Undefined Reference when compiling C++ - c++

Question

My code is similar to this one, but the problem is exactly the same: I'm getting an "undefined reference to `Test1::v" in Test1.cpp and in Test2.cpp when compilling the program in VSCode. What am I doing wrong? I'm a bit new on c++ so I just downloaded an extension that made me a project in c++ automatically. When I run the program using Ctrl + Shift + B it gives me this error, but when I do it with the Code Runner extension it doesn't detect the .cpp files.
// Test1.h
#include <iostream>
#include <vector>
using namespace std;
#ifndef TEST1_H
#define TEST1_H
class Test1{
public:
Test1();
static vector<Test1> v;
int a;
};
#endif
//Test1.cpp
#include "Test1.h"
Test1::Test1(){
a = 2;
v.push_back(*this);
}
//Test2.h
#include <iostream>
#include <vector>
using namespace std;
#ifndef TEST2_H
#define TEST2_H
class Test2{
public:
Test2();
double var;
};
#endif
//Test2.cpp
#include "Test2.h"
#include "Test1.h"
Test2::Test2(){
var = 5;
Test1::v[0].a += var;
}
//main.cpp
#include <iostream>
#include "Test1.h"
#include "Test2.h"
using namespace std;
int main(int argc, char *argv[])
{
cout << "Hello world!" << endl;
}

Answer 1

You have declared the static vector in the header file, but you need to define it in a cpp file. Add:
vector<Test1> Test1::v;
to your test1.cpp file. You can learn more about definition vs declaration here.
Also make sure you read this: Why is "using namespace std;" considered bad practice?

Answer 2

You could prepend the class name to call the variable directly since it's static. So, you could do something like:
Test1::Test1(){
// v.push__back(*this); // previous
Test1::v.push_back(*this); // now
}
in Test1.cpp. You'll then get a reference tooltip on your VS Code:
static std::vector<Test1> Test1::v
Which proves it's done.