导类型确认
+    auto get_type_size = [](auto x) {
+        return sizeof(D2X_YOUR_ANSWER);
+    };
+
+    d2x_assert_eq(get_type_size(42), sizeof(int));
+    d2x_assert_eq(get_type_size('c'), sizeof(char));
+
+    D2X_WAIT
+
+    return 0;
+}

dslings/cpp14/00-generic-lambdas-1.cpp

@@ -0,0 +1,67 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// file: dslings/cpp14/00-generic-lambdas-1.cpp
+//
+// Exercise/练习: cpp14 | 00 - generic lambdas | 泛型 lambda 与 STL 算法
+//
+// Tips/提示:
+// - 泛型 lambda 可复用于不同元素类型的容器, 同一个 lambda 传给多种 STL 算法
+// - 捕获的变量类型不变, 只有参数用 auto
+//
+// Docs/文档:
+//   - https://en.cppreference.com/w/cpp/language/lambda
+//   - https://github.com/mcpp-community/d2mcpp/blob/main/book/src/cpp14/00-generic-lambdas.md
+//
+// 练习交流讨论: http://forum.d2learn.org/category/20
+//
+// Auto-Checker/自动检测命令:
+//
+//   d2x checker generic-lambdas
+//
+
+#include <d2x/cpp/common.hpp>
+#include <vector>
+#include <algorithm>
+
+int main() {
+
+    // 0. 同一个泛型 lambda 用于不同元素类型的容器排序
+    std::vector<int> v1 = {5, 1, 4, 2, 8};
+    std::vector<double> v2 = {3.1, 2.7, 8.5, 1.9};
+
+    auto desc = [](D2X_YOUR_ANSWER a, D2X_YOUR_ANSWER b) {
+        return a > b;
+    };
+
+    std::sort(v1.begin(), v1.end(), desc);
+    d2x_assert_eq(v1[0], 8);
+    d2x_assert_eq(v1[4], 1);
+
+    std::sort(v2.begin(), v2.end(), desc);
+    d2x_assert_eq(v2[0], D2X_YOUR_ANSWER);
+
+    // 1. 带捕获的泛型 lambda — find_if
+    int threshold = 3;
+    auto above = [threshold](auto x) {
+        return x > threshold;
+    };
+
+    auto it1 = std::find_if(v1.begin(), v1.end(), above);
+    d2x_assert(*it1 == D2X_YOUR_ANSWER);
+
+    auto it2 = std::find_if(v2.begin(), v2.end(), above);
+    d2x_assert(*it2 == 8.5);
+
+    // 2. 泛型 lambda 返回 lambda — 函数工厂
+    auto make_multiplier = [](auto factor) {
+        return [factor](auto x) { return x * D2X_YOUR_ANSWER; };
+    };
+
+    auto times2 = make_multiplier(2);
+    d2x_assert_eq(times2(10), 20);
+    d2x_assert_eq(times2(0.5), 1.0);
+
+    D2X_WAIT
+
+    return 0;
+}

dslings/cpp14/xmake.lua

@@ -0,0 +1,11 @@
+set_languages("cxx14")
+
+-- target: cpp14-00-generic-lambdas
+
+target("cpp14-00-generic-lambdas-0")
+    set_kind("binary")
+    add_files("00-generic-lambdas-0.cpp")
+
+target("cpp14-00-generic-lambdas-1")
+    set_kind("binary")
+    add_files("00-generic-lambdas-1.cpp")

dslings/en/cpp14/00-generic-lambdas-0.cpp

@@ -0,0 +1,57 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// file: dslings/en/cpp14/00-generic-lambdas-0.cpp
+//
+// Exercise: cpp14 | 00 - generic lambdas | basic generic lambda
+//
+// Tips:
+// - Use auto in lambda parameters; the compiler generates an implicit
+//   template for operator()
+// - The same generic lambda can accept arguments of different types
+//
+// Docs:
+//   - https://en.cppreference.com/w/cpp/language/lambda
+//   - https://github.com/mcpp-community/d2mcpp/blob/main/book/en/src/cpp14/00-generic-lambdas.md
+//
+// Discussion Forum: http://forum.d2learn.org/category/20
+//
+// Auto-Checker:
+//
+//   d2x checker generic-lambdas
+//
+
+#include <d2x/cpp/common.hpp>
+#include <string>
+
+int main() {
+
+    // 0. Simple generic lambda — identity
+    auto identity = [](D2X_YOUR_ANSWER x) {
+        return x;
+    };
+
+    d2x_assert_eq(identity(42), 42);
+    d2x_assert(identity(std::string("hello")) == "hello");
+    d2x_assert_eq(identity(3.14), D2X_YOUR_ANSWER);
+
+    // 1. Generic lambda as comparator
+    auto greater = [](auto a, auto b) {
+        return D2X_YOUR_ANSWER;
+    };
+
+    d2x_assert(greater(5, 3));
+    d2x_assert(greater(2.5, 1.2));
+    d2x_assert(greater(std::string("z"), std::string("a")));
+
+    // 2. Verify deduced types
+    auto get_type_size = [](auto x) {
+        return sizeof(D2X_YOUR_ANSWER);
+    };
+
+    d2x_assert_eq(get_type_size(42), sizeof(int));
+    d2x_assert_eq(get_type_size('c'), sizeof(char));
+
+    D2X_WAIT
+
+    return 0;
+}

dslings/en/cpp14/00-generic-lambdas-1.cpp

@@ -0,0 +1,68 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// file: dslings/en/cpp14/00-generic-lambdas-1.cpp
+//
+// Exercise: cpp14 | 00 - generic lambdas | generic lambda with STL algorithms
+//
+// Tips:
+// - A single generic lambda can be reused with containers of different
+//   element types
+// - Captured variables retain their concrete types; only parameters use auto
+//
+// Docs:
+//   - https://en.cppreference.com/w/cpp/language/lambda
+//   - https://github.com/mcpp-community/d2mcpp/blob/main/book/en/src/cpp14/00-generic-lambdas.md
+//
+// Discussion Forum: http://forum.d2learn.org/category/20
+//
+// Auto-Checker:
+//
+//   d2x checker generic-lambdas
+//
+
+#include <d2x/cpp/common.hpp>
+#include <vector>
+#include <algorithm>
+
+int main() {
+
+    // 0. Same generic lambda used to sort containers of different element types
+    std::vector<int> v1 = {5, 1, 4, 2, 8};
+    std::vector<double> v2 = {3.1, 2.7, 8.5, 1.9};
+
+    auto desc = [](D2X_YOUR_ANSWER a, D2X_YOUR_ANSWER b) {
+        return a > b;
+    };
+
+    std::sort(v1.begin(), v1.end(), desc);
+    d2x_assert_eq(v1[0], 8);
+    d2x_assert_eq(v1[4], 1);
+
+    std::sort(v2.begin(), v2.end(), desc);
+    d2x_assert_eq(v2[0], D2X_YOUR_ANSWER);
+
+    // 1. Generic lambda with capture — find_if
+    int threshold = 3;
+    auto above = [threshold](auto x) {
+        return x > threshold;
+    };
+
+    auto it1 = std::find_if(v1.begin(), v1.end(), above);
+    d2x_assert(*it1 == D2X_YOUR_ANSWER);
+
+    auto it2 = std::find_if(v2.begin(), v2.end(), above);
+    d2x_assert(*it2 == 8.5);
+
+    // 2. Generic lambda returning lambda — factory function
+    auto make_multiplier = [](auto factor) {
+        return [factor](auto x) { return x * D2X_YOUR_ANSWER; };
+    };
+
+    auto times2 = make_multiplier(2);
+    d2x_assert_eq(times2(10), 20);
+    d2x_assert_eq(times2(0.5), 1.0);
+
+    D2X_WAIT
+
+    return 0;
+}

dslings/en/cpp14/xmake.lua

@@ -0,0 +1,11 @@
+set_languages("cxx14")
+
+-- target: cpp14-00-generic-lambdas
+
+target("cpp14-00-generic-lambdas-0")
+    set_kind("binary")
+    add_files("00-generic-lambdas-0.cpp")
+
+target("cpp14-00-generic-lambdas-1")
+    set_kind("binary")
+    add_files("00-generic-lambdas-1.cpp")

solutions/cpp14/00-generic-lambdas-0.cpp

@@ -0,0 +1,41 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// reference solution for: dslings/cpp14/00-generic-lambdas-0.cpp
+//
+// 用途: 仅给 CI 与维护者参考使用，不是教程入口。
+// 教程练习入口: dslings/cpp14/00-generic-lambdas-0.cpp
+//
+
+#include <d2x/cpp/common.hpp>
+#include <string>
+
+int main() {
+
+    // 0. 简单泛型 lambda — identity
+    auto identity = [](auto x) {
+        return x;
+    };
+
+    d2x_assert_eq(identity(42), 42);
+    d2x_assert(identity(std::string("hello")) == "hello");
+    d2x_assert_eq(identity(3.14), 3.14);
+
+    // 1. 泛型 lambda 做比较
+    auto greater = [](auto a, auto b) {
+        return a > b;
+    };
+
+    d2x_assert(greater(5, 3));
+    d2x_assert(greater(2.5, 1.2));
+    d2x_assert(greater(std::string("z"), std::string("a")));
+
+    // 2. 推导类型确认
+    auto get_type_size = [](auto x) {
+        return sizeof(x);
+    };
+
+    d2x_assert_eq(get_type_size(42), sizeof(int));
+    d2x_assert_eq(get_type_size('c'), sizeof(char));
+
+    return 0;
+}

solutions/cpp14/00-generic-lambdas-1.cpp

@@ -0,0 +1,52 @@
+// d2mcpp: https://github.com/mcpp-community/d2mcpp
+// license: Apache-2.0
+// reference solution for: dslings/cpp14/00-generic-lambdas-1.cpp
+//
+// 用途: 仅给 CI 与维护者参考使用，不是教程入口。
+// 教程练习入口: dslings/cpp14/00-generic-lambdas-1.cpp
+//
+
+#include <d2x/cpp/common.hpp>
+#include <vector>
+#include <algorithm>
+
+int main() {
+
+    // 0. 同一个泛型 lambda 用于不同元素类型的容器排序
+    std::vector<int> v1 = {5, 1, 4, 2, 8};
+    std::vector<double> v2 = {3.1, 2.7, 8.5, 1.9};
+
+    auto desc = [](auto a, auto b) {
+        return a > b;
+    };
+
+    std::sort(v1.begin(), v1.end(), desc);
+    d2x_assert_eq(v1[0], 8);
+    d2x_assert_eq(v1[4], 1);
+
+    std::sort(v2.begin(), v2.end(), desc);
+    d2x_assert_eq(v2[0], 8.5);
+
+    // 1. 带捕获的泛型 lambda — find_if
+    int threshold = 3;
+    auto above = [threshold](auto x) {
+        return x > threshold;
+    };
+
+    auto it1 = std::find_if(v1.begin(), v1.end(), above);
+    d2x_assert(*it1 == 8);
+
+    auto it2 = std::find_if(v2.begin(), v2.end(), above);
+    d2x_assert(*it2 == 8.5);
+
+    // 2. 泛型 lambda 返回 lambda — 函数工厂
+    auto make_multiplier = [](auto factor) {
+        return [factor](auto x) { return x * factor; };
+    };
+
+    auto times2 = make_multiplier(2);
+    d2x_assert_eq(times2(10), 20);
+    d2x_assert_eq(times2(0.5), 1.0);
+
+    return 0;
+}

solutions/cpp14/xmake.lua

@@ -0,0 +1,11 @@
+set_languages("cxx14")
+
+-- target: cpp14-00-generic-lambdas
+
+target("cpp14-00-generic-lambdas-0-ref")
+    set_kind("binary")
+    add_files("00-generic-lambdas-0.cpp")
+
+target("cpp14-00-generic-lambdas-1-ref")
+    set_kind("binary")
+    add_files("00-generic-lambdas-1.cpp")