没有用于初始化'mapped_type'std :: map错误的匹配构造函数

Question

我有一个名为'Card'的类,我试图将它的一些对象存储在std :: map Card.hpp中:

class Card
{
public:
    enum ValueType { NOVALUE, ACE };
    enum FaceType { NOFACE, CLUBS };

    Card(const ValueType & _value, const FaceType & _face);
    Card(const Card & _card);
private:
    ValueType m_value;
    FaceType m_face;
};

以下是我存储和访问它的方法:Deck.hpp:

#include <map>

class Card;

class Deck
{
    public:
        Deck();

        std::size_t length() const;

        Card get_card(const int & _num);

    private:
        std::map<int, Card> m_deck;
};

Deck.cpp:

#include "Card.hpp"

Deck::Deck()
{
    m_deck.insert(std::pair<int, Card>(0, Card(Card::NOVALUE, Card::NOFACE)));
    m_deck.insert(std::pair<int, Card>(1, Card(Card::ACE, Card::CLUBS)));
}
std::size_t Deck::length() const
{
    return m_deck.size();
}

Card Deck::get_card(const int & _num)
{
    return m_deck[_num];
}

现在,当我编译它时,我得到以下错误:

/usr/include/c++/4.6/bits/stl_map.h:453:45: error: no matching constructor for initialization of 'mapped_type' (aka 'Card')
          __i = insert(__i, value_type(__k, mapped_type()));
                                            ^
Deck.cpp:69:18: note: in instantiation of member function 'std::map<int, Card, std::less<int>, std::allocator<std::pair<const int, Card> > >::operator[]' requested here
    return m_deck[_num];
                 ^
./Card.hpp:30:2: note: candidate constructor not viable: requires 2 arguments, but 0 were provided
        Card(const ValueType & _value, const FaceType & _face);
        ^
./Card.hpp:32:2: note: candidate constructor not viable: requires 1 argument, but 0 were provided
        Card(const Card & _card);
        ^
1 error generated.

为什么我收到此错误？我只使用卡作为价值!

Answer 1

问题是对于地图,如果元素不存在m_deck[_num]则设计为插入默认构造Card.并且Card没有默认构造函数.

您可以使用map::find以查看条目是否存在,而无需创建条目.