std :: map<>的自定义键 [英] Custom Key for std::map<>

问题描述

我正在尝试使用以下struct作为std::map的自定义键:

I am trying to use the following struct as a custom key for std::map:

struct ActionId {
        // ENCAPSULATED MEMBERS
    private:
        size_t _id;
        static size_t _root;
        static size_t incrementedRoot() {
            return (_root += 1);
        }

        // INTERFACE
    public:
        ActionId() :
            _id(incrementedRoot()) { }
        ActionId(const ActionId& that) :
            _id(that._id) { }
        ActionId& operator=(const ActionId& that) {
            this->_id = that._id;
            return *this;
        }
        bool operator==(const ActionId& that) const {
            return this->_id == that._id;
        }
        bool operator!=(const ActionId& that) const {
            return this->_id != that._id;
        }
        bool operator<(const ActionId& that) const {
            return this->_id < that._id;
        } 
};

以下字典是单独的InputManager类的成员:

The following dictionary is a member of a separate InputManager class:

std::map<ActionId, std::set<sf::Keyboard::Key>> _keyBindings;

在该成员函数中访问的

:

which is accessed in this member function:

std::set<sf::Keyboard::Key> InputManager::keysBoundTo(ActionId action) const {
    return _keyBindings[action];
}

不幸的是，该函数抛出此编译器错误:

Unfortunately, the function is throwing this compiler error:

错误C2678:二进制'[':未找到采用'const std::map<Game2D::ActionId,std::set<sf::Keyboard::Key,std::less<_Kty>,std::allocator<_Kty>>,std::less<Game2D::ActionId>,std::allocator<std::pair<const Game2D::ActionId,_Ty>>>'类型的左操作数(或没有可接受的转换)的运算符

error C2678: binary '[' : no operator found which takes a left-hand operand of type 'const std::map<Game2D::ActionId,std::set<sf::Keyboard::Key,std::less<_Kty>,std::allocator<_Kty>>,std::less<Game2D::ActionId>,std::allocator<std::pair<const Game2D::ActionId,_Ty>>>' (or there is no acceptable conversion)

根据本文，具有const资格的ActionId的operator<()成员应足以将其设置为自定义地图键，而本文说，我需要做的只是使ActionId可复制和可分配.显然，我的结构同时满足这两个条件，那么为什么InputManager::keysBoundTo()不能编译?

According to this article, the operator<() member of ActionId with const qualification should be sufficient to make it a custom map key, while this article says that all I need is to make ActionId copiable and assignable. Clearly, my struct meets both of these criteria, so why won't InputManager::keysBoundTo() compile?

推荐答案

索引运算符("[]")是std :: map的非常量成员函数.而您已经清楚地表明keysBoundTo是const成员.

The index operator (the "[]") is a non-const member function of std::map. Whereas, you have clearly indicated that keysBoundTo is a const member.

将keysBoundTo重写为

std::set<sf::Keyboard::Key> InputManager::keysBoundTo(ActionId action) const
{
    auto it = keyBindigs_.find(action);
    if ( it == keyBindings_.end() )
        return std::set<sf::Keyboard::Key>();
    else
        return it->second;
}

请注意，我已将您的成员变量重命名为带有下划线.请勿在下划线前使用标识符.

Notice that I renamed your member variable to have a trailing underscore. Do not use identifiers with leading underscores.

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