尝试使用该类的结构中的变量创建自定义类的优先级队列 [英] Trying to make a priority queue of a custom class using a variable in a struct of that class

问题描述

所以这是我的类，目标是使 bnode 的优先级队列按顺序排列，以便具有最低计数符号的 bnode 具有最高优先级.这是我的代码:

So here are my classes, the goal is to make a priority queue of bnode that go in order, so that the bnode with the symbol with the lowest count has the highest priority. Here's my code:

struct symbol {
    explicit symbol(char av = 0, int ac = 0) : value(av), count(ac) { }
    char value; // actual symbol, by default 0 (empty)
    int count;  // count of the symbol, by default 0
}; // symbol

// compare two symbols
// symbol with a lower count is "less than" symbol with a higher count
inline bool operator<(const symbol& lhs, const symbol& rhs) {
    return ((lhs.count < rhs.count) || (!(rhs.count < lhs.count) && (lhs.value < rhs.value)));
} // operator<

template <typename T> struct bnode {
    explicit bnode(const T& t = T(), bnode* l = 0, bnode* r = 0)
        : value(t), left(l), right(r) { }

    T value;      // payload

    bnode* left;  // left child
    bnode* right; // right child
}; // struct bnode

#endif // SYMBOL_HPP

//and here is me trying to make a priority queue:

std::priority_queue<bnode<symbol>,std::vector<bnode<symbol> >, std::less<std::vector<bnode<symbol> >::value_type::value> > queue;

这会导致错误:错误:bnode::value"不能出现在常量表达式中

this results in the error: error: ‘bnode::value’ cannot appear in a constant-expression

推荐答案

你的priority_queue是存储bnode，所以需要提供operator< 代表 bnode.

Your priority_queue is storing bnode<symbol>, so you need to provide operator< for bnode.

template <typename T>
bool operator<(bnode<T> const& l, bnode<T> const& r)
{
    // perform comparison
}

完成此操作后，无需为 priority_queue 提供第二个和第三个模板参数，默认值即可.

Once you've done that, there's no need to provide the second and third template arguments for priority_queue, the defaults work.

std::priority_queue<bnode<symbol> > queue;

或者如果你想指定所有的模板参数

Or if you want to specify all the template arguments

std::priority_queue<bnode<symbol>, 
                    std::vector<bnode<symbol> >, 
                    std::less<bnode<symbol> > > queue;

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