C++ Introduction

Programming Workshop 2 (CSCI 1061U)

Faisal Qureshi

Faculty of Science, UOIT

http://vclab.science.uoit.ca


Compiled Languages and C++

Why Use a Language Like C++?

At its core, a computer is just a processor with some memory, capable of running tiny instructions like "store 5 in memory location 23459." Why would we express a program as a text file in a programming language, instead of writing processor instructions?

The advantages:

The Compilation Process

A program goes from text files (or source files) to processor instructions as follows:

Cpp compilation process

Cpp compilation process

Object files are intermediate files that represent an incomplete copy of the program: each source file only expresses a piece of the program, so when it is compiled into an object file, the object file has some markers indicating which missing pieces it depends on. The linker takes those object files and the compiled libraries of predefined code that they rely on, fills in all the gaps, and spits out the final program, which can then be run by the operating system (OS).

The compiler and linker are just regular programs. The step in the compilation process in which the compiler reads the file is called parsing.

In C++, all these steps are performed ahead of time, before you start running a program. In some languages, they are done during the execution process, which takes time. This is one of the reasons C++ code runs far faster than code in many more recent languages.

C++ actually adds an extra step to the compilation process: the code is run through a preprocessor, which applies some modifications to the source code, before being fed to the compiler. Thus, the modified diagram is:

Cpp preprocessor

Cpp preprocessor

Commandline tools

Cpp files are compiled using a C++ compiler/linker to create the executable.

Case 1

Using g++ to create an executable from a single file cpp program

$ g++ hello.cpp -o hello

The above creates a program hello from hello.cpp file.

Case 2

Using g++ to create an object file from a cpp file, and then linking the object file to create the executable.

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

The above creates a program hello from hello.o file. File hello.o is created by compiling the cpp file. Notice that we use the -c flag.

Case 3

Using g++ to compile/link a multi-file cpp program.

$ g++ -c file1.cpp
$ g++ -c file2.cpp
$ g++ file1.o file2.o -o myprogram

The above program creates two .o files and then link these to create the executable myprogram.

General Notes on C++

C++ is immensely popular, particularly for applications that require speed and/or access to some low-level features. It was created in 1979 by Bjarne Stroustrup, at first as a set of extensions to the C programming language. C++ extends C; our first few lectures will basically be on the C parts of the language.

Though you can write graphical programs in C++, it is much hairier and less portable than text-based (console) programs. We will be sticking to console programs in this course.

Everything in C++ is case sensitive: someName is not the same as SomeName.

Hello World

In the tradition of programmers everywhere, we'll use a "Hello, world!" program as an entry point into the basic features of C++.

The code

1 // A Hello World program 
2 #include <iostream>
3
4 int main() {
5   std::cout << "Hello, world!\n"; 
6
7 return 0;
8 }

Tokens

Tokens are the minimals chunk of program that have meaning to the compiler - the smallest meaningful symbols in the language. Our code displays all 6 kinds of tokens, though the usual use of operators is not present here:

Token type Description/Purpose Examples
Keywords Words with special meaning to the compiler int, double, for, auto
Identifiers Names of things that are not built into the language cout, std, x, myFunction
Literals Basic constant values whose value is specified directly in the source code "Hello, world!", 24.3, 0, 'c'
Operators Mathematical or logical operations +, -, &&, %, <<
Separators Separator/Punctuation defining the structure of a program {}(),;
Whitespace Spaces of various sorts; ignored by the compiler Spaces, tabs, newlines, comments

Line-By-Line Explanation

  1. // indicates that everything following it until the end of the line is a comment: it is ignored by the compiler. Another way to write a comment is to put it between /* and */ (e.g. x = 1 + /*sneaky comment here*/ 1;). A comment of this form may span multiple lines. Comments exist to explain non-obvious things going on in the code. Use them: document your code well!
Escape Sequence Represented Character
\a System bell (beep sound)
\b Backspace
\f Formfeed (page break)
\n Newline (line break)
\r "Carriage return" (returns cursor to start of line)
\t Tab
\\ Backslash
\| Single quote character
\" Double quote character
\some integer x, \67 The character represented by x
  1. return 0 indicates that the program should tell the operating system it has completed successfully. This syntax will be explained in the context of functions; for now, just include it as the last line in the main block.
  2. Note that every statement ends with a semicolon (except preprocessor commands and blocks using {}). Forgetting these semicolons is a common mistake among new C++ programmers.

Basic Language Features

So far our program doesn't do very much. Let's tweak it in various ways to demonstrate some more interesting constructs.

Values and Statements

First, a few definitions:

Not every statement is an expression. It makes no sense to talk about the value of an #include statement, for instance.

Operators

We can perform arithmetic calculations with operators. Operators act on expressions to form a new expression. For example, we could replace "Hello, world!\n" with (4 + 2) / 3, which would cause the program to print the number 2. In this case, the + operator acts on the expressions 4 and 2 (its operands).

Operator types:

Bitwise Operators

C++ supports the following 6 bitwise operators. Unlike logical operators, bitwise operators deal with individual bits in a variable. Consider the the following code

#include <iostream>
using namespace std;

int main(int argc, char** argv)
{
    unsigned char c1 = 'A';
    unsigned char c2 = 'B';

    cout << c1 << endl;
    cout << static_cast<int>(c1) << endl;
    cout << hex << static_cast<int>(c1) << endl;

    cout << c2 << endl;
    cout << static_cast<int>(c2) << endl;
    cout << hex << static_cast<int>(c2) << endl;

    return 0;
}

Here variable c1 stores letter A. We know that the ascii code for letter A is 65 decimal or 01000001 in binary. Similarly, variable c2 stores letter B. We know that the ascii code for letter B is 66 decimal or 01000010 in binary. Say we apply a bitwise operator NOT on variable c1 as follows

unsigned char not_c1 = ~c1;

NOT operator will flip every bit in c1, so not_c1 will contain 10111110.

Operator Descriptions
& AND - 1 if both bits are 1
| OR - 1 if one or both bits are 1
^ XOR - 1 if one bit is 1 and the other bit is 0
~ NOT - Flip a bit
<< Left shift - Shifts all bits left
>> Right shift - Shifts all bits right

AND

unsigned char c1_and_c2 = c1 & c2;

c1_and_c2 is 01000000

OR

unsigned char c1_or_c2 = c1 | c2;

c1_or_c2 is 01000011

XOR

unsigned char c1_xor_c2 = c1 ^ c2;

c1_xor_c2 is 00000011

Left shift

unsigned char left_shift_c1_by_1 = c1 << 1;

left_shift_c1_by_1 is 10000010

Right shift

unsigned char right_shift_c1_by_1 = c1 >> 1;

right_shift_c1_by_1 is 00100000

Data Types

Every expression has a type - a formal description of what kind of data its value is. For instance, 0 is an integer, 3.142 is a floating-point (decimal) number, and "Hello, world!\n" is a string value (a sequence of characters). Data of different types take a different amounts of memory to store. Here are the built-in datatypes we will use most often:

Type Names Description Size Range
char Single text character or small integer. Indicated with single quotes ('a', '3'). 1 byte signed: -128 to 127, unsigned: 0 to 255
int Larger integer. 4 bytes signed: -2147483648 to 2147483647, unsigned: 0 to 4294967295
bool Boolean (true/false). Indicated with the keywords true and false. 1 byte Just true (1) or false (0).
double "Doubly" precise floating point number. 8 bytes +/- 1.7e +/- 308 ( 15 digits)

Notes on this table:

An operation can only be performed on compatible types. You can add 34 and 3, but you can't take the remainder of an integer and a floating-point number.

An operator also normally produces a value of the same type as its operands; thus, 1 / 4 evaluates to 0 because with two integer operands, / truncates the result to an integer. To get 0.25, you'd need to write something like 1 / 4.0.

A text string, for reasons we will learn in Lecture 5, has the type char *.

Variables

We might want to give a value a name so we can refer to it later. We do this using variables. A variable is a named location in memory.

For example, say we wanted to use the value 4 + 2 multiple times. We might call it x and use it as follows:

1 #include <iostream> 
2 using namespace std;
3
4 int main() { 
5   int x;
6   x=4+2; 
7   cout << x/3 << '' << x*2;
8   return 0; 
9 }

(Note how we can print a sequence of values by "chaining" the << symbol.)

The name of a variable is an identifier token. Identifiers may contain numbers, letters, and underscores (_), and may not start with a number.

Line 5 is the declaration of the variable x. We must tell the compiler what type x will be so that it knows how much memory to reserve for it and what kinds of operations may be performed on it.

Line 6 is the initialization of x, where we specify an initial value for it. This introduces a new operator: =, the assignment operator. We can also change the value of x later on in the code using this operator.

We could replace lines 5 and 6 with a single statement that does both declaration and initialization: int x = 4 + 2;

This form of declaration/initialization is cleaner, so it is to be preferred.

Input

Now that we know how to give names to values, we can have the user of the program input values. This is demonstrated in line 6 below:

1 #include <iostream> 
2 using namespace std;
3
4 int main() { 
5   int x;
6   cin >> x;
7   cout << x/3 << '' << x*2;
8   return 0; 
9 }

Just as cout << is the syntax for outputting values, cin >> (line 6) is the syntax for inputting values.

Memory trick: if you have trouble remembering which way the angle brackets go for cout and cin, think of them as arrows pointing in the direction of data flow. cin represents the terminal, with data flowing from it to your variables; cout likewise represents the terminal, and your data flows to it.

Debugging

There are two kinds of errors you'll run into when writing C++ programs: compilation errors and runtime errors. Compilation errors are problems raised by the compiler, generally resulting from violations of the syntax rules or misuse of types. These are often caused by typos and the like. Runtime errors are problems that you only spot when you run the program: you did specify a legal program, but it doesn't do what you wanted it to. These are usually more tricky to catch, since the compiler won't tell you about them.

Credits