error: no type named ‘value_type’ in ‘class
c++, templates
Solution
The comparison class should be the 3rd parameter of the `std::priority_queue` template, the second being a container (like `std::vector<typename Graph::graph<U>::vertex>`) which will have a `value_type` member.
Problem
I am getting the following compiler error: (`graph_algorithms.h:59:111: error: no type named ‘value_type’ in ‘class Graph::graph_algorithms<int>::vertex_comparer’`) What does this mean? The following line gives me a whole slew of compiler errors ``` std::priority_queue<typename Graph::graph<U>::vertex, typename Graph::graph_algorithms<U>::vertex_comparer> pri_que; ``` ``` #ifndef GRAPH_ALGORIUHMS_H_ #define GRAPH_ALGORIUHMS_H_ #include <queue> #include "graph.h" namespace Graph { template <class U> class graph_algorithms { // Forward declarations of // internal classes private : class vertex_comparer; public : graph_algorithms(graph<U> &graph) : m_Graph(graph) {} // List of algorithms to perform on graph typename std::queue<U> &find_key_breadth_first(U key); // Definition of internal classes private : class vertex_comparer { public: bool operator()(typename graph<U>::vertex &v1, typename graph<U>::vertex &v2) { return (v1.key() < v2.key()) ? true : false; } }; private : // Private member variables and methods graph<U> &m_Graph; std::queue<U> m_Queue; void breadth_first_search(U key); }; } ```