02 Lesson2 Notes.md

Lesson 2 - The Anatomy of a C++ Program

// Preprocessor directive that includes header iostream 
#include <iostream> 
// Start of your program: function block main()
int main(){
  /* Write to the screen */
  std::cout << "Hello World" << std::endl;

  // Return a value to the OS 
  return 0;
}

Preprocessor Directive #include

Preprocessor directives are commands to the preprocessor and always start with a pound sign #.

• #include <filename> <> brackets are typically used when including standard headers.
• #include "...relative path to FileB\FileB" we use quotes " " when including our self-programmed headers.

The Body of Your Program main() Following

The execution of a C++ program always starts here. It is a standardized convention that function main() is declared with an int preceding it. int is the return value type of the function main() and stands for integer. Conventionally programmers return 0 in the event of success or –1 in the event of error.

The Concept of Namespaces

Namespaces are names given to parts of code that help in reducing the potential for a naming conflict. By invoking std::cout, you are telling the compiler to use that one unique cout that is available in the std namespace.
i.e. maybe there is another namespace called stdddd, and there is also a cout function in it, so that when you just use cout, the compiler will be confused that which cout you used, std::cout or stdddd::cout. Then if you tell the compiler that I will using namespace std, the compiler will know that that cout is std::cout.
There are three ways to use namespace:

• #include <iostream>
  using namespace std;  // That works on all functions.
  int func(){
    cout << "balabala";
  }
  int main(){
    cout << "balabala";
    // ...
  }

• #include <iostream>
  int func(){
    cout << "balabala";  // Error, can't use cout directly, must std::cout here.
  }
  int main(){
    using namespace std;  // That works only on the main() function.
    cout << "balabala";
    // ...
  }

• #include <iostream>
  int main(){
    using std::cout;  // Only use some specific name;
    cout << "balabala";
    // ...
  }

Function Declaration

When you define a function under the main() function and you want to invoke it in main(), then you need a function declaration above the main() function:

#include <iostream>
int func(int num);  // function declaration

int main(){
  int num = 1;
  func(num);
  // ...
  }
  
int func(int num){
  // ...
}

CAUTION

C++ is case-sensitive. So, expect compilation to fail if you write Int instead of int and Std::Cout instead of std::cout.

Anytime you want to use type string inside main(), you should include <string>:

#include <iostream>
#include <string>
using namespace std;

int main(){
  string name = "Tom";
  // ...
}