I was writing a parser where my code had a structure like this (removed all the non-important parts for this question):

class Token
{
    private:
        char kind;
    
    public:
        Token(char kind) : kind(kind) {}
        inline char getKind() const {return kind;}
};

class Parser
{
    private:
        const std::vector<Token>& tokens;
    
    public:
        Parser(const std::vector<Token>& tokens) : tokens(tokens) {}
};

The & after std::vector<Token> was added by accident. The result was that the following returned a undefined value:

auto it = tokens.begin();
(*it).getKind();

After finding the problem I removed the & and the code worked as intended. What I am wondering about is what const std::vector<Token>& tokens; really does? The code did not give any compiler warning or error so does that mean that mean using & like this is useful in some cases?