typedef struct {
int i = 0;
int g = 0;

//to be compatible with vector.size() return data type
unsigned long int leftoff = 0;

char *path = nullptr; <-- I'm assuming that is right instead of NULL.

//holds file names
std::vector<std::string> dfile; <-- I have no idea how to initialize that

}options;


extern options opts;

options opts;

#include <iostream>
#include "strt.h"
int main(){
options opts;
return 0;
}

#ifndef STRT_H
#define STRT_H

#include <vector>
typedef struct {
int i = 0;
int g = 0;

//to be compatible with vector.size() return data type
unsigned long int leftoff = 0;
char *path = NULL;
//holds file names
std::vector<std::string> dfile;
}options;

extern options opts;
#endif

g++ -Weffc++ *.cpp

[userx@void gurp]$ g++ -Weffc++ *.cpp
In file included from dummy.cpp:2:0:
strt.h: In constructor 'options::options()':
strt.h:18:2: warning: 'options::dfile' should be initialized in the member initialization list [-Weffc++]
 }options;
  ^~~~~~~
dummy.cpp: In function 'int main()':
dummy.cpp:5:10: note: synthesized method 'options::options()' first required here
  options opts;
          ^~~~