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20.02. CPP/Structured binding declaration.md

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+---
+tags: cpp17
+aliases: 
+date: 2025-02-10 
+time: 17:34:18
+description: 
+---
+
+Structured binding declaration 可以把對應的 tuple、pair、vector 展開，讓 code 更好讀。
+
+## 展開 tuple
+假設我們有一個 tuple：
+```cpp
+std::tuple<std::string, uint32_t, uint32_t> person{ "John", 32, 170 };
+
+auto& [name, age, tall] = person;
+```
+
+name 會是 "John"
+age 是 32
+tall 是 170
+
+但比較好用的時候還是用來展開 function 的回傳值，假設我們有一個 function 會回傳 tuple：
+```cpp
+std::tuple<std::string, uint32_t, uint32_t> getPersonData(uint32_t id) {
+    return std::make_tuple("John", 32, 170);
+}
+
+auto [name, age, tall] = getPersonData(id);
+```
+
+## 展開 array
+`std::vector` 也是一樣的用法：
+```cpp
+#include <array>
+
+std::array<int, 3> my_vec = { 5, 7, 10 };
+auto& [num1, num2, num3] = my_vec;
+```
+
+或是：
+```cpp
+float rect[4]{ 5.0f, 6.0f, 120.0f, 200.0f };
+auto& [x, y, w, h] = rect;
+```
+
+但是不能用來展開 `std::vector`。
+
+## 展開 pair
+```cpp
+std::pair<std::string, int32_t> name_phone{ "John", 912345678 };
+auto& [name, phone_number] = name_phone;
+```
+
+用在 for-loop 裡也比較好懂，假設我們有一個 vector 用來存剛剛的姓名跟電話：
+```cpp
+std::vector<std::pair<std::string, uint32_t>> phoneBook = {
+    { "John", 912345678 },
+    { "Andy", 912345679 },
+};
+
+for (const auto& [name, phone] : phoneBook) {
+    std::cout << "Name: " << name << ", phone: " << phone << std::endl;
+}
+```
+
+
+# 參考來源