为什么在“形成对引用类型的引用”映射中出现错误？

Question

如果我需要使用引用，那么替代方法是什么，而我传递的数据我无法更改类型，因此我无法真正存储指向它的指针？

Code:

    #include <map>     
    #include<iostream>
    #include<string>     

    using namespace std;

    int main()
    {
       string test;
       pair<string, string> p=pair<string, string>("Foo","Bar");
       map<pair<string, string>, string&> m;
       m[make_pair("aa","bb")]=test;

       return 0;
}

Error:

$ g ++ MapPair.cpp /usr/include/c++/3.2.3/bits/stl_map.h：在

std::map<std::pair<std::string,
  std::string>, std::string&,
  std::less<std::pair<std::string,
  std::string> >,
  std::allocator<std::pair<const
  std::pair<std::string, std::string>,
  std::string&> > >': MapPair.cpp:15:
  instantiated from here
  /usr/include/c++/3.2.3/bits/stl_map.h:221:
  forming reference to reference type
  

std :: string＆'MapPair.cpp的实例化中：在函数

int main()': MapPair.cpp:16:
  no match for
  

std :: map，std :: string＆，std :: less > 的std ::分配器， std :: string＆>>>＆[std :: pair]'operator /usr/include/c++/3.2.3/bits/stl_pair.h：在全球范围内：/usr/include/c++/3.2.3/bits/ stl_pair.h：在

std::pair<const
  std::pair<std::string, std::string>,
  std::string&>':
  /usr/include/c++/3.2.3/bits/stl_tree.h:122:
  instantiated from
  

std :: _ Rb_tree_node的实例化中

我做错了什么导致这个错误？

Answer 1

您无法存储参考。参考文献只是aliases to another variable。

地图需要存储的字符串副本：

map<pair<string, string>, string> m;

你得到那个特殊错误的原因是因为地图中的某个地方，它将在mapped_type上进行操作，在你的情况下是string&。其中一个操作（例如在operator[]中）将返回对mapped_type的引用：

mapped_type& operator[](const key_type&)

哪，与你的mapped_type，将是：

string&& operator[](const key_type& _Keyval)

你不能参考参考：

标准8.3.4：

不应引用引用，不引用引用数组，也不引用引用指针。

---

另外，我建议您使用typedef，以便您的代码更易于阅读：

int main()
{
    typedef pair<string, string> StringPair;
    typedef map<StringPair, string> StringPairMap;

    string test;

    StringPair p("Foo","Bar");
    StringPairMap m;
    m[make_pair("aa","bb")] = test;

   return 0;

}

Answer 2

此处的答案已过时。今天我们将std::reference_wrapper作为C ++ 11标准的一部分：

#include <map>
#include <iostream>
#include <string>

using namespace std;

int main()
{
    string test;
    pair<string, string> p = pair<string, string>("Foo", "Bar");
    map<pair<string, string>, reference_wrapper<string>> m;
    m[make_pair("aa", "bb")] = test;

    return 0;
}

std :: reference_wrapper将隐式转换为对其内部类型的引用，但这在某些上下文中不起作用，在这种情况下，您调用.get()进行访问。

http://en.cppreference.com/w/cpp/utility/functional/reference_wrapper

Answer 3

您可以使用boost :: reference_wrapper在STL容器中存储引用。以下是您修改的示例（未经过测试，绝对写得不是很好，只是说明了一点）

#include <map>     
#include<iostream>
#include<string>   
#include <boost/ref.hpp>



int main()
{
   typedef std::pair< std::string, std::string> PairType;
   typedef std::map< PairType, boost::reference_wrapper<std::string> > MapType;
   std::string test = "Hello there!!";
   MapType m;
   PairType pp =  std::make_pair("aa","bb");
   m.insert(std::make_pair(pp , boost::ref(test) ) );

   MapType::iterator it (m.find( pp ) );
   if(it != m.end())
   {
       std::cout << it->second.get() << std::endl;
   }

   //change test
   test = "I am different now";
   std::cout << it->second.get() << std::endl;

   return 0;
}

Answer 4

由于模板的构建方式，您不能将引用用作val。您也可以使用指针代替。

Answer 5

基本上，问题是你是否可以在容器中使用引用。当然，如果你准备好你的课程和你的容器，你可以。我在下面用两个简单的向量容器演示它：vectoref修改std::vector<>，另一个是vec，它是从头开始实现的。

#include <iostream>
#include <vector>

// requires compilation with --std=c++11 (at least)

using namespace std;

class A {
  int _a; // this is our true data
  A *_p; // this is to cheat the compiler

  public:
  A(int n = 0) : _a(n), _p(0)
  { cout << "A constructor (" << this << "," << _a << ")\n"; }
  // constructor used by the initializer_list (cheating the compiler)
  A(const A& r) : _p(const_cast<A *>(&r))
  { cout << "A copy constructor (" << this << "<-" << &r << ")\n"; }
  void print() const {cout << "A instance: " << this << "," << _a << "\n";}
  ~A() {cout << "A(" << this << "," << _a << ") destructor.\n";}
  // just to see what is copied implicitly
  A& operator=(const A& r) {
    cout << "A instance copied (" << this << "," << _a << ")\n";
    _a = r._a; _p = r._p;
    return *this;
  }
  // just in case you want to check if instance is pure or fake
  bool is_fake() const {return _p != 0;}
  A *ptr() const {return _p;}
};

template<typename T, int sz>
class vec { // vector class using initializer_list of A-references!!
  public:
  const T *a[sz]; // store as pointers, retrieve as references
  // because asignment to a reference causes copy operator to be invoked
  int cur;
  vec() : cur(0) {}
  vec(std::initializer_list<T> l) : cur(0) {
    cout << "construct using initializer list.\n";
    for (auto& t : l) // expecting fake elements
      a[cur++] = t.ptr();
  }
  const T& operator[](int i) {return *a[i];}
  // expecting pure elements
  vec& push_back(const T& r) {a[cur++] = &r; return *this;}
  void copy_from(vec&& r) {
    for (int i = 0; i < r.cur; ++i)
      push_back(r[i]);
  }
};

template<typename T>
class vectoref : public vector<T *> { // similar to vec but extending std::vector<>
  using size_type = typename vector<T*>::size_type;
  public:
  vectoref() {}
  vectoref(std::initializer_list<T> l) {
    cout << "construct using initializer list.\n";
    for (auto& t : l) // expecting fake elements
      vector<T*>::push_back(t.ptr());
  }
  const T& operator[](size_type i) {return *vector<T*>::at(i);}
  // expecting pure elements
  vectoref& push_back(const T& r)
  { vector<T*>::push_back(&r); return *this; }
  void copy_from(const vectoref&& r) {
    for (size_type i = 0; i < r.size(); ++i)
      vectoref<T>::push_back(r[i]);
  }
};

class X { // user of initializer_list of A
  public:
  X() {}
  void f(initializer_list<A> l) const {
    cout << "In f({...}):\n";
    for (auto& a : l)
      a.ptr()->print();
  }
};

int main()
{
  A a(7), b(24), c(80);
  cout << "----------------------------------\n";
  vectoref<A> w{a,a,b,c}; // alternatively, use next line
  // vec<A,5> w{a,a,b,c}; // 5-th element undefined
  w[0].print();
  w[3].print();
  cout << "----------------------------------\n";
  X x;
  x.f({a,b,c,a,b,c,b,a});
  cout << "==================================\n";
  return 0;
}