C++ Fundamentals: Hit the ground running with C++, the language that supports tech giants globally

Source files contain the actual implementation code. Source files typically have the extension .cpp, although other extensions such as .cc, .ccx, or .c++ are also quite common.

On the other hand, header files contain code that describes the functionalities that are available. These functionalities can be referred to and used by the executable code in the source files, allowing source files to know what functionality is defined in other source files. The most common extensions for header files are .hpp, .hxx, and .h.

To create an executable file from the header and the source files, the compiler starts by preprocessing the directives (preceded by a # sign and generally at the top of the files) that are contained in them. In the preceding HelloUniverse program, the directive would be #include. It is preprocessed by the compiler before actual compilation and replaced with the content of the iostream header, which describes standard functionality for reading and writing from streams.

The second step is to process each source file and produce an object file that contains the machine code relative to that source file. Finally, the compilers link all the object files into a single executable program.

We saw that the preprocessor converts the content of the directives into the source files. Headers can also include other headers, which will be expanded, creating a chain of expansions.

For example, let's assume that the content of the logger.hpp header is as follows:

// implementation of logger

Let's also assume that the content of the calculator.hpp header is as follows:

#include <logger.hpp>
// implementation of calculator

In the main.cpp file, we include both directives, as shown in the following code snippet:

#include <logger.hpp>
#include <calculator.hpp>

int main() {
  // use both the logger and the calculator
}

The result of the expansion will be as follows:

// implementation of logger
// implementation of logger
// implementation of calculator
int main() {
  // use both the logger and the calculator
}

As we can see, the logger has been added in the resulting file twice:

Included files that are not directly specified in a #include directive in the file we are compiling, but are instead included by some other included file, are called transitive included files.

Often, including the same header file multiple times creates a problem with multiple definitions, as we will see in Lesson 2, Functions, and the Lesson 03, Classes.

Including the same file multiple times is very likely because of the transitive included files we explained before, and will often result in a compilation error. In C++, there is a convention to prevent problems that originate from including a header file multiple times: include guards.

An include guard is a specific pattern of instructing the preprocessor to ignore the content of the header if it has been included before.

It consists of writing all the header code inside the following structure:

#ifndef <unique_name>
#define <unique_name>
// all the header code should go here
#endif /* <unique_name> */

Here, <unique_name> is a name unique throughout the C++ project; it typically consists of the header file name, such as LOGGER_HPP for the logger.hpp header.

The preceding code checks whether a special preprocessor variable, <unique_name>, exists. If it does not exist, it defines it and it proceeds to read the content of the header. If it exists, it will skip all the code until the #endif part.

Since initially the special variable does not exist, the first time the preprocessor includes a header, it creates the variable and proceeds to read the file. The subsequent times, the variable is already defined, so the preprocessor jumps to the #endif directive, skipping all the content of the header file.

Compilation is a process that ensures that a program is syntactically correct, but it does not perform any checks regarding its logical correctness. This means that a program that compiles correctly might still produce undesired results:

Figure 1.2: Compilation and linking processes for an executable file

Every C++ program needs to define a starting point, that is, the part of the code the execution should start from. The convention is to have a uniquely named main function in the source code, which will be the first thing to be executed. This function is called by the operating system, so it needs to return a value that indicates the status of the program; for this reason, it is also referred to as the exit status code.

Let's see how we can compile a program.

Together with C, C++ is the language with the most supported hardware and platforms. This means that there are many C++ compilers, produced by many different vendors. Each compiler can accept parameters in a different way, and it's important to consult the manual of the compiler you are using when developing in C++ to understand the available options and their meaning.

We'll now see how to compile a program with two of the most common compilers: the Microsoft Visual Studio compiler and GCC.

To compile the myfile.cpp file in to an object file named myfile.obj, we can run the following commands:

Figure 1.3: Compiling the CPP file

When we compile, it is common to include some headers.

We can include the headers defined in the C++ standard without performing any action, but in case we want to include user-defined headers, we need to tell the compiler in which folders to look up the header files.

For MSVC, you need to pass the parameter as /I path, where path is the path to the directory to look in for the header.

For GCC, you need to pass the parameter as -I path, where path has the same meaning as in MSVC.

If myfile.cpp is including a header in the include directory, we would compile the file with the following commands:

Figure 1.4: Compiling the CPP file with the include directory

We can compile several files in their respective object files, and then link them all together to create the final application.

To link together two object files called main.obj and mylib.obj into an executable, we can run the following commands:

Figure 1.5: Compiling two object files

With MSVC, we will create an executable named main.exe, while with g++, the executable will be named main.

For convenience, MSVC and GCC offer a way to compile several files into an executable, without the need to create an object file for each file, and then link the files together.

Even in this case, if the files are including any user-defined header, you need to specify the header location with the /I or -I flags.

To compile the main.cpp and mylib.cpp files together, which uses some headers from the include folder, you can use the following commands:

Figure 1.6: Compiling files with include folder

In the next chapter, we will discuss functions in more depth; for now, we can define the main function, which does nothing, apart from returning a successful status code in the following way:

int main() 
{
  return 0;
}

The first line contains the definition of the function, constituted by the return type int, the name of the main function, and the list of arguments, which in this case is an empty list. Then, we have the body of the function, delimited by curly braces. Finally, the body is composed of a single instruction that will return a successful status code.

We will discuss these topics in more detail later; what is important to know is that this is a valid C++ program that can be compiled and executed.

In this exercise, we will create a source file named main.cpp containing the code. Compile the file and run the program. We will be using it to explore the C++ environment:

Primitive data types consist of the following types:

The character types char and wchar_t hold numeric values corresponding to the characters in the machine's character set.

The numeric types offered by the C++ programming language fall into three categories:

The signed and unsigned types come with different sizes, which means each of them can represent a smaller or larger range of values.

Integer types can be signed or unsigned, where signed types can be used to distinguish between negative or positive numbers, while unsigned can only represent numbers greater than or equal to zero.

The signed keyword is optional; the programmer only needs to specify it if the type is unsigned. Thus, signed int and int are the same types, but they are different from unsigned int, or just unsigned for brevity. Indeed, if it is not specified, an unsigned type always defaults to int.

Integers, as previously mentioned, can come in different sizes:

The short int type, or just short, is guaranteed to be at least 16 bits according to the standard. This means it can hold values in the range of -32768 to 32767. If it was also unsigned, so unsigned short int or just unsigned int, this range would be 0 to 65535.

A variable is named storage that refers to a location in memory that can be used to hold a value. C++ is a strongly-typed language and it requires every variable to be declared with its type before its first use.

The type of the variable is used by the compiler to determine the memory that needs to be reserved and the way to interpret its value.

The following syntax is used to declare a new variable:

type variable_name;

Variable names in C++ can contain letters from the alphabet, both upper and lower case, digits and underscores (_). While digits are allowed, they cannot be the first character of a variable name. Multiple variables of the same type can all be declared in the same statement by listing their variable names, separated by commas:

type variable_name1, variable_name2, …;

This is equivalent to the following:

type variable_name1;
type variable_name2;
type ...;

When declaring a variable, its value is left undetermined until an assignment is performed. It is also possible to declare a variable with a given value; this operation is also referred to as variable initialization.

One way – and probably the most common one – to initialize a variable, also referred to as C-like initialization, uses the following syntax:

type variable_name = value;

Another solution is constructor initialization, which we will see in detail in Lesson 3, Classes. Constructor initialization looks like this:

type variable_name (value);

Uniform initialization or list initialization introduces brace initialization, which allows for the initialization of variables and objects of different types:

type variable_name {value};

When a variable is initialized, the compiler can figure out the type needed to store the value provided, which means that it is not necessary to specify the type of the variable. The compiler is indeed able to deduct the type of the variable, so this feature is also referred to as type deduction. For this reason, the auto keyword has been introduced to replace the type name during initialization. The initialization syntax becomes this:

auto vvariable_name = value;

Another way to avoid directly providing a type is to use the decltype specifier. It is used to deduce a type of a given entity and is written with the following syntax:

type variable_name1;
decltype(variable_name1) variable_name2;

Here, variable_name2 is declared according to the type deducted from variable_name1.

In the following code, we will check a valid statement for variables by creating a new source file named main.cpp and analyzing the code one line at a time.

Which one of the following is a valid statement?

int foo;
auto foo2;
int bar = 10;
sum = 0;
float price = 5.3 , cost = 10.1;
auto val = 5.6;
auto val = 5.6f;
auto var = val;
int  a = 0, b = {1} , c(0);