58 lines
1.4 KiB
Markdown
58 lines
1.4 KiB
Markdown
Structured binding declaration 可以把對應的 tuple、pair、vector 展開，讓 code 更好讀。
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## 展開 tuple
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假設我們有一個 tuple：
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```cpp
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std::tuple<std::string, uint32_t, uint32_t> person{ "John", 32, 170 };
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auto& [name, age, tall] = person;
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```
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name 會是 "John"
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age 是 32
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tall 是 170
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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：
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```cpp
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std::tuple<std::string, uint32_t, uint32_t> getPersonData(uint32_t id) {
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return std::make_tuple("John", 32, 170);
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}
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auto [name, age, tall] = getPersonData(id);
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```
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## 展開 array
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`std::vector` 也是一樣的用法：
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```cpp
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#include <array>
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std::array<int, 3> my_vec = { 5, 7, 10 };
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auto& [num1, num2, num3] = my_vec;
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```
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或是：
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```cpp
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float rect[4]{ 5.0f, 6.0f, 120.0f, 200.0f };
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auto& [x, y, w, h] = rect;
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```
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但是不能用來展開 `std::vector`。
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## 展開 pair
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```cpp
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std::pair<std::string, int32_t> name_phone{ "John", 912345678 };
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auto& [name, phone_number] = name_phone;
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```
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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：
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```cpp
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std::vector<std::pair<std::string, uint32_t>> phoneBook = {
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{ "John", 912345678 },
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{ "Andy", 912345679 },
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};
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for (const auto& [name, phone] : phoneBook) {
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std::cout << "Name: " << name << ", phone: " << phone << std::endl;
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}
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```
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