Static in the house - C++ tales #1

What is static

In C++, static is a keyword that primarily affects a variable's storage duration. A variable declared as static exists for the entire lifetime of the program.

However, there is a little more to it. Depending on where it is used, static can also affect a symbol's visibility, which is known as linkage.

Static: The Local Hero

When a variable is declared inside a function, it can only be accessed within that function. This is true for both automatic and static local variables. The difference lies in their lifetime. Automatic variables are created every time the function is called and destroyed when the function returns. A local static variable, on the other hand, is initialized only once and remains alive until the program terminates.

Although the variable continues to exist after the function returns, it is still only accessible from within the function where it was declared. Because its lifetime extends beyond the function call, a local static variable is not typically stored on the stack. It also preserves its value between function calls.

Example time

Here is a little showcase code:

#include <cstdio>



void test_static() {

  static int count{};

  int other_count{};



  count++;

  other_count++;



  printf("The test_static func is called %d times!!!\n", count);

  printf("This is other_count >> %d\n", other_count);

}



int main() {

  test_static();

  test_static();

  test_static();



  return 0;

}

...and the output:

The test_static func is called 1 times!!!

This is other_count >> 1

-------------------------------------------------------

The test_static func is called 2 times!!!

This is other_count >> 1

-------------------------------------------------------

The test_static func is called 3 times!!!

This is other_count >> 1

-------------------------------------------------------

Here, we have a test_static function with a static and automatic variable: count and other_count. In the output, it can be easily what happens with this two little guy. The count - what is a static variable of the test_static funtion - keeps the value and in every function call it is incremented and keeps and increase the value while the other_count is incremented, too although it cannot keep the own value between the function calls.

Static, the member

The behavior of the static is pretty similar in classes but have some differencies, too.

Let's check this:

class PlanetOfTheSolarSystem {

  public:

    static std::string our_star;

    std::string planet_name;



    PlanetOfTheSolarSystem(std::string n) {

      this->planet_name = n;

    }



    std::string get_planet_name() {

      return this->planet_name;

    }

};



std::string PlanetOfTheSolarSystem::our_star{ "Sun (other name is Sol)" };

The PlanetOfTheSolarSystem class has two variable our_star and the planet_name. While the planet_name is declared in the constructor, the static varialbe - our_star - is declared outside of the class, and a different syntax is used to it. The reason is the static member belongs to class and not to the object.

void print_planets_star(std::string planet_name, std::string star_of_the_planet) {

  printf("Star of the %s is %s\n", planet_name.c_str(), star_of_the_planet.c_str());

}



int main() {

  PlanetOfTheSolarSystem venus{ "Venus" };

  PlanetOfTheSolarSystem earth{ "Earth" };



  print_planets_star(venus.get_planet_name(), venus.our_star);

  print_planets_star(earth.get_planet_name(), earth.our_star);



  return 0;

}

Star of the Venus is Sun (other name is Sol)

Star of the Earth is Sun (other name is Sol)

This example code and the output show us the essence of the static member. The venus and the earth objects point to same variable.

Conclusion

In a nutshell, static is an essential C++ feature for creating data that must persist throughout the program's execution. When used inside functions, it allows values to survive between function calls. When used as a class member, it creates data shared by all instances of the class. In other contexts, it can also affect symbol visibility and linkage.

Understanding static is important because it appears frequently in real-world C++ code and forms the basis of several common programming techniques and design patterns.

Thanks for reading, and keep learning!