static vs extern

file.cpp

#include "header.h"

std::string A::name = "David";
std::string surname = "Gavilan";

header.h

#include <string>

class A {
public:
    static std::string name;
};

extern std::string surname;

main.cpp

#include <iostream>
#include "header.h"

// g++-13 -std=c++17 -Wall -Wextra -o main *.cpp
int main() {
    std::cout << "Hello " << A::name << " " << surname << std::endl;
    return 0;
}