Chapter 2: Introduction

This document offers an introduction to the C++ programming language. It is a guide for C/C++ programming courses, yearly presented by Frank at the University of Groningen. This document is not a complete C/C++ handbook, as much of the C-background of C++ is not covered. Other sources should be referred to for that (e.g., the Dutch book De programmeertaal C, Brokken and Kubat, University of Groningen, 1996) or the on-line book suggested to me by George Danchev (danchev at spnet dot net).

The reader should be forewarned that extensive knowledge of the C programming language is actually assumed. The C++ Annotations continue where topics of the C programming language end, such as pointers, basic flow control and the construction of functions.

Some elements of the language, like specific lexical tokens (like digraphs (e.g., <: for [, and >: for ])) are not covered by the C++ Annotations, as these tokens occur extremely seldom in C++ source code. In addition, trigraphs (using ??< for {, and ??> for }) were removed from C++ in the C++17 standard.

The version number of the C++ Annotations (currently 10.8.1) is updated when the contents of the document change. The first number is the major number, and is probably not going to change for some time: it indicates a major rewriting. The middle number is increased when new information is added to the document. The last number only indicates small changes; it is increased when, e.g., series of typos are corrected.

This document is published by the Center of Information Technology, University of Groningen, the Netherlands under the GNU General Public License.

The C++ Annotations were typeset using the yodl formatting system.

All correspondence concerning suggestions, additions, improvements or changes to this document should be directed to the author:

Frank B. Brokken
Center of Information Technology,
University of Groningen
Nettelbosje 1,
P.O. Box 11044,
9700 CA Groningen
The Netherlands

In this chapter an overview of C++'s defining features is presented. A few extensions to C are reviewed and the concepts of object based and object oriented programming (OOP) are briefly introduced.

2.1: What's new in the C++ Annotations

This section is modified when the first or second part of the version number changes (and occasionally also for the third field of the version number). At a major version upgrade the entries of the previous major version are kept, and entries referring to older releases are removed.

2.2: C++'s history

The first implementation of C++ was developed in the 1980s at the AT&T Bell Labs, where the Unix operating system was created.

C++ was originally a `pre-compiler', similar to the preprocessor of C, converting special constructions in its source code to plain C. Back then this code was compiled by a standard C compiler. The `pre-code', which was read by the C++ pre-compiler, was usually located in a file with the extension .cc, .C or .cpp. This file would then be converted to a C source file with the extension .c, which was thereupon compiled and linked.

The nomenclature of C++ source files remains: the extensions .cc and .cpp are still used. However, the preliminary work of a C++ pre-compiler is nowadays usually performed during the actual compilation process. Often compilers determine the language used in a source file from its extension. This holds true for Borland's and Microsoft's C++ compilers, which assume a C++ source for an extension .cpp. The Gnu compiler g++, which is available on many Unix platforms, assumes for C++ the extension .cc.

The fact that C++ used to be compiled into C code is also visible from the fact that C++ is a superset of C: C++ offers the full C grammar and supports all C-library functions, and adds to this features of its own. This makes the transition from C to C++ quite easy. Programmers familiar with C may start `programming in C++' by using source files having extensions .cc or .cpp instead of .c, and may then comfortably slip into all the possibilities offered by C++. No abrupt change of habits is required.

2.2.1: History of the C++ Annotations

The original version of the C++ Annotations was written by Frank Brokken and Karel Kubat in Dutch using LaTeX. After some time, Karel rewrote the text and converted the guide to a more suitable format and (of course) to English in September 1994.

The first version of the guide appeared on the net in October 1994. By then it was converted to SGML.

Gradually new chapters were added, and the contents were modified and further improved (thanks to countless readers who sent us their comment).

In major version four Frank added new chapters and converted the document from SGML to yodl.

The C++ Annotations are freely distributable. Be sure to read the legal notes.

Reading the annotations beyond this point implies that you are aware of these notes and that you agree with them.

If you like this document, tell your friends about it. Even better, let us know by sending email to Frank.

2.2.2: Compiling a C program using a C++ compiler

Prospective C++ programmers should realize that C++ is not a perfect superset of C. There are some differences you might encounter when you simply rename a file to a file having the extension .cc and run it through a C++ compiler:

2.2.3: Compiling a C++ program

To compile a C++ program, a C++ compiler is required. Considering the free nature of this document, it won't come as a surprise that a free compiler is suggested here. The Free Software Foundation (FSF) provides at a free C++ compiler which is, among other places, also part of the Debian ( distribution of Linux (

To use the features of the C++14 standard the compiler flag --std=c++14 must currently be provided. In the C++ Annotations it is assumed that this flag is used when compiling the examples. To use the C++17 standard Gnu's g++-6 compiler can be used. To activate the C++17 standard specify the flag --std=c++1z. C++ under MS-Windows

For MS-Windows Cygnus ( provides the foundation for installing the Windows port of the Gnu g++ compiler.

When visiting the above URL to obtain a free g++ compiler, click on install now. This will download the file setup.exe, which can be run to install cygwin. The software to be installed can be downloaded by setup.exe from the internet. There are alternatives (e.g., using a CD-ROM), which are described on the Cygwin page. Installation proceeds interactively. The offered defaults are sensible and should be accepted unless you have reasons to divert.

The most recent Gnu g++ compiler can be obtained from If the compiler that is made available in the Cygnus distribution lags behind the latest version, the sources of the latest version can be downloaded after which the compiler can be built using an already available compiler. The compiler's webpage (mentioned above) contains detailed instructions on how to proceed. In our experience building a new compiler within the Cygnus environment works flawlessly. Compiling a C++ source text

Generally the following command can be used to compile a C++ source file `':
This produces a binary program (a.out or a.exe). If the default name is inappropriate, the name of the executable can be specified using the -o flag (here producing the program source):
        g++ -o source

If a mere compilation is required, the compiled module can be produced using the -c flag:

        g++ -c
This generates the file source.o, which can later on be linked to other modules. As pointed out, provide the compiler option --std=c++14 to activate the features of the C++14 standard.

C++ programs quickly become too complex to maintain `by hand'. With all serious programming projects program maintenance tools are used. Usually the standard make program is used to maintain C++ programs, but good alternatives exist, like the icmake or ccbuild program maintenance utilities.

It is strongly advised to start using maintenance utilities early in the study of C++.

2.3: C++: advantages and claims

Often it is said that programming in C++ leads to `better' programs. Some of the claimed advantages of C++ are: Which of these allegations are true? Originally, our impression was that the C++ language was somewhat overrated; the same holding true for the entire object-oriented programming (OOP) approach. The enthusiasm for the C++ language resembles the once uttered allegations about Artificial-Intelligence (AI) languages like Lisp and Prolog: these languages were supposed to solve the most difficult AI-problems `almost without effort'. New languages are often oversold: in the end, each problem can be coded in any programming language (say BASIC or assembly language). The advantages and disadvantages of a given programming language aren't in `what you can do with them', but rather in `which tools the language offers to implement an efficient and understandable solution to a programming problem'. Often these tools take the form of syntactic restrictions, enforcing or promoting certain constructions or which simply suggest intentions by applying or `embracing' such syntactic forms. Rather than a long list of plain assembly instructions we now use flow control statements, functions, objects or even (with C++) so-called templates to structure and organize code and to express oneself `eloquently' in the language of one's choice.

Concerning the above allegations of C++, we support the following, however.

C++ in particular (and OOP in general) is of course not the solution to all programming problems. However, the language does offer various new and elegant facilities which are worth investigating. At the downside, the level of grammatical complexity of C++ has increased significantly as compared to C. This may be considered a serious drawback of the language. Although we got used to this increased level of complexity over time, the transition was neither fast nor painless.

With the C++ Annotations we hope to help the reader when transiting from C to C++ by focusing on the additions of C++ as compared to C and by leaving out plain C. It is our hope that you like this document and may benefit from it.

Enjoy and good luck on your journey into C++!

2.4: What is Object-Oriented Programming?

Object-oriented (and object-based) programming propagates a slightly different approach to programming problems than the strategy usually used in C programs. In C programming problems are usually solved using a `procedural approach': a problem is decomposed into subproblems and this process is repeated until the subtasks can be coded. Thus a conglomerate of functions is created, communicating through arguments and variables, global or local (or static).

In contrast (or maybe better: in addition) to this, an object-based approach identifies the keywords used in a problem statement. These keywords are then depicted in a diagram where arrows are drawn between those keywords to depict an internal hierarchy. The keywords become the objects in the implementation and the hierarchy defines the relationship between these objects. The term object is used here to describe a limited, well-defined structure, containing all information about an entity: data types and functions to manipulate the data. As an example of an object oriented approach, an illustration follows:

The employees and owner of a car dealer and auto garage company are paid as follows. First, mechanics who work in the garage are paid a certain sum each month. Second, the owner of the company receives a fixed amount each month. Third, there are car salesmen who work in the showroom and receive their salary each month plus a bonus per sold car. Finally, the company employs second-hand car purchasers who travel around; these employees receive their monthly salary, a bonus per bought car, and a restitution of their travel expenses.
When representing the above salary administration, the keywords could be mechanics, owner, salesmen and purchasers. The properties of such units are: a monthly salary, sometimes a bonus per purchase or sale, and sometimes restitution of travel expenses. When analyzing the problem in this manner we arrive at the following representation: The hierarchy of the identified objects are further illustrated in Figure 1.

Figure 1: Hierarchy of objects in the salary administration.

The overall process in the definition of a hierarchy such as the above starts with the description of the most simple type. Traditionally (and still in vogue with some popular object oriented languages) more complex types are then derived from the basic set, with each derivation adding a little extra functionality. From these derived types, more complex types can be derived ad infinitum, until a representation of the entire problem can be made. Over the years, however, this approach has become less popular in C++ as it typically results in overly tight coupling, which in turns reduces rather than enhances the understanding, maintainability and testability of complex programs. In C++ object oriented program more and more favors small, easy to understand hierarchies, limited coupling and a developmental process where design patterns (cf. Gamma et al. (1995)) play a central role.

Nonetheless, in C++ classes are frequently used to define the characteristics of objects. Classes contain the necessary functionality to do useful things. Classes generally do not offer all their functionality (and typically none of their data) to objects of other classes. As we will see, classes tend to hide their properties in such a way that they are not directly modifiable by the outside world. Instead, dedicated functions are used to reach or modify the properties of objects. Thus class-type objects are able to uphold their own integrity. The core concept here is encapsulation of which data hiding is just an example. These concepts are further explained in chapter 7.

2.5: Differences between C and C++

In this section some examples of C++ code are shown. Some differences between C and C++ are highlighted.

2.5.1: The function `main'

In C++ there are only two variants of the function main: int main() and int main(int argc, char **argv).


2.5.2: End-of-line comment

According to the ANSI/ISO definition, `end of line comment' is implemented in the syntax of C++. This comment starts with // and ends at the end-of-line marker. The standard C comment, delimited by /* and */ can still be used in C++:
    int main()
        // this is end-of-line comment
        // one comment per line

            this is standard-C comment, covering
            multiple lines
Despite the example, it is advised not to use C type comment inside the body of C++ functions. Sometimes existing code must temporarily be suppressed, e.g., for testing purposes. In those cases it's very practical to be able to use standard C comment. If such suppressed code itself contains such comment, it would result in nested comment-lines, resulting in compiler errors. Therefore, the rule of thumb is not to use C type comment inside the body of C++ functions (alternatively, #if 0 until #endif pair of preprocessor directives could of course also be used).

2.5.3: Strict type checking

C++ uses very strict type checking. A prototype must be known for each function before it is called, and the call must match the prototype. The program
    int main()
        printf("Hello World\n");
often compiles under C, albeit with a warning that printf() is an unknown function. But C++ compilers (should) fail to produce code in such cases. The error is of course caused by the missing #include <stdio.h> (which in C++ is more commonly included as #include <cstdio> directive).

And while we're at it: as we've seen in C++ main always uses the int return value. Although it is possible to define int main() without explicitly defining a return statement, within main it is not possible to use a return statement without an explicit int-expression. For example:

    int main()
        return;     // won't compile: expects int expression, e.g.
                    // return 1;

2.5.4: Function Overloading

In C++ it is possible to define functions having identical names but performing different actions. The functions must differ in their parameter lists (and/or in their const attribute). An example is given below:
    #include <stdio.h>

    void show(int val)
        printf("Integer: %d\n", val);

    void show(double val)
        printf("Double: %lf\n", val);

    void show(char const *val)
        printf("String: %s\n", val);

    int main()
        show("Hello World!\n");
In the above program three functions show are defined, only differing in their parameter lists, expecting an int, double and char *, respectively. The functions have identical names. Functions having identical names but different parameter lists are called overloaded. The act of defining such functions is called `function overloading'.

The C++ compiler implements function overloading in a rather simple way. Although the functions share their names (in this example show), the compiler (and hence the linker) use quite different names. The conversion of a name in the source file to an internally used name is called `name mangling'. E.g., the C++ compiler might convert the prototype void show (int) to the internal name VshowI, while an analogous function having a char * argument might be called VshowCP. The actual names that are used internally depend on the compiler and are not relevant for the programmer, except where these names show up in e.g., a listing of the contents of a library.

Some additional remarks with respect to function overloading:

2.5.5: Default function arguments

In C++ it is possible to provide `default arguments' when defining a function. These arguments are supplied by the compiler when they are not specified by the programmer. For example:
    #include <stdio.h>

    void showstring(char *str = "Hello World!\n");

    int main()
        showstring("Here's an explicit argument.\n");

        showstring();           // in fact this says:
                                // showstring("Hello World!\n");
The possibility to omit arguments in situations where default arguments are defined is just a nice touch: it is the compiler who supplies the lacking argument unless it is explicitly specified at the call. The code of the program will neither be shorter nor more efficient when default arguments are used.

Functions may be defined with more than one default argument:

    void two_ints(int a = 1, int b = 4);

    int main()
        two_ints();            // arguments:  1, 4
        two_ints(20);          // arguments: 20, 4
        two_ints(20, 5);       // arguments: 20, 5
When the function two_ints is called, the compiler supplies one or two arguments whenever necessary. A statement like two_ints(,6) is, however, not allowed: when arguments are omitted they must be on the right-hand side.

Default arguments must be known at compile-time since at that moment arguments are supplied to functions. Therefore, the default arguments must be mentioned at the function's declaration, rather than at its implementation:

    // sample header file
    extern void two_ints(int a = 1, int b = 4);

    // code of function in, say,
    void two_ints(int a, int b)
It is an error to supply default arguments in function definitions. When the function is used by other sources the compiler reads the header file rather than the function definition. Consequently the compiler has no way to determine the values of default function arguments. Current compilers generate compile-time errors when detecting default arguments in function definitions.

2.5.6: NULL-pointers vs. 0-pointers and nullptr

In C++ all zero values are coded as 0. In C NULL is often used in the context of pointers. This difference is purely stylistic, though one that is widely adopted. In C++ NULL should be avoided (as it is a macro, and macros can --and therefore should-- easily be avoided in C++, see also section 8.1.4). Instead 0 can almost always be used.

Almost always, but not always. As C++ allows function overloading (cf. section 2.5.4) the programmer might be confronted with an unexpected function selection in the situation shown in section 2.5.4:

    #include <stdio.h>

    void show(int val)
        printf("Integer: %d\n", val);

    void show(double val)
        printf("Double: %lf\n", val);

    void show(char const *val)
        printf("String: %s\n", val);

    int main()
        show("Hello World!\n");

In this situation a programmer intending to call show(char const *) might call show(0). But this doesn't work, as 0 is interpreted as int and so show(int) is called. But calling show(NULL) doesn't work either, as C++ usually defines NULL as 0, rather than ((void *)0). So, show(int) is called once again. To solve these kinds of problems the new C++ standard introduces the keyword nullptr representing the 0 pointer. In the current example the programmer should call show(nullptr) to avoid the selection of the wrong function. The nullptr value can also be used to initialize pointer variables. E.g.,

    int *ip = nullptr;      // OK
    int value = nullptr;    // error: value is no pointer

2.5.7: The `void' parameter list

In C, a function prototype with an empty parameter list, such as
    void func();
means that the argument list of the declared function is not prototyped: for functions using this prototype the compiler does not warn against calling func with any set of arguments. In C the keyword void is used when it is the explicit intent to declare a function with no arguments at all, as in:
    void func(void);
As C++ enforces strict type checking, in C++ an empty parameter list indicates the total absence of parameters. The keyword void is thus omitted.

2.5.8: The `#define __cplusplus'

Each C++ compiler which conforms to the ANSI/ISO standard defines the symbol __cplusplus: it is as if each source file were prefixed with the preprocessor directive #define __cplusplus.

We shall see examples of the usage of this symbol in the following sections.

2.5.9: Using standard C functions

Normal C functions, e.g., which are compiled and collected in a run-time library, can also be used in C++ programs. Such functions, however, must be declared as C functions.

As an example, the following code fragment declares a function xmalloc as a C function:

    extern "C" void *xmalloc(int size);
This declaration is analogous to a declaration in C, except that the prototype is prefixed with extern "C".

A slightly different way to declare C functions is the following:

    extern "C"
        // C-declarations go in here
It is also possible to place preprocessor directives at the location of the declarations. E.g., a C header file myheader.h which declares C functions can be included in a C++ source file as follows:
    extern "C"
        #include <myheader.h>
Although these two approaches may be used, they are actually seldom encountered in C++ sources. A more frequently used method to declare external C functions is encountered in the next section.

2.5.10: Header files for both C and C++

The combination of the predefined symbol __cplusplus and the possibility to define extern "C" functions offers the ability to create header files for both C and C++. Such a header file might, e.g., declare a group of functions which are to be used in both C and C++ programs.

The setup of such a header file is as follows:

    #ifdef __cplusplus
    extern "C"

        /* declaration of C-data and functions are inserted here. E.g., */
        void *xmalloc(int size);

    #ifdef __cplusplus
Using this setup, a normal C header file is enclosed by extern "C" { which occurs near the top of the file and by }, which occurs near the bottom of the file. The #ifdef directives test for the type of the compilation: C or C++. The `standard' C header files, such as stdio.h, are built in this manner and are therefore usable for both C and C++.

In addition C++ headers should support include guards. In C++ it is usually undesirable to include the same header file twice in the same source file. Such multiple inclusions can easily be avoided by including an #ifndef directive in the header file. For example:

    #ifndef MYHEADER_H_
    #define MYHEADER_H_
        // declarations of the header file is inserted here,
        // using #ifdef __cplusplus etc. directives
When this file is initially scanned by the preprocessor, the symbol MYHEADER_H_ is not yet defined. The #ifndef condition succeeds and all declarations are scanned. In addition, the symbol MYHEADER_H_ is defined.

When this file is scanned next while compiling the same source file, the symbol MYHEADER_H_ has been defined and consequently all information between the #ifndef and #endif directives is skipped by the compiler.

In this context the symbol name MYHEADER_H_ serves only for recognition purposes. E.g., the name of the header file can be used for this purpose, in capitals, with an underscore character instead of a dot.

Apart from all this, the custom has evolved to give C header files the extension .h, and to give C++ header files no extension. For example, the standard iostreams cin, cout and cerr are available after including the header file iostream, rather than iostream.h. In the Annotations this convention is used with the standard C++ header files, but not necessarily everywhere else.

There is more to be said about header files. Section 7.11 provides an in-depth discussion of the preferred organization of C++ header files.

2.5.11: Defining local variables

Although already available in the C programming language, local variables should only be defined once they're needed. Although doing so requires a little getting used to, eventually it tends to produce more readable, maintainable and often more efficient code than defining variables at the beginning of compound statements. We suggest to apply the following rules of thumb when defining local variables:

If considered appropriate, nested blocks can be used to localize auxiliary variables. However, situations exist where local variables are considered appropriate inside nested statements. The just mentioned for statement is of course a case in point, but local variables can also be defined within the condition clauses of if-else statements, within selection clauses of switch statements and condition clauses of while statements. Variables thus defined are available to the full statement, including its nested statements. For example, consider the following switch statement:

    #include <stdio.h>

    int main()
        switch (int c = getchar())
            case 'a':
            case 'e':
            case 'i':
            case 'o':
            case 'u':
                printf("Saw vowel %c\n", c);

            case EOF:
                printf("Saw EOF\n");

            case '0' ... '9':
                printf("Saw number character %c\n", c);

                printf("Saw other character, hex value 0x%2x\n", c);
Note the location of the definition of the character `c': it is defined in the expression part of the switch statement. This implies that `c' is available only to the switch statement itself, including its nested (sub)statements, but not outside the scope of the switch.

The same approach can be used with if and while statements: a variable that is defined in the condition part of an if and while statement is available in their nested statements. There are some caveats, though:

The latter point of attention should come as no big surprise: in order to be able to evaluate the logical condition of an if or while statement, the value of the variable must be interpretable as either zero (false) or non-zero (true). Usually this is no problem, but in C++ objects (like objects of the type std::string (cf. chapter 5)) are often returned by functions. Such objects may or may not be interpretable as numeric values. If not (as is the case with std::string objects), then such variables can not be defined at the condition or expression clauses of condition- or repetition statements. The following example will therefore not compile:
    if (std::string myString = getString())     // assume getString returns
    {                                           // a std::string value
        // process myString

The above example requires additional clarification. Often a variable can profitably be given local scope, but an extra check is required immediately following its initialization. The initialization and the test cannot both be combined in one expression. Instead two nested statements are required. Consequently, the following example won't compile either:

    if ((int c = getchar()) && strchr("aeiou", c))
        printf("Saw a vowel\n");
If such a situation occurs, either use two nested if statements, or localize the definition of int c using a nested compound statement:
    if (int c = getchar())             // nested if-statements
        if (strchr("aeiou", c))
            printf("Saw a vowel\n");

    {                                  // nested compound statement
        int c = getchar();
        if (c && strchr("aeiou", c))
           printf("Saw a vowel\n");

2.5.12: The keyword `typedef'

The keyword typedef is still used in C++, but is not required anymore when defining union, struct or enum definitions. This is illustrated in the following example:
    struct SomeStruct
        int     a;
        double  d;
        char    string[80];
When a struct, union or other compound type is defined, the tag of this type can be used as type name (this is SomeStruct in the above example):
    SomeStruct what;

    what.d = 3.1415;

2.5.13: Functions as part of a struct

In C++ we may define functions as members of structs. Here we encounter the first concrete example of an object: as previously described (see section 2.4), an object is a structure containing data while specialized functions exist to manipulate those data.

A definition of a struct Point is provided by the code fragment below. In this structure, two int data fields and one function draw are declared.

    struct Point            // definition of a screen-dot
        int x;              // coordinates
        int y;              // x/y
        void draw();        // drawing function
A similar structure could be part of a painting program and could, e.g., represent a pixel. With respect to this struct it should be noted that: The Point structure could be used as follows:
    Point a;                // two points on
    Point b;                // the screen

    a.x = 0;                // define first dot
    a.y = 10;               // and draw it

    b = a;                  // copy a to b
    b.y = 20;               // redefine y-coord
    b.draw();               // and draw it
As shown in the above example a function that is part of the structure may be selected using the dot (.) (the arrow (->) operator is used when pointers to objects are available). This is therefore identical to the way data fields of structures are selected.

The idea behind this syntactic construction is that several types may contain functions having identical names. E.g., a structure representing a circle might contain three int values: two values for the coordinates of the center of the circle and one value for the radius. Analogously to the Point structure, a Circle may now have a function draw to draw the circle.

2.5.14: Features introduced at the C++17 standard

The C++17 (also referred to as C++1z) standard will be implemented in the next major version upgrade of the Gnu g++ compiler (version 7.0.0 or beyond).

The working draft of the standard is freely available, and can be cloned from the git-repository at

In time the C++ Annotations will reflect the changes related to the C++17 standard. Several sections in the C++ Annotations directly relate to the C++17 standard. These sections can easily be located by looking for he C++17 entry in the table of contents or in the index.

In addition to the topics mentioned in separate sections the C++17 standard introduces the following features:

Evaluation order of operands of operators

Up to C++17, the evaluation order of expressions of operands of binary operators is, except for the boolean operators and and or, not defined. C++17 changes this for postfix expressions, assignment expressions (including compound assignments), and shift operators:

In the following examples first is evaluated before second, before third, before fourth, whether they are single variables, parenthesized expressions, or function calls:
    fourth += third = second += first
    first << second << third << fourth
    first >> second >> third >> fourth
In addition, when overloading an operator, the function implementing the overloaded operator is evaluated like the built-in operator it overloads, and not in the way function calls are generally ordered.


When statements that are nested under case entries in switch statements continue into subsequent case or default entries the compiler will issue a `falling through' warning. If falling through is intentional the attribute [[fallthrough]] should be used. Here is an annotated example:

void function(int selector)
    switch (selector)
        case 1:
        case 2:             // no falling through, but merged entry points
            cout << "cases 1 and 2\n";
        case 3:
            cout << "case 3\n";

        case 4:             // a warning is issued
            cout << "case 4\n";
        [[fallthrough]];    // error: nothing beyond


This attribute can be applied to a class, typedef-name, variable, non-static data member, a function, an enumeration or an enumerator. When it is applied to an entity no warning is generated when the entity is not used. Example:

void fun([[maybe_unused]] size_t argument)
    // argument isn't used, but no warning 
    // telling you so is issued


The attribute [[nodiscard]] may be specified when declaring a function, class or enumeration. If a function is declared [[nodiscard]] or if a function returns an entity previously declared using [[nodiscard]] then the return value of such a function may only be ignored when explicitly cast to void. Otherwise, when the return value is not used a warning is issued. Example:

int [[nodiscard]] importantInt();
struct [[nodiscard]] ImportantStruct { ... };

ImportantStruct factory();
int main()
    importantInt();         // warning issued
    factory();              // warning issued