Error when coding a graph class in C++ -

i know debugging other people's code can frustrating want find out what's wrong mine. it's incomplete graph lot of functions not implemented yet. got stage , got stuck.

it's graph of adjacent list. has vertices , edges. got error cannot understand. can explain how error had come about? error

c:\users\jialee\documents\codeblocks\shortestpath\graph.cpp: in constructor 'edge::edge(vertex, weight)': c:\users\jialee\documents\codeblocks\shortestpath\graph.cpp:34:33: error: no matching function call 'vertex::vertex()'

and code

#include <forward_list> #include <string>  using namespace std;  const int max_size = 10000; typedef int weight;  class vertex { public:     vertex(string name, int num);     string city_name;     int city_num; };  class edge{ public:     edge(vertex v, weight w);     vertex associated_vertex;     weight weight; };  class graph{ public:     graph(int size); };  vertex::vertex(string name, int num){     city_name = name;     city_num = num; }  edge::edge(vertex v, weight cost){     associated_vertex = v;     weight = cost; }  graph::graph(int size = max_size){     forward_list<edge> g[size]; } 

the error says missing default constructor (constructor no arguments) vertex, required during construction of edge.
basically, edge constructor tries first default initialize members , assign passed values.

you can either add default constructor vertex class or (better) use initializer lists in edge constructor:

edge::edge(vertex v, weight cost):    associated_vertex{v},    weight{cost} { }