How to include global library in C++ class? [closed] - c++

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I have the main.cpp, a class Student and a global.h library.
I want the functions of global.h to be acessible everywhere, so I did this.
global.h
#include <iostream>
#include <cstdlib>
#include <string>
#include <vector>
using namespace std;
#ifndef GLOBAL_H
#define GLOBAL_H
int min(vector<int> v) {
int min = -99999999;
for (unsigned int i = 0; i < v.size(); i++) {
if (v[i] > min) min = v[i];
}
return min;
}
double average(vector<int> v) {
int sum = 0;
for (unsigned int i = 0; i < v.size(); i++) {
sum += v[i];
}
return (double)sum / v.size();
}
#endif /* GLOBAL_H */
Student.h
#include "global.h"
#ifndef STUDENT_H
#define STUDENT_H
class Student {
private:
string name;
vector<int> grades;
public:
Student();
void setName(string name);
void addGrade(int grade);
int getBestGrade();
double getAverageGrade();
};
#endif /* STUDENT_H */
Student.cpp
#include "Student.h"
Student::Student() {
}
void Student::setName(string name) {
this->name = name;
}
void Student::addGrade(int grade) {
this->grades.push_back(grade);
}
int Student::getBestGrade() {
return min(this->grades);
}
double Student::getAverageGrade() {
return average(this->grades);
}
main.cpp
#include "Student.h"
using namespace std;
int main(int argc, char** argv) {
Student a;
a.setName("John");
a.addGrade(15);
a.addGrade(13);
a.addGrade(20);
cout << a.getAverageGrade() << endl;
cout << a.getBestGrade() << endl;
return 0;
}
I get this error:
multiple definition of min(...)
multiple definition of average(...)
It seems I am including "global.h" multiple times. But I don't know where. Indeed, I use include "Student.h" two times. But I think the class won't work if I don't to it like this.
Please, help me find out how to include a global library inside a class.
Thanks
##############################
SOLUTION
Thanks to WhiteViking, I have now a solution.
The global.h must have a global.cpp.
global.h
#include <iostream>
#include <cstdlib>
#include <string>
#include <vector>
using namespace std;
#ifndef GLOBAL_H
#define GLOBAL_H
int min(vector<int> v);
double average(vector<int> v);
#endif /* GLOBAL_H */
global.cpp
#include "global.h"
int min(vector<int> v) {
int min = -99999999;
for (unsigned int i = 0; i < v.size(); i++) {
if (v[i] > min) min = v[i];
}
return min;
}
double average(vector<int> v) {
int sum = 0;
for (unsigned int i = 0; i < v.size(); i++) {
sum += v[i];
}
return (double)sum / v.size();
}

Answer 1

Your question does not show this in detail, but it seems you are defining funcX and funcY in global.h instead of just declaring them.
The preprocessor will replace all #include statements with the verbatim contents of those include files. This happens recursively. So after preprocessing, the compiler sees a "A.cpp" that includes the contents of global.h with the full definitions of funcX and funcY. (global.h was included indirectly via A.h. ) The same thing happens for Main.cpp.
After compilation, the object files for A.cpp as well as for Main.cpp will contain the compiled definitions of funcX and funcY. The error then happens when these object files are linked together in order to build the final executable. The linker will see multiple definitions of these functions and will error out. (It doesn't know/check/care if these definitions are actually identical.)
The solution is to only declare these functions in global.h and put their definition in a separate .cpp file, say global.cpp. For example:
In global.h:
// declarations only here
int funcX(int x);
int funcY(int x);
In global.cpp:
int funcX(int x)
{
return 2 * x;
}
int funcY(int x)
{
return x + 42;
}
In short: you were violating the so-called One Definition Rule (ODR).