IO and Namespaces

Output

The keyword cout will output. Use the << operator to output the data following it.

cout << "Output sentence";

Either write std::cout or use the std namespace as follows:

#include <iostream>
using namespace std;
cout << "hello";

For simplicity I will always implicitly include the std namespace

The output can also be chained into a single statement

cout << "This " << " is a " << "single C++ statement";

This is useful when you mix variables and literals together.

int age = 20;
int zipcode = 99999;
cout << "I am " << age << " years old and my zipcode is " << zipcode;

The std::endl or endl manipulator function (there are no arguments) will enter a newline and flush the output stream. The following code appears on two different lines.

cout << "My name is Nissan" << endl;
cout << "How are you doing?" << endl;

Input

The keyword cin or std::cin is for input

int age;
cin >> age;

The following will enter the input into the variable age when the ENTER key has been pressed. Notice >>

Strings are defined using std::string or string keyword and then variables' name. Don't forget to include string

#include <string> 
std::string str1;
string str2;

Putting it all together we get

string name;
cout << "What is your name? ";
cin >> name;
cout << endl << "Hello " << name << "!" << endl;

A little bit more difficult example is the following

#include <iostream>
#include <string>
int main(){
    std::string str;
    int a;
    double b;
    std::cin >> str >> a >> b;
    if(std::cin.fail()){
        std::cerr << "input problem\n";
        return 1;
    }
    std::cout << "I got: "<< str << ' '
    << a << ' ' << b << std::endl;
}

valid input

hi
4
6.7
output: I got: hi 4 6.7

invalid input

a
b
output: input problem   

Namespaces

Namespaces allow us to group named entities that otherwise would have global scope into narrower scopes, giving them namespace scope. This allows organizing the elements of programs into different logical scopes referred to by names.

To create a namespace use the namespace keyword. To use the namespace created use the :: operator

#include <iostream> 
using namespace std; 

// Variable created inside namespace 
namespace first { 
    int val = 500; 
} 


int main() { 
    int val = 200; // Local variable 
    cout << first::val << '\n';  //namespace var
    return 0; 
}

output

500  

Namespaces can also be used to access functions

#include <iostream>
using namespace std;
namespace ns  { 
    const double x = 100; 
    double value() {  return 2*x; } 
} 

int main() { 
    // Access value function within ns 
    cout << ns::value() << endl;  
    // Access variable x directly 
    cout << ns::x << endl;        
    return 0; 
}

output

200 
100

To access a namespace without explicitly calling each the the ns::variable you may do the use the using keyword

using namespace std;
namespace ns{
    const double x = 100;
    double value() {  return 2*x; }
}

using namespace ns;

int main(){
    cout << value() << endl; 
    cout << x << endl;
    return 0;
}

notice using namespace ns; on line 8. This allows us to use x and value in the namespace without using the :: operator

Note: It's actually best practice not to import the entire namespace into your program because it pollutes your namespace. This can lead to naming collisions. It's best to import only what you are using. So use std::cout instead of cout. A better solution is to use the namespace only where you need it, for example inside the function.

void foo(){
  using namespace std;
  using ns::context;
  // some implementation
};