How to Declare Variables

beginner c++11

In C++ every variable has its own type. There are built-in types such as int, float, and bool and standard library types such as std::string, std::map, and std::vector. You can also declare your own types. The type of a variable cannot change after it has been declared, an int will always be an int and a std::string will be a std::string. Variables are mutable by default in C++, meaning the value of a variable can change after it has been declared.

#include <string>
#include <iostream>

int main() {
  int x = 5;
  std::string name = "Baker";

  std::cout << x << " " << name << "\n";
  
  x = 40 + 2;
  name = "Rainier";

  std::cout << x << " " << name << "\n";
}

5 Baker
42 Rainier

Variables in C++ are mutable, meaning they can change their values over time. To declare a variable as immutable, or unchanging, in C++ we use const.

#include <string>
#include <iostream>

int main() {
  const int x = 42;
  const std::string name = "Rainier";

  std::cout << x << " " << name << "\n";
}

42 Rainier

You can use the keyword auto to let the compiler infer the type of a variable for you, but it will still always be the type the compiler infers. You can still declare auto variables as const.

Be careful with strings! The type of characters in quotes ("like this") is const char* not std::string.

#include <string>
#include <iostream>

int main() {
  auto x = 5; // int
  auto name = "Baker"; // const char*
  auto place = std::string{"Washington"}; // std::string

  const auto answer = 42; // const int

  std::cout << x << " " << name << " " << place << " " 
            << answer << "\n";

  x = 40 + 2; // still int
  name = "Rainier"; // still const char*
  place.append(", USA"); // still std::string
  
  std::cout << x << " " << name << " " << place << " " 
            << answer << "\n";
}

5 Baker Washington 42
42 Rainier Washington, USA 42