We're always interested in getting feedback. E-mail us if you like this guide, if you think that important material is omitted, if you encounter errors in the code examples or in the documentation, if you find any typos, or generally just if you feel like e-mailing. Send your email to Frank Brokken.Please state the document version you're referring to, as found in the title (in this document: 5.2.0a) and please state the paragraph you're referring to.
All mail received is seriously considered, and new (sub)releases of the Annotations will normally reflect your suggestions for improvements. Except for the incidental case I will not otherwise acknowledge the receipt of suggestions for improvements. Please don't misinterpret this for lack of appreciation.
This document presents an introduction to programming in C++. It is a guide for C/C++ programming courses, that Frank gives yearly at the University of Groningen. As such, this document is not a complete C/C++ handbook, but rather serves as an addition to other documentation sources (e.g., the Dutch book De programmeertaal C, Brokken and Kubat, University of Groningen, 1996).
The reader should realize that extensive knowledge of the C programming language is assumed and required. This document continues where topics of the C programming language end, such as pointers, memory allocation and compound types.
The version number of this document (currently 5.2.0a) is updated when the contents of the document change. The first number is the major number, and will probably not be changed 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 Computing Center, University of Groningen, the Netherlands. This document was typeset using the yodl formatting system.
All rights reserved. No part of this document may be published or changed without prior consent of the author. Direct all correspondence concerning suggestions, additions, improvements or changes to this document to the author:
Frank B. Brokken
Computing Center, University of Groningen
Nettelbosje 1,
P.O. Box 11044,
9700 CA Groningen
The Netherlands
(email: f.b.brokken@rc.rug.nl)
In this chapter a first impression of C++ is presented. A few extensions to C are reviewed and a tip of the mysterious veil surrounding object oriented programming (OOP) is lifted.
mutable
keyword (section 6.6), and after thoroughly changing the
discussion of the Fork()
abstract base class (section 19.4). All
examples should now be up-to-date with respect to the use of the std
namespace.
fork()
system call in chapter 19. Under the
ANSI/ISO
standard many of the previously available extensions (like
procbuf
, and
vform()
) applied to streams were discontinued. Starting with version
5.1.1. ways of constructing these facilities under the ANSI/ISO standard are
discussed in the C++ Annotations. I consider the involved subject
sufficiently complex to warrant the upgrade to a new subversion.
g++
compiler version 3.00, a more
strict implementation of the
ANSI/ISO C++ standard became
available. This resulted in version 5.1.0 of the Annotations, appearing
shortly after version 5.0.0. In version 5.1.0 chapter 5 was
modified and several cosmetic changes took place (e.g., removing class
from template type parameter lists, see chapter 18). Intermediate
versions (like 5.0.0a, 5.0.0b) were not further documented, but were mere
intermediate releases while approaching version 5.1.0. Code
examples will gradually be adapted to the new release of the compiler.
In the meantime the reader should be prepared to insert
using namespace std;
in many code examples, just beyond the
#include
preprocessor directives as a temporary measure to make the example accepted by the compiler.
stringstream
class, replacing the
strstream
class is now
covered too (sections 5.4.3 and 5.5.3). Actually,
the chapter on input and output was completely rewritten. Furthermore, the
operators new
and delete
are now discussed in chapter 7,
where they fit better than in a chapter on classes, where they previously were
discussed. Chapters were moved, split and reordered, so that subjects could
generally be introduced without forward references. Finally, the
html
,
PostScript and
pdf versions of the C++ Annotations now contain an
index (
sigh of relief ?) All in, considering the volume and nature of the
modifications, it seemded right to upgrade to a full major version. So here it
is.
Considering the volume of the annotations, I'm sure there will be typos found every now and then. Please do not hesitate to send me mail containing any mistakes you find or corrections you would like to suggest.
4.4.1b
the pagesize in the LaTeX file was defined to
be din A4
. In countries where other pagesizes are standard the conversion
the default pagesize might be a better choice. In that case, remove the
dina4
option from cplusplus.tex
(or cplusplus.yo
if you have
yodl
installed), and reconstruct the annotations from the TeX-file or
Yodl
-files.
The Annotations mailing lists was stopped at release 4.4.1d
. From this
point on only minor modifications were expected, which are not anymore
generally announced.
At some point, I considered version 4.4.1
to be the final version of the
C++ annotations. However, a section on special I/O functions was added to
cover unformatted I/O, and the section about the string
datatype had its
layout improved and was, due to its volume, given a chapter of its own
(chapter 4). All this eventually resulted in version 4.4.2
.
Version 4.4.1
again contains new material, and reflects the
ANSI/ISO standard (well, I try to
have it reflect the
ANSI/ISO standard). In version 4.4.1. several new
sections and chapters were added, amon which a chapter about the
Standard Template Library (
STL) and
generic algorithms.
Version 4.4.0
(and subletters) was a mere construction version and was
never made available.
The version 4.3.1a
is a precursor of 4.3.2
. In 4.3.1a
most of the
typos I've received since the last update have been processed. In version
4.3.2.
extra attention was paid to the syntax for
function address
function addresses and
pointers to member functions.
The decision to upgrade from version 4.2.* to 4.3.* was made after realizing
that the lexical scanner function yylex()
can be defined in the
scanner class that is derived from yyFlexLexer
. Under this approach
the yylex()
function can access the members of the class derived from
yyFlexLexer
as well as the public and protected members of
yyFlexLexer
. The result of all this is a clean implementation of the rules
defined in the flex++
specification file.
The upgrade from version 4.1.* to 4.2.* was the result of the inclusion of section 3.3.1 about the bool data type in chapter 3. The distinction between differences between C and C++ and extensions of the C programming languages is (albeit a bit fuzzy) reflected in the introduction chapter and the chapter on first impressions of C++: The introduction chapter covers some differences between C and C++, whereas the chapter about first impressions of C++ covers some extensions of the C programming language as found in C++.
Major version 4 represents a major rewrite of the previous version 3.4.14: The document was rewritten from SGML to Yodl and many new sections were added. All sections got a tune-up. The distribution basis, however, hasn't changed: see the introduction.
Modifications in versions 1.*.*, 2.*.*, and 3.*.* were not logged.
Subreleases like 4.4.2a
etc. contain bugfixes and typographical
corrections.
C++ was originally a `pre-compiler', similar to the preprocessor of
C, which converted special constructions in its source code to plain
C. This code was then compiled by a normal 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 compiled and linked.
The nomenclature of C++ source files remains: the extensions .cc
and
.cpp
are usually still used. However, the preliminary work of a C++
pre-compiler is in modern compilers usually included in the actual compilation
process. Often compilers will determine the type of a source file by the
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 all
possibilities of C, and more. This makes the transition from C to
C++ quite easy. Programmers who are familiar with C may start
`programming in C++' by using source files with an extension .cc
or
.cpp
instead of .c
, and can then just comfortably slide into all the
possibilities that C++ offers. No abrupt change of habits is required.
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
.
In time several chapters were added, and the contents were modified thanks to countless readers who sent us their comment, due to which we were able to correct some typos and improve unclear parts.
The transition from major version three to major version four was realized by
Frank: again new chapters were added, and the source-document was converted
from SGML
to
Yodl(http://www.xs4all.nl/~jantien/yodl/).
The C++ Annotations are not freely distributable. Be sure to read the legal notes.If you like this document, tell your friends about it. Even better, let us know by sending email to Frank.Reading the annotations beyond this point implies that you are aware of the restrictions that we pose and that you agree with them.
In the Internet, many useful hyperlinks exist to C++. Without even suggesting completeness (and without being checked regularly for existence: they might have died by the time you read this), the following might be worthwhile visiting:
.cc
and
run it through a C++ compiler:
sizeof
('c')
equals sizeof(int)
, 'c'
being
any ASCII character. The underlying philosophy is probably that char
's,
when passed as arguments to functions, are passed as integers
anyway. Furthermore, the C compiler handles a character constant like
'c'
as an integer constant. Hence, in C, the function calls
putchar(10);and
putchar('\n');are synonyms.
In contrast, in C++, sizeof('c')
is always 1 (but see also section
3.3.2), while an int
is still an int
. As we shall see later (see
section 2.5.11), two function calls
somefunc(10);and
somefunc('\n');are quite separate functions: C++ discriminates functions by their arguments, which are different in these two calls: one function requires an
int
while the other one requires a char
.
extern void func();means in C that a function
func()
exists, which returns no
value. However, in C, the declaration doesn't specify which arguments (if
any) the function takes.
In contrast, such a declaration in C++ means that the function
func()
takes no arguments at all.
For
MS-Windows
Cygnus
(http://sources.redhat.com/cygwin) provides the foundation
for installing the Windows port of the
Gnu
g++
compiler.
When going to the above URL for 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 normally what you would want.
The most recent Gnu g++
compiler can be obtained from
http://gcc.gnu.org. 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 the 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.
2.2.3.2: Compiling a C++ source text
In general, compiling a C++ source source.cc
is done as follows:
g++ source.cc
This produces a binary program (a.out
or a.exe
). If the default
name is not wanted, the name of the executable can be specified using the
-o
flag:
g++ -o source source.cc
If only a compilation is required, the compiled module can be generated
using the -c
flag:
g++ -c source.cc
This produces the file source.o
, which can be linked to other modules
later on.
Using the
icmake program a
maintenance script can be used to assist in the construction and maintenance
of C++ programs. This script has been tested on
Linux platforms for
several years now. Its description and components are found in a file named
icmake-C1.61.tar.gz
(or comparably), which is found in the same location
as the icmake
program. Alternatively, the standard
make
program can
be used for maintenance of C++ programs. It is strongly advised to start
using maintenance scripts or programs early in the study of the C++
programming language.
Concerning the above allegations of C++, we think that the following can be concluded. The development of new programs while existing code is reused can also be realized in C by, e.g., using function libraries: thus, handy functions can be collected in a library and need not be re-invented with each new program. Still, C++ offers its specific syntax possibilities for code reuse, apart from function libraries (see chapter 13).
Creating and using new data types is also very well possible in C; e.g.,
by using struct
s, typedef
s etc.. From these types other types can be
derived, thus leading to struct
s containing struct
s and so on.
Memory management is in principle in C++ as easy or as difficult as in
C. Especially when dedicated C functions such as xmalloc()
and
xrealloc()
are used (these functions are often present in our
C-programs, they allocate or abort the program when the memory pool is
exhausted). In short, memory management in C or in
C++ can be coded `elegantly', `ugly' or anything in between --
this depends on the developer rather than on the language.
Concerning `bug proneness' we can say that C++ indeed uses stricter type checking than C. However, most modern C compilers implement `warning levels'; it is then the programmer's choice to disregard or heed a generated warning. In C++ many of such warnings become fatal errors (the compilation stops).
As far as `data hiding' is concerned, C does offer some tools. E.g.,
where possible, local or
static
variables can be used and special data
types such as struct
s can be manipulated by dedicated functions. Using
such techniques, data hiding can be realized even in C; though it must be
admitted that C++ offers special syntactical constructions. In contrast,
programmers who prefer to use a global variable int
i
for each counter
variable will quite likely not benefit from the concept of data hiding, be it
in C or C++.
Concluding, C++ in particular and OOP in general are not solutions to all programming problems. C++, however, does offer some elegant syntactical possibilities which are worthwhile investigating. At the same time, the level of grammatical complexity of C++ has increased significantly compared to C. In time we got used to this increased level of complexity, but the transition didn't take place fast or painless. With the Annotations we hope to help the reader to make the transition from C to C++ by providing, indeed, our annotations to what is found in some textbooks on C++. We hope you like this document and may benefit from it: Good luck!
static
).
In contrast, or maybe better: in addition to this, an object-oriented approach identifies the keywords in the problem. These keywords are then depicted in a diagram and arrows are drawn between these keywords to define an internal hierarchy. The keywords will be 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 some 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:
In the hierarchy of objects we would define the dependency between the first two objects by letting the car salesmen be `derived' from the owner and mechanics.
The overall process in the definition of a hierarchy such as the above starts with the description of the most simple type. Subsequently more complex types are derived, while each derivation adds a little functionality. From these derived types, more complex types can be derived ad infinitum, until a representation of the entire problem can be made.
In C++ each of the objects can be represented in a class, containing the necessary functionality to do useful things with the variables (called objects) of these classes. Not all of the functionality and not all of the properties of a class is usually available to objects of other classes. As we will see, classes tend to encapsulate their properties in such a way that they are not immediately accessible from the outside world. Instead, dedicated functions are normally used to reach or modify the properties of objects.
sin()
, operating on degrees without losing the
capability of using the standard sin()
function, operating on radians.
Namespaces are covered extensively in section 3.6. For now it
should be noted that most compilers require the explicit declaration of a
standard namespace:
std
. So, unless otherwise indicated, it is
stressed that all examples in the Annotations now implicitly use the
using namespace std;
declaration. So, if you intend to actually compile the examples given in
the Annotations, make sure that the sources start with the above using
declaration.
//
and ends with 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, over more than one line */ }
0
. In C, where pointers are
concerned,
NULL
is often used. This difference is purely stylistic, though
one that is widely adopted. In C++ there's no need anymore to use
NULL
. Indeed, according to the descriptions of the pointer-returning
operator new
0 rather than NULL
is returned when memory allocation
fails.
int main() { printf("Hello World\n"); }does often compile under C, though with a warning that
printf()
is
not a known function. Many C++ compilers will fail to produce code in such
a situation (When Gnu's
g++
compiler encounters an unknown
function, it assumes that an `ordinary' C function is meant. It does complain
however.). The error is of course the missing
#include <stdio.h>
directive.
Although, while we're at it: in C++ the function
main()
always
uses the int
return value. It is possible to define
int main()
,
without an
explicit return statement, but a
return
statement without an
expression cannot be given inside the main()
function: a return
statement in main()
must always be given an int
-expression. For
example:
int main() { return; // won't compile: expects int expression // omitting the above statement is ok too }
(typename)expression
in which typename
is the name of a valid type, and expression
an
expression. Following that, C++ initially also supported the function
call style cast notation:
typename(expression)
But, these casts are now called old-style casts, and they are deprecated. Instead, four new-style casts were introduced:
static_cast<type>(expression)
const
type-modification:
const_cast<type>(expression)
reinterpret_cast<type>(expression)
dynamic_cast<type>(expression)
is performed run-time to convert, e.g., a pointer to an object of a certain
class to a pointer to an object in its so-called class hierarchy. At this
point in the Annotations it is a bit premature to discuss the
dynamic_cast
, but we will return to this topic in section
14.5.1.
2.5.5.1: The `static_cast'-operator
The
static_cast<type>(expression)
operator is used to convert one type
to an acceptable other type. E.g., double
to int
. An example of such a
cast is, assuming intVar
is of type int
:
intVar = static_cast<int>(12.45);
Another nice example of code in which it is a good idea to use the
static_cast<>()
-operator is in situations where the arithmetic assignment
operators are used in mixed-type situations. E.g., consider the following
expression (assume doubleVar
is a variable of type double
):
intVar += doubleVar;
Here, the evaluated expression actually is:
intVar = static_cast<int>(static_cast<double>(intVar) + doubleVar);
IntVar
is first promoted to a double
, and is then added as
double
to doubleVar
. Next, the sum is cast back to an int
. These
two conversions are a bit overdone. The same result is obtained by explicitly
casting the doubleVar
to an int
, thus obtaining an int
-value for
the right-hand side of the expression:
intVar += static_cast<int>(doubleVar);
2.5.5.2: The `const_cast'-operator
The
const_cast<type>(expression)
operator is used to do away with the
const
-ness of a (pointer) type. Assume that a function
fun(char *s)
is available, which performs some operation on its
char *s
parameter. Furthermore, assume that it's known that the function
does not actually alter the string it receives as its argument. How can we use
the function with a string like char const hello[] = "Hello world"
?
Passing hello
to fun()
produces the warning
passing `const char *' as argument 1 of `fun(char *)' discards const
which can be prevented using the call
fun(const_cast<char *>(hello));
2.5.5.3: The `reinterpret_cast'-operator
The
reinterpret_cast<type>(expression)
operator is used to reinterpret
byte patterns. For example, the individual bytes making up a double
value
can easily be reached using a reinterpret_cast<>()
. Assume doubleVar
is a variable of type double
, then the individual bytes can be reached
using
reinterpret_cast<char *>(&doubleVar)
This particular example also suggests the danger of the cast: it looks as
though a standard C
-string is produced, but there is not normally a
trailing 0-byte. It's just a way to reach the individual bytes of the memory
holding a double value.
More in general: using the cast-operators is a dangerous habit, as it suppresses the normal type-checking mechanism of the compiler. It is suggested to prevent casts if at all possible. If circumstances arise in which casts have to be used, document the reasons for their use well in your code, to make double sure that the cast is not the underlying cause for a program to misbehave.
2.5.5.4: The `dynamic_cast'-operator
The
dynamic_cast<>()
operator is used in the context of
polymorphism. The discussion of this cast is postponed until section
14.5.1.
extern void func();means in C that the argument list of the declared function is not prototyped: the compiler will not be able to warn against improper argument usage. When declaring a function in C which has no arguments, the keyword
void
is used, as in:
extern void func(void);Because C++ enforces strict type checking, an empty parameter list is interpreted as the absence of any parameter. The keyword
void
can then
be omitted: in C++ the above two declarations are equivalent.
__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.
As an example, the following code fragment declares a function xmalloc()
which is a C function:
extern "C" void *xmalloc(unsigned 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> }The above presented methods can be used without problem, but are not generally used. A more frequently used method to declare external C functions is presented next.
__cplusplus
and of 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" { #endif // declaration of C-data and functions are inserted here. E.g., extern void *xmalloc(unsigned size); #ifdef __cplusplus } #endifUsing this setup, a normal C header file is enclosed by
extern "C"
{
which occurs at the start of the file and by }
,
which occurs at the end 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++.
An extra addition which is often seen is the following. Usually it is
desirable to avoid
multiple inclusions of the same header file. This can
easily be achieved by including an
#ifndef
directive in the header file.
An example of a file myheader.h
would then be:
#ifndef _MYHEADER_H_ #define _MYHEADER_H_ // declarations of the header file is inserted here, // using #ifdef __cplusplus etc. directives #endifWhen this file is scanned for the first time 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 for a second time during the same compilation,
the symbol _MYHEADER_H_
is defined. All information between the
#ifndef
and #endif
directives is skipped.
The symbol name _MYHEADER_H_
serves in this context 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 preprocessor directive
#include <iostream>
, rather
than #include <iostream.h>
in a source. In the Annotations this convention
is used with the standard C++ header files, but not everywhere else
(Frankly, we tend not to follow this convention: our C++ header files
still have the .h
extension, and apparently nobody cares...).
There is more to be said about header files. In section 6.5 the preferred organization of header files with C++ classes is discussed.
Furthermore, local variables can be defined in some statements, just prior to
their usage. A typical example is the for
statement:
#include <stdio.h> int main() { for (register int i = 0; i < 20; i++) printf("%d\n", i); return (0); }In this code fragment the variable
i
is created inside the for
statement. According to the ANSI-standard, the variable does not exist prior
to the for
-statement and not beyond the for
-statement. With some
compilers, the variable continues to exist after the execution of the
for
-statement, but a warning like
warning: name lookup of `i' changed for new ANSI `for' scoping using obsolete binding at `i'will be issued when the variable is used outside of the
for
-loop. The
implication seems clear: define a variable just before the for
-statement
if it's to be used after that statement, otherwise the variable can be
defined at the for
-statement itself.
Defining local variables when they're needed requires a little getting used to. However, eventually it tends to produce more readable code than defining variables at the beginning of compound statements. We suggest the following rules of thumb for defining local variables:
{
,
for
-statement, but
also all situations where a variable is only needed, say, half-way through the
function.
#include <stdio.h> void show(int val) { printf("Integer: %d\n", val); } void show(double val) { printf("Double: %lf\n", val); } void show(char *val) { printf("String: %s\n", val); } int main() { show(12); show(3.1415); show("Hello World\n!"); }In the above fragment three functions
show()
are defined, which only
differ in their parameter lists: int
, double
and char *
. The
functions have the same names. The definition of several functions having
identical names is called `
function overloading'.
It is interesting that the way in which the C++ compiler implements
function overloading is quite simple. Although the functions share the same
name in the source text (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 name void
show
(int)
to the
internal name VshowI
, while an analogous function with a char*
argument might be called VshowCP
. The actual names which are internally
used 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.
A few remarks concerning function overloading are:
show()
are still somewhat related (they print information to the screen).
However, it is also quite possible to define two functions lookup()
,
one of which would find a name in a list while the other would determine the
video mode. In this case the two functions have nothing in common except for
their name. It would therefore be more practical to use names which suggest
the action; say, findname()
and getvidmode()
.
printf("Hello World!\n");holds no information concerning the return value of the function
printf()
(The return value is, by the way, an integer which states
the number of printed characters. This return value is practically never
inspected.). Two functions printf()
which would only differ in their
return type could therefore not be distinguished by the compiler.
show(0);given the three functions
show()
above. The zero could be interpreted
here as a NULL
pointer to a char
, i.e., a (char *)0
, or as an
integer with the value zero. C++ will choose to call the function
expecting an integer argument, which might not be what one expects.
#include <stdio.h> void showstring(char *str = "Hello World!\n") { printf(str); } 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: the compiler will supply the missing argument when not specified. The code of the program becomes by no means shorter or more efficient.
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 when necessary. A statement as two_ints(,6)
is however not
allowed: when arguments are omitted they must be on the right-hand side.
Default arguments must be known to the compiler when the code is generated where the arguments may have to be supplied. Often this means that the default arguments are present in a header file:
// sample header file extern void two_ints(int a = 1, int b = 4); // code of function in, say, two.cc void two_ints(int a, int b) { ... }Note that supplying the default arguments in function definitions instead of in the header file is not the correct approach: the compiler will read the header file and not the function definition when the function is used in other sources. Consequently, in that case no default arguments can be inserted by the compiler.
typedef
is still allowed in C++, but no longer necessary
when used as a prefix in
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;
A definition of a struct point
is given in 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 y; // x/y void draw(void); // drawing function };A similar structure could be part of a painting program and could, e.g., represent a pixel in the drawing. Concerning this
struct
it should be
noted that:
draw()
which occurs in the struct
definition is
only a declaration. The actual code of the function, or in other words the
actions which the function should perform, are located elsewhere: in the code
section of the program, where all code is collected. We will describe the
actual definitions of functions inside struct
s later (see section
3.2).
struct
point
is just two int
s. Even
though a function is declared in the structure, its size is not affected
by this. The compiler implements this behavior by allowing the function
draw()
to be known only in the context of a point
.
point
structure could be used as follows:
point // two points on a, // screen b; a.x = 0; // define first dot a.y = 10; // and draw it a.draw(); b = a; // copy a to b b.y = 20; // redefine y-coord b.draw(); // and draw itThe function that is part of the structure is selected in a similar manner in which data fields are selected; i.e., using the field selector operator (
.
). When pointers to struct
s are used,
->
can be used.
The idea of this syntactical 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 function draw()
could be declared which would draw
the circle.