C ++模板类：实例化错误

Question

I am doing a project using template classes. I am almost finish. However, I am getting warnings and errors about instantiation when I compile my program.

In file included from /usr/include/c++/4.7/utility:72:0,
             from test.cpp:2:
/usr/include/c++/4.7/bits/stl_pair.h: In instantiation of ‘std::pair<_T1, _T2>&  
std::pair<_T1, _T2>::operator=(std::pair<_U1, _U2>&&) [with _U1 = int; _U2 = char; _T1 = const int; _T2 = char; std::pair<_T1, _T2> = std::pair<const int, char>]’:
map1.hpp:30:5:   required from ‘cs540::Node<Key, Value>::Node(std::pair<const Key, 
Value>, cs540::Node<Key, Value>*, cs540::Node<Key, Value>*) [with Key = int; Value = char; cs540::Node<Key, Value> = cs540::Node<int, char>]’
map1.hpp:121:7:   required from ‘void cs540::Map<Key, Value>::insert_node(cs540::Node<Key, Value>*, std::pair<const Key, Value>) [with Key = int; Value = char]’
map1.hpp:433:5:   required from ‘cs540::Map<Key, Value>::Iterator cs540::Map<Key, Value>::insert(const std::pair<const Key, Value>&) [with Key = int; Value = char]’
test.cpp:12:23:   required from here

I feel as though Key and Value are already instantiate once I create an object of that class. I created a custom Map class that is suppose to have the functionality of std::map. I have all of my classes under a cs540 namespace.

Here is my Node class:

template<class Key, class Value> class Node{
    /*Class member variables*/
    private:
        std::pair<const Key,Value> data;
    public: 
        Node<Key,Value>* rightChild;
        Node<Key,Value>* leftChild;
        Node<Key,Value>* next;
        Node<Key,Value>* previous;
        Node() :rightChild(nullptr),
            leftChild(nullptr),
            next(nullptr),
            previous(nullptr){/*Default constructor*/ }
        Node(std::pair<const Key,Value> dataIn, Node* rightIn, Node* leftIn):rightChild(rightIn),leftChild(leftIn){
            data = std::make_pair(dataIn.first, dataIn.second);     
        }       
        ~Node(){/*Destructor*/ }
        Key getKey(){ return data.first;}
        Value getValue(){ return data.second;}
        void setData(const Key key, Value value){ data = std::make_pair(key,value); }
        std::pair<const Key,Value> getData(){ return data;}
};

And the functions that call the Node constructor

void insert_node(Node<Key,Value>* current, const std::pair<const Key,Value> newPair){

                if(current == nullptr){
                    num_size++;
                    current = new Node<Key,Value>(newPair,nullptr,nullptr);
                    return;
                }

                if(current->getKey() > newPair.first){
                    if(current->leftChild == nullptr){
                        num_size++;
                        current->leftChild = new Node<Key,Value>(newPair,nullptr,nullptr);
                        return;
                    }else{ insert_node(current->leftChild,newPair);}
                }else if(current->getKey() < newPair.first){
                    if(current->rightChild == nullptr){
                        num_size++;
                        current->rightChild = new Node<Key,Value>(newPair,nullptr,nullptr);
                        return;
                    }else{ insert_node(current->rightChild,newPair);}
                }else{  return;}
            }

Last but not least, my main function

int main(void){

   cs540::Map<int,char> mapper;
   mapper.insert({1,'g'});
   return 0;
}

Answer 1

You try to assign to data which has the first element const, so you cannot modify it. Instead of assigning in the constructor body use initialisation list in the same way you used for rightChild and leftChild:

data(dataIn.first, dataIn.second)