title: finding an element in a multimap ONLY with its value c++
body: <p>First of all sorry if I am asking stupid questions, but I am a beginer in c++.</p>
  
  <p>I am writing a system that represents a library and there is a member function of my Library class that is supposed to allow us to remove a book. Now, if the book is loaned by a user, means there is an element in my <code>_usersLoaningMultimap</code> (<code>multimap&lt;UserId,LoanInfo&gt;</code>). How can I find the LoanInfo that I want without knowing the key (UserId)?</p>
  
  <pre><code>bool Library::removeBook(const BookId&amp; bookId){
  //how to find my book in my library without knowing who loaned it.
  </code></pre>
  
  <p>}</p>
  
  <p>Just to make it clearer, my class Library is like that:</p>
  
  <pre><code>class Library {
  public:
  Library();
  void addUser(const UserId&amp;, const string&amp;);
  Optional&lt;string&gt;&amp; getUserInfo(const UserId&amp; userId);
  void addBook(const BookId&amp; bookId, const string&amp; description);
  Optional&lt;string&gt;&amp; getBookInfo(const BookId&amp; bookId);
  bool returnBook(const UserId&amp; userId, const BookId&amp; bookId);
  void loanBook(const UserId&amp; userId,LoanInfo&amp; loan);
  bool removeUser(const UserId&amp; userId);
  void getLoansSortedByDate(const UserId,std::vector&lt;LoanInfo&gt;&amp; loanVector);
  
  ~Library() {}
  private:
  map&lt;BookId, string&gt; _bookMap;
  map&lt;UserId, string&gt; _userMap;
  multimap&lt;UserId, LoanInfo&gt; _usersLoaningMultimap;
  
  };
  </code></pre>