structured bindings

• Since C++17. 適用在各種 container 以及 object(class, struct) 上。
• Structured binding only works if the structure is known at compile time. This is not the case for the vector. 請見下方例子，當 array 被傳入到 function 中，若沒有提供長度參數，是不可能得知長度的，所以對 arr3 進行 structure binding 會導致 error。

#include <iostream>
#include <tuple>
#include <map>
#include <array>
 
void f(std::array<int, 3> arr1, int (&arr2)[3], int (&arr3)[])
{
    auto [a1,b1,c1] = arr1;
    auto [a2,b2,c2] = arr2;
    auto [a3,b3,c3] = arr3; //  error: deduced type 'int []' for '<structured bindings>' is incomplete
};
 
int main() {
    std::tuple<int, double, int> t{1, 2.0, 1};
    auto [x, y, z] = t;
    std::cout << x << " " << y << " " << z << std::endl; // 1 2 1
 
    std::pair<int, int> p{1, 2};
    auto [p1, p2] = p;
    std::cout << p1 << " " << p2 << std::endl; // 1 2
 
    std::map<int, int> s;
    s[1] = 2;
    auto [key, value] = *s.begin();
    std::cout << key << " " << value << std::endl; // 1 2
 
    std::array<int, 3> arr1{1, 2, 3};
    auto [a1, b1, c1] = arr1;
    std::cout << a1 << " " << b1 << " " << c1 << std::endl; // 1 2 3
 
    int arr2[3]{1, 2 ,3};
    auto [aa, bb, cc] = arr2;
    std::cout << aa << " " << bb << " " << cc << std::endl;
 
    int arr3[] = {1, 2, 3};
    f(arr1, arr2, arr3);
 
}
 

#include <map>
#include <string>
#include <iostream>
 
template <typename Key, typename Value, typename F>
void update(std::map<Key, Value>& m, F foo) {
    for(auto& [key, value] : m) {
        m[key] = foo(key);
    }
}
int main() {
    std::map<std::string, long long int> m {
        {"a", 1},
        {"b", 2},
        {"c", 3}
    };
    update(m, [](std::string key) {
        return std::hash<std::string>{}(key);
    });
    for (auto&& [key, value] : m)
        std::cout << key << ":" << value << std::endl;
}

Reference

• Structured binding declaration (since C++17)