C++ Run down Series: variables, types and casting

 

Abstract 

This week we will discuss variables, types and casting between types. We'll discuss how C++ sees variables, and what are types for. Variables are named positions of memory used by the programmer to hold values, during the lifetime of a process. The positions may be constant, and thus hold the same value throughout the process lifetime or be modifiable (thus the name variable).

Variables

In the previous post we looked at the requirements to write C++ software on all the major systems, using a simple "Hello world" as a motivation.
This week, we'll look into what makes a variables, types and conversions.
Let us start with variables; variables are nothing but named positions on virtual memory. These names allow us to know what goes where while the compiler will deal with the actual position in memory.
In most languages the usage of a variable is transparent. Let us look at an assignment operation:

<variable> = <expression>;

The left side of a assignment operation must be a variable. This is both expected and reasonable: if we want to save the result of a expression's computation, we'll save it onto a variable.
In typed programming languages, which means that each variable has a associated type, before we can use a variable we must declare it.

int iResult;

iResult = 1+1; 

In the above example, iResult is a name which is declared to be a integer position. When the compiler encounters this line, it'll reserve a virtual memory position to hold a single integer.
The second line is an assignment. The compiler's syntax checker will look at the line and decide if it is well-formed instruction: if all the symbols are recognized, if the line ends in with a semi-colon, if the right side is a valid expression and if the left side is a known name. If any of these conditions fail, the compilation will fail.

Types

Types are named concepts for variables. They specify how the compiler encodes instructions to deal with the memory holding the variable.
There are several types already defined on the language. There are LiteralTypes, which consist on bool, char, int, float, long and double.
Types that are not LiteralTypes, like user-defined types also exist. The standard defines several, for instance std::string, std::fstream, or std::iterator.

A program may use all available types to define new variables. It is possible also for a programmer to specify her own types.

Every variable must have an associated type; this information is used by the compiler to check the correctness of the program: you'll be warned if you try to save a char in a int, and the program's compilation would halt if you tried to save a byte in a string.

However, other conversions are possible. A char can be saved into a byte. A integer can be narrowed to a char. And a long can hold a int.
By and large, all LiteralTypes can be converted into another LiteralType. Let us call the preexistant variable value's type a "predecessor type" and the destination type the "successor type".
Whenever a predecessor type is stored with less bytes than the successor type, there is no cause of worry. However, when the successor type is itself stored with less bytes than the predecessor, we have a narrowing conversion. Narrowing conversions may yield unexpected results and in extreme cases, these conversions may be the source of very strange behavior in runtime. The most common cases of such problems are the conversions of floating point types (float, double) to other literals of lesser storage (byte, int). The most dramatic situation involving a casting problem was the explosion of the Ariene 5, on the 4º of July in '95. A 64 bit floating point variable was narrowingly cast to a 16 bit integer, which led the guidance system astray.

Conversions

The conversions between types may be implicit or explicit. Implicit conversions are those which the compiler deduces the intervening types. The deduction takes into account the predecessor and successor types, and the compiler does its best to create code that does the conversion. If the compiler does not know of a way to do the conversion, the compiler stops the compilation process yielding an error.
An implicit conversion:

float a = 0.5f;

double b = a;

Explicit conversions are more programmer involved. The programmer specifies a conversion to be done through a casting operation.
An explicit casting operation:

int a = 1;

float b = (float) a;

In these operations, the type is specified before the predecessor variable appears. The (float) operator tells the compiler that a should be converted into a temporary float value and that value should be used to alter the memory that b holds.


There are other operators to handle type conversion. These are: dynamic_cast, reinterpret_cast, static_cast, and const_cast.
The general syntax is

<variable> = [dynamic,reinterpret,static,const]_cast< sucessor_type >( predecessor_value );

ex:

const float a = const_cast< const float >( 1 );

Each of these conversion primitives are similar to the explicit conversion above, however they have advantages over the explicit method. Most of the advantages are related to C++ virtual polymorphism but there three operators still relevant with LiteralTypes: const_cast, static_cast and reinterpret_cast which we'll talk about in another post.

If you liked the post, have any questions or suggestions, feel free to comment.
Next post, we'll deal with program control flow through conditionals.

C++ Run Down Series: requirements and "hello world"

C++ Run Down Series: requirements and "hello world"

Abstract  

In this series I'll explore C++ language and how it applies to modern software development; this week I'll present the basis of the language and a few historic notes. This week we will look into what is necessary to compile a C++ "Hello world" program, and discuss one example.

What is C++ 

C++ is a language developed by Bjarne Stroustrup in 1979, that is one of the major computer programming languages in the world. Based on C, it has similar syntax with its predecessor and several other C-inspired languages.
However, unlike C, C++ is a object oriented language that supports also procedural programming and can be even adapted to a functional programming pattern, which means it is a overall specially suited language for many realms of software development from embedded system development through kernel development up to client applications.

The major users of C++ are performance-critical system developers, those who need the systems to respond in real (or near real time). Thus, we can find C++ being employed in financial trading where a single microsecond may impact the net result of a transaction (check this TED talk for a discussion on this subject; check this C++ API of a financial trader for its clients) to game industry where one has multiprocessor systems to compute and react to world changes, user input, task management all within a 30 fps limit.

These high demanding environments motivated the development of the language into a high level language with multi-paradigm support with concerns with performance.

For a wannabe game developer like myself, this is a must-learn, must-use language.
I've been fortunate to develop my skills in many projects, each with different challenges than the next, and I'll try to guide through an awesome, but also gigantic language; hopefully, by the end of this C++ course, you'll be able to write your own programs with C++, meaning that you'll know what to search for when looking to accomplish a task.

Requirements for following the series: a Linux or OSX machine (preferred) or a Windows box.
If you have Linux, I advise you to install your package manager's build tools. All distros package managers have a package that represent your distro's build tools. For most Debian-derived distros something like "apt-get install build-essential g++" would suffice.

For OS X, you'll need the XCode command line tools, and you'll be good to go. You can install the package through XCode, in the Preferences->Downloads, choosing Components, and clicking Install next to Command Line Tools. Alternatively you can use Terminal, and use the command string "xcode-select --install".

If you use Windows you can choose one (or more) paths. The Microsoft's way would be to use Visual Studio. There was segment of the product called Visual Studio Express which was for non commercial usage. Nowadays you can get a Community Edition which is promoted as a full featured IDE.

If you use Windows but which to make cross compatible software from a free software tool set, I would recommend using MingW (with MSYS) and a open source IDE like CodeBlocks (I used this software for years, it is one hell of a IDE, props to the team). The MingW (with the MSYS) project allows you to use many open source tools like Make which are not native to Windows.

The other way you can go about this is using CygWin. CygWin project creates a Unix like environment on your Windows box, allowing you to create code that can be compiled on a CygWin-able Windows and a Unix machine.


If the difference between CygWin and MingW isn't clear, think about it this way: if you compile code with CygWin, you'll only be able to run the binary in a Windows with CygWin; if you compile code with MingW, all windows can run the binary. On the other hand, you can write fully POSIX compliant binaries with CygWin, while on MingW you'll have access to all POSIX support Microsoft mustered, and Win C++ API.

In doubt: CodeBlocks and MingW + MSYS because I can actually respond questions about those tools.

The rest of the series will present the code from a Linux's user perspective; if you have a Mac you'll probably be able to follow all the steps in order. If you have a Windows, CodeBlocks is straight forward.


So, the hello world example is overdue.

Open your terminal, and check if g++ is installed with
 

g++ --version

which should give the g++ version information if the installation was completed successfully. Open your tool of preference, either it being an IDE or a text editor (Linux users are encouraged to use vim or gvim). Lets begin. Our first program will be a "hello world" example. This is a simple, standard way, to approach a language. The main idea is to write a program that outputs a "Hello world" message.

#include <iostream>

int main() {
  std::cout << "Hello world" << std::endl;
  return 0;
}

The above C++ code has several interesting properties for such a simple program, each of which we'll explore in future posts of the series, as we'll dwell deeper into language.

In the first line, we are including part of the standard library of C++. This line gives the rest of the program access to the defined primitives in the iostream header.
The iostream header contains all the standard, cross-platform, definitions of Input and Output. It is this header file that declares the "std::cout" name we use a few lines down.

The "int main() {" line, declares and starts the definition of a function called "main". This function is the entry point of a program; execution starts at this function (unless otherwise configured) for all C and C++ programs.


The next line uses a name not declared on our program: std::cout. This names the "STandarD Console OUTput". The console is another name for the system's text terminal.
The name is also a object, a class instantiation -- we will explore object oriented programming at a later time -- which exports a way for the programmer to write to the console.
The "std::endl" is a cross platform name for the line terminator which is "\n" or "\r\n" depending on the system. This name also causes the program to write to the screen immediately .

The last instruction causes the control flow to terminate the program with the result 0.

Next week, we will start discussing variables, control flow conditionals and loops.
See you next week!