video(cpp11/00): manim animation for auto/decltype

videos/cpp11/00-auto-and-decltype.py

@@ -0,0 +1,174 @@
+import sys, os
+from manim import *
+
+"""
+Manim Community v0.18.1
+
+# 用 uv 运行(无需预装 manim):
+uv run --with "manim==0.18.1" manim -pql videos/cpp11/00-auto-and-decltype.py
+uv run --with "manim==0.18.1" manim -pqh videos/cpp11/00-auto-and-decltype.py
+"""
+
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..")))
+
+from d2x import *
+
+class AutoAndDecltype(MovingCameraScene):
+    def construct(self):
+
+        title, logo = mcpp_video_start(self, "{ auto / decltype }")
+
+        self.wait(0.5)
+
+        self.play(Transform(title, Text("{ 类型自动推导 }", t2c={'类型自动推导': BLUE})))
+
+        self.wait(0.5)
+
+        # 场景1: 声明定义 —— auto 从初始值推导, decltype 取声明类型
+        self.play(Transform(title, Text("{ 1 - 声明定义 }", t2c={'声明定义': BLUE})))
+
+        self.wait(0.5)
+
+        code1 = self.create_code_helper("""int b = 2;
+auto b1 = b;          // b1: int
+decltype(b) b2 = b;   // b2: int""")
+
+        self.play(ReplacementTransform(title, code1))
+
+        self.wait(0.5)
+
+        self.play(DHighlight(code1.code[1]))            # auto: 从初始值推导
+
+        self.wait(0.5)
+
+        self.play(DHighlight(code1.code[2]))            # decltype: 取声明类型
+
+        self.wait(0.5)
+
+        # 场景2: 表达式类型推导 —— 保留计算精度
+        title = Text("{ 2 - 表达式类型推导 }", t2c={'表达式': BLUE})
+        self.play(ReplacementTransform(code1, title))
+
+        self.wait(0.5)
+
+        code2 = self.create_code_helper("""int a = 1;
+auto x = a + 2 + 1.1;            // double
+decltype(a + 2 + 1.1) y = x;     // double""")
+
+        self.play(ReplacementTransform(title, code2))
+
+        self.wait(0.5)
+
+        self.play(DHighlight(code2.code[1], color=GREEN))
+        self.play(DHighlight(code2.code[2], color=GREEN))
+
+        self.wait(0.5)
+
+        # 场景3: 复杂类型推导 —— auto 驯服冗长的迭代器类型(STL 真实场景)
+        title = Text("{ 3 - 复杂类型推导 }", t2c={'复杂类型': BLUE})
+        self.play(ReplacementTransform(code2, title))
+
+        self.wait(0.5)
+
+        code3a = self.create_code_helper("""std::vector<int> v = {1, 2, 3};
+std::vector<int>::iterator it = v.begin();""")
+
+        self.play(ReplacementTransform(title, code3a))
+
+        self.wait(0.5)
+
+        self.play(DHighlight(code3a.code[1], color=PURE_RED))   # 又长又难写
+
+        self.wait(0.5)
+
+        code3b = self.create_code_helper("""std::vector<int> v = {1, 2, 3};
+auto it = v.begin();""")
+
+        self.play(ReplacementTransform(code3a, code3b))
+
+        self.wait(0.5)
+
+        self.play(DHighlight(code3b.code[1], color=GREEN))      # auto 一步到位
+
+        self.wait(0.5)
+
+        # 场景4: 函数返回值类型推导 —— 后置返回 + decltype
+        title = Text("{ 4 - 函数返回值推导 }", t2c={'函数返回值': BLUE})
+        self.play(ReplacementTransform(code3b, title))
+
+        self.wait(0.5)
+
+        code4 = self.create_code_helper("""template<class T1, class T2>
+auto add(T1 a, T2 b) -> decltype(a + b) {
+    return a + b;
+}""")
+
+        self.play(ReplacementTransform(title, code4))
+
+        self.wait(0.5)
+
+        self.play(DHighlight(code4.code[1]))            # auto ... -> decltype(a + b)
+
+        self.wait(0.5)
+
+        # 场景5: 注意事项 —— auto 剥离 const / 引用, 想保留要显式写
+        title = Text("{ 5 - const / 引用剥离 }", t2c={'const / 引用': PURE_RED})
+        self.play(ReplacementTransform(code4, title))
+
+        self.wait(0.5)
+
+        code5 = self.create_code_helper("""const int ci = 1;
+auto a = ci;          // int   (const lost!)
+const auto& r = ci;   // const int&
+decltype(ci) d = ci;  // const int""")
+
+        self.play(ReplacementTransform(title, code5))
+
+        self.wait(0.5)
+
+        self.play(DHighlight(code5.code[1], color=PURE_RED))    # 顶层 const 被剥离
+
+        self.wait(0.5)
+
+        self.play(DHighlight(code5.code[2], color=GREEN))       # const auto& 保留
+        self.play(DHighlight(code5.code[3], color=GREEN))       # decltype 精确保留
+
+        self.wait(0.5)
+
+        # 场景6: 注意事项 —— decltype 的括号陷阱
+        title = Text("{ 6 - decltype 括号陷阱 }", t2c={'括号陷阱': PURE_RED})
+        self.play(ReplacementTransform(code5, title))
+
+        self.wait(0.5)
+
+        code6 = self.create_code_helper("""int a = 1;
+decltype(a)   b;   // int
+decltype((a)) c;   // int&""")
+
+        self.play(ReplacementTransform(title, code6))
+
+        self.wait(0.5)
+
+        self.play(DHighlight(code6.code[1]))            # decltype(a): 声明类型 int
+
+        self.wait(0.5)
+
+        select_box = SurroundingRectangle(code6.code[2], color=PURE_RED, buff=0.1)
+        self.play(Create(select_box))
+        self.play(DHighlight(code6.code[2], color=PURE_RED))    # decltype((a)): 左值表达式 -> int&
+
+        self.wait(0.5)
+
+        mcpp_video_end(self, logo, VGroup(code6, select_box))
+
+    @staticmethod
+    def create_code_helper(code: str):
+        return Code(
+            code=code,
+            background="",
+            language="cpp",
+        )
+
+if __name__ == "__main__":
+    scene = AutoAndDecltype()
+    scene.render()