铛：错误：链接器命令失败，退出代码为1（使用-v查看调用）*关于全局变量

Question

I have a problem with compiling my files. I compiled and got this message

"Undefined symbols for architecture x86_64:
"_airport_label", referenced from:
  FlightMap::pathfind_before(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> > const&,             
std::__1::basic_string<char, std::__1::char_traits<char>, 
std::__1::allocator<char> > const&) in FlightMap-9a7ab0.o
  FlightMap::pathfind(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> > const&,  
std::__1::basic_string<char, std::__1::char_traits<char>, 
std::__1::allocator<char> > const&) in FlightMap-9a7ab0.o
"_dijkstra", referenced from:
  FlightMap::findShortestRoute(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> > const&, 
std::__1::basic_string<char, std::__1::char_traits<char>, 
std::__1::allocator<char> > const&) in FlightMap-9a7ab0.o
  FlightMap::pathfind_before(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> > const&, 
std::__1::basic_string<char, std::__1::char_traits<char>, 
std::__1::allocator<char> > const&) in FlightMap-9a7ab0.o
  FlightMap::fun_dijkstra(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> > const&) in FlightMap-9a7ab0.o
 (maybe you meant: __ZN9FlightMap12fun_dijkstraERKNSt3__112basic_stringIcNS0_11char_traitsIcEENS0_9allocatorIcEEEE)
"_distance_label", referenced from:
  FlightMap::pathfind_before(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> > const&, 
std::__1::basic_string<char, std::__1::char_traits<char>, 
std::__1::allocator<char> > const&) in FlightMap-9a7ab0.o
  FlightMap::pathfind(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> > const&, 
std::__1::basic_string<char, std::__1::char_traits<char>, 
std::__1::allocator<char> > const&) in FlightMap-9a7ab0.o
"_path", referenced from:
FlightMap::pathfind(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> > const&, 
std::__1::basic_string<char, std::__1::char_traits<char>, 
std::__1::allocator<char> > const&) in FlightMap-9a7ab0.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)"

I think there are some problems in global variable in FlightMap.h file.
(I edited Flight.h -> FlightMap.h) I declared 4 global variables like

stack<string> path;
map<string, string > airport_label;
map<double, string > distance_label;
map<string, double > dijkstra;

I have already tried to put "extern" in front of each global variable but it does not work.

In FlightMap.h file code(Edited Flight.h -> FlightMap.h)

#ifndef LAB6_FLIGHTMAP_H
#define LAB6_FLIGHTMAP_H

#include <iostream>
#include <string>
#include <stdexcept>
#include <map>
#include <list>
#include <vector>
#include <stack>
#include "AdjacencyListDirectedGraph.h"

 extern stack<string> path;
 extern map<string, string > airport_label;
 extern map<double, string > distance_label;
 extern map<string, double > dijkstra;
 .
 .
 .

Also, those global variables are used in FlightMap.cpp file.(I edited!)

I think that is where problems occur

Answer 1

Use fully qualified names in your header file's declarations:

#ifndef LAB6_FLIGHTMAP_H
#define LAB6_FLIGHTMAP_H

//#includes ...

extern std::stack<std::string> path;
extern std::map<std::string, std::string> airport_label;
extern std::map<double, std::string> distance_label;
extern std::map<std::string, double> dijkstra;
...
#endif

And in FlightMap.cpp you need to define these global variabes:

std::stack<std::string> path;
std::map<std::string, std::string> airport_label;
std::map<double, std::string> distance_label;
std::map<std::string, double> dijkstra;

extern tells the compiler that the variable will be defined somewhere, in another translation unit, and leaves resolving this up to the linker. The linker now has to find the definition in any of the compiled files, but can't since there is no definition anywhere. By actually putting the definition (that is, without extern ) in one file (and one file only), the linker will be happy.