In file included from main.cpp:55:0:
visitor.h: In instantiation of 'std::map<int, long unsigned int, std::less<int>, std::allocator<std::pair<const int, long unsigned int> > > BfsVisitor<int>::distance':
visitor.h:9:19:   required from 'BfsVisitor<Vertex>::BfsVisitor() [with Vertex = int]'
main.cpp:63:21:   required from here
visitor.h:40:26: error: conversion from 'std::map<int, int, std::less<int>, std::allocator<std::pair<const int, int> > >' to non-scalar type 'std::map<int, long unsigned int, std::less<int>, std::allocator<std::pair<const int, long unsigned int> > >' requested
 std::map<Vertex, size_t> BfsVisitor<Vertex>::distance
                          ^~~~~~~~~~~~~~~~~~