c++ Dijkstra's stupid question

I am getting stuck with my code in 1 last function:

unsigned long shortestPath(std::string startLabel, std::string endLabel, std::vector &path)

This function calculates the shortest distance from startLabel to endLabel. startLabel and endLabel are something like "USA" or "CAN" or "RUS" . Calculates the shortest path between the vertex with startLabel and the vertex with endLabel using Dijkstra's Algorithm. A vector is passed into the method that stores the shortest path between the vertices. The return value is the sum of the edges between the start and end vertices on the shortest path.

Please dont ask me to provide more information. My project has 5 functions, I have finished 4 of them which are:

void graph::addVertex(std::string label)

void graph::removeVertex(std::string label)

void graph::addEdge(std::string label1, std::string label2, unsigned long weight)

void removeEdge(std::string label1, std::string label2)

and

unsigned long shortestPath(std::string startLabel, std::string endLabel, std::vector &path)