First C++ program

Overview

Teaching: 5 min
Exercises: 5 min

Questions

• How do you create a C++ program?

Objectives

• Create, compile, and run your first C++ program

Here we will create our first C++ program and dissect its components and in doing so will introduce some important concepts that we will dive into in more detail as we go.

Creating a first C++ program

$ nano hello.cpp

#include <iostream>

int main(){
  std::cout<<"Hello, world!\n";
}

Use the GNU C++ compiler g++ from the GNU Compiler Collection (GCC) to build it:

$ g++ hello.cpp -o hello
$ ./hello

Hello, world!

Dissecting your first C++ program

Objects and classes

• cout is an object, an object can be thought of as a particular instantiation of a class.
  Think: int a; a is an instantiation of an int. In this example int would be considered the class and a would be the object. Though in this case int is a basic type and not actually a class but the relationship between int and a in the example is exactly the same as the relationship between an class and an object.
• A class is very much like a C struct with some associated functions.
• Classes can be thought of as a data type just like a struct can.
• cout is of type ostream, ostream is a class(see: https://www.cplusplus.com/reference/iostream/cout/)
• #include <iostream> includes a file that defines the ostream class and the cout object

Namespaces

• Namespaces are a way of grouping names:
  • names of functions, names of constants, names of objects, etc.
  • std is the “standard” namespace which contains the C++ standard library https://www.cplusplus.com/reference/
  • Helps to avoid name clashes, e.g. std::list versus some custom made list class

Operators

• << is an operator. Like +, -, etc.
• Operators can behave differently depending on the data type/class they operate on.
• For example the << operator will shift all the bits of a basic type (e.g. int, char etc.) to the left a given amount.
• For a complete list of operators with explanations: http://www.cplusplus.com/doc/tutorial/operators/

Nano line numbers

When compiling programs a compiler usually reports errors it encounters by line number so it is particularly important to be able to find a particular line in your editor.

There are a couple ways that nano can display line number/column information. Which methods are available will depend on your version of Nano.

1. While in nano press alt+c to display line number and column information for the line the cursor is currently on. This works for this particular nano session and on the version of nano on our training cluster.

2. Line numbers can be toggled on/off using alt+shift+#. This is nice, but only works on newer versions of nano.

3. When starting nano the -l option can be used to tell nano to display line numbers along the left hand side. Again only works with newer versions of nano.

4. Tell nano to always display line numbers and current line information:

   $ nano ~/.nanorc
   

   and enter the following two lines and save and exit

   set linenumbers
   set constantshow
   

   Note that set linenumbers will only work on newer versions of nano while set constantshow will work on older versions of nano also.

Useful Nano shortcuts

Here are some shortcuts that are helpful while editing text in nano

• alt+a: mark or select text starting from where the cursor is when these keys are pressed to where you move the cursor. Then selected text can be cut, or pasted over.

• ctl+k: cut text

• ctl+u: uncut (e.g. paste) previously cut text either from nano or elsewhere

• alt+u: undo last changes

What is a class?

Is a class:

1. a variable which can have multiple members like a struct

2. an object which can have multiple members like a struct

3. a datatype which can have multiple members like a struct

4. a datatype which can contain only a single value like int,float,etc.

Solution

1. NO: while a class can have multiple members like a struct, a class defines a new datatype where as a variable or object are the instantiation of that class or datatype.

2. NO: while an object has a class type, the object is not the class its self.

3. Yes: a class is a kind of datatype that can have multiple members.

4. No: classes can have multiple members, it is possible that they only have one member but they are not restricted to holding a single value.

Key Points

• g++ can be used to compiler and link C++ programs.

• Program execution starts in the main function.

• std::cout allows you to write strings to the terminal using the << operator.

• A class is a datatype.

• An object is a particular instance of that datatype or class.

• Namespaces group names and help to avoid name clashes.