vault backup: 2022-06-13 18:24:14

Affected files:
02. PARA/03. Resources（資源）/C++17/lambda.md

02. PARA/03. Resources（資源）/C++17/lambda.md

@@ -122,10 +122,10 @@ private:
 ```cpp
 void testLambda() {
     float notUsed = 1.0f;
-    std::vector<int32_t> numlist{10, 20, 30, 50, 60};
-    auto findInRange = [&](int32_t start, int32_t end) {
+    std::vector<int32_t> numlist{ 10, 20, 30, 50, 60 };
+    auto findInRange = [&](int32_t start, int32_t end) {    // Use & here
         numlist.push_back(100); // OK
-        
+
         for (auto num : numlist) {
             if (num >= start && num <= end) return true;
         }
@@ -133,6 +133,47 @@ void testLambda() {
     };
 
     std::cout << "Result: " << findInRange(25, 35) << "\n";
-    std::cout << "numlist: " << numlist << "\n";
+    std::cout << "numlist: ";
+    for (auto n : numlist) {
+        std::cout << n << " ";
+    }
+    std::cout << "\n"; // Output numlist: 10 20 30 50 60 100
 }
-```
+```
+
+但是直接參考全部的外部變數不是好的作法，這讓你有機會做出一些意外的修改，所以請擷取有需要的變數就好：
+```cpp
+void testLambda() {
+    float notUsed = 1.0f;
+    std::vector<int32_t> numlist{ 10, 20, 30, 50, 60 };
+    
+    auto findInRange = [&numlist](int32_t start, int32_t end) {
+        numlist.push_back(100); // OK
+
+        for (auto num : numlist) {
+            if (num >= start && num <= end) return true;
+        }
+        return false;
+    };
+
+    ...
+}
+```
+
+如果有多個變數需要擷取，那就用 `,` 分開：
+```cpp
+auto findInRange = [&numlist, &var1, &var2](int32_t start, int32_t end) {
+    ...
+};
+```
+
+### 混合
+以值擷取跟參考擷取也可以寫在一起：
+```cpp
+auto findInRange = [=, &numlist](int32_t start, int32_t end) {
+    ...
+};
+```
+
+上面的例子中，`numlist` 會是一個參考擷取，其他的外部變數則是以值擷取。
+或是：