在 c++ 中对 std::pair 使用 const

Question

For implementing binary search tree I am wandering why we put Const before K in std::pair?

template<class K,class V>
class BST {
public:
    class Node {
    public:
        pair<const K, V> data; // pair < data_type1, data_type2 > Pair_name;
        unique_ptr<Node> left;   // Create a unique_ptr object through raw pointer
        unique_ptr<Node> right;
        unique_ptr<Node> parent; 

        Node(const K& k, const V& v): //constructor
        data{std::move(k),std::move(v)}, right(nullptr),left(nullptr),parent(nullptr)
        {
        }
        ~Node() noexcept = default;

Answer 1

Because, logically, the node keys in your data structure must be immutable : immune from change. That's to maintain the proper structural layout, which depends on the relative ordering (by key.) of the nodes.

If a node needs to be renamed, higher-level algorithms need to be deployed in order to do that.

Sure, there's no code in your snippet that would even try to modify a key — and why would there be? It wouldn't compile! — but putting the const there enforces that at compile-time if anybody forgets and tries to do it. So, it's for the same reason as any other usage of const : enforcing immutability .

std::map containers also store std::pair<const K, V> , for just the same reason.