I Human Network: January 2011

char_traits

template <class charT> struct char_traits;

Character traits

C++ strings can be based on different types of characters. And each type of character can also have different properties, which are known as character traits.

Type Casting C++

Converting an expression of a given type into another type is known as type-casting. We have already seen some ways to type cast:

Implicit conversion

Implicit conversions do not require any operator. They are automatically performed when a value is copied to a compatible type. For example:

Exceptions provide a way to react to exceptional circumstances (like runtime errors) in our program by transferring control to special functions called handlers.

To catch exceptions we must place a portion of code under exception inspection. This is done by enclosing that portion of code in a try block. When an exceptional circumstance arises within that block, an exception is thrown that transfers the control to the exception handler. If no exception is thrown, the code continues normally and all handlers are ignored.

Namespaces C++

Namespaces allow to group entities like classes, objects and functions under a name. This way the global scope can be divided in "sub-scopes", each one with its own name.

The format of namespaces is:

namespace identifier { entities }

Function templates

Function templates are special functions that can operate with generic types. This allows us to create a function template whose functionality can be adapted to more than one type or class without repeating the entire code for each type.

Defined data types (typedef)

C++ allows the definition of our own types based on other existing data types. We can do this using the keyword typedef, whose format is:

streampos

Stream position type

Type to represent positions in a stream.

Objects of this class must support construction and conversion from int, and allow consistent conversions to/from streamoff objects, as well as the addition and subtraction of streamoff objects.

Another requirement is that two streampos objects must be able to be compared with == or != yielding a value convertible to bool, or be subtracted yielding an object of type streamoff.

It is a synonym of the instance of fpos with mbstate_t state type as template parameter.

See also

fpos

Stream position class template (class template)

streamoff

Stream offset type (type)

streamsize

Stream size type (types)

Polymorphism C++

Before getting into this section, it is recommended that you have a proper understanding of pointers and class inheritance. If any of the following statements seem strange to you, you should review the indicated sections:

Statement:	Explained in:
int a::b(int c) { }	Classes
a->b	Data Structures
class a: public b { };	Friendship and inheritance

Friend functions

In principle, private and protected members of a class cannot be accessed from outside the same class in which they are declared. However, this rule does not affect friends.

Friends are functions or classes declared with the friend keyword.

Overloading operators

C++ incorporates the option to use standard operators to perform operations with classes in addition to with fundamental types. For example:

1 2	`int a, b, c; a = b + c;`

Classes (I) C++

A class is an expanded concept of a data structure: instead of holding only data, it can hold both data and functions.

An object is an instantiation of a class. In terms of variables, a class would be the type, and an object would be the variable.

Data Structures

We have already learned how groups of sequential data can be used in C++. But this is somewhat restrictive, since in many occasions what we want to store are not mere sequences of elements all of the same data type, but sets of different elements with different data types.

template <class stateT> class fpos;

Stream position class template

Class template used as a template for types to indicate positions in streams. The template depends on the state type stateT.

The class is implementation-defined, but has at least two members:

1 2	`stateT state() const; void state(stateT);`

streamsize

Stream size type

Type to represent sizes in streams.

The type is an implementation-defined synonym of one of the signed basic integral types (generally signed intsigned long).
or

size_t

Unsigned integral type

size_t corresponds to the integral data type returned by the language operator sizeof and is defined in the <cstddef> header file (among others) as an unsigned integral type.

It expresses a size or count in bytes.

Dynamic Memory

Until now, in all our programs, we have only had as much memory available as we declared for our variables, having the size of all of them to be determined in the source code, before the execution of the program. But, what if we need a variable amount of memory that can only be determined during runtime? For example, in the case that we need some user input to determine the necessary amount of memory space.

streamoff

Stream offset type

Type to represent position offsets in a stream.

The underlying type is implementation defined, but must be able to be consistently converted to both streamsize and fpos (thus, to streampos too).

Construct string object C++

string ( );
string ( const string& str );
string ( const string& str, size_t pos, size_t n = npos );
string ( const char * s, size_t n );
string ( const char * s );
string ( size_t n, char c );
template<class InputIterator> string (InputIterator begin, InputIterator end);

Pointers

within memory, we simply used its identifier whenever we wanted to refer to our variable.

The memory of your computer can be imagined as a succession of memory cells, each one of the minimal size that computers manage (one byte). These single-byte memory cells are numbered in a consecutive way, so as, within any block of memory, every cell has the same number as the previous one plus one.

string

String class

String objects are a special type of container, specifically designed to operate with sequences of characters.

Character Sequences C++

As you may already know, the C++ Standard Library implements a powerful string class, which is very useful to handle and manipulate strings of characters. However, because strings are in fact sequences of characters, we can represent them also as plain arrays of char elements.

Arrays C++

An array is a series of elements of the same type placed in contiguous memory locations that can be individually referenced by adding an index to a unique identifier.

That means that, for example, we can store 5 values of type int in an array without having to

Arguments passed by value and by reference.

Until now, in all the functions we have seen, the arguments passed to the functions have been passed by value. This means that when calling a function with parameters, what we have passed to the function were copies of their values but never the variables themselves. For example, suppose that we called our first function addition using the following code:

Functions (I) C++

Using functions we can structure our programs in a more modular way, accessing all the potential that structured programming can offer to us in C++.

A function is a group of statements that is executed when it is called from some point of the program. The following is its format:

Control Structures C++

A program is usually not limited to a linear sequence of instructions. During its process it may bifurcate, repeat code or take decisions. For that purpose, C++ provides control structures that serve to specify what has to be done by our program, when and under which circumstances.

Basic Input/Output C++

Until now, the example programs of previous sections provided very little interaction with the user, if any at all. Using the standard input and output library, we will be able to interact with the user by printing messages on the screen and getting the user's input from the keyboard.

Operators C++

Once we know of the existence of variables and constants, we can begin to operate with them. For that purpose, C++ integrates operators. Unlike other languages whose operators are mainly keywords, operators in C++ are mostly made of signs that are not part of the alphabet but are available in all keyboards. This makes C++ code shorter and more international, since it relies less on English words, but requires a little of learning effort in the beginning.

Constants C++

Constants are expressions with a fixed value.

Literals

Literals are the most obvious kind of constants. They are used to express particular values within the source code of a program. We have already used these previously to give concrete values to variables or to express messages we wanted our programs to print out, for example,

Variables. Data Types. C++

The usefulness of the "Hello World" programs shown in the previous section is quite questionable. We had to write several lines of code, compile them, and then execute the resulting program just to obtain a simple sentence written on the screen as result. It certainly would have been much faster to type the output sentence by ourselves. However, programming

Structure of a program C++

Probably the best way to start learning a programming language is by writing a program. Therefore, here is our first program:

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// my first program in C++

#include <iostream>
using namespace std;

int main ()
{
  cout << "Hello World!";
  return 0;
}

Output: Hello World!

To whom is this tutorial directed?

This tutorial is for those people who want to learn programming in C++ and do not necessarily have any previous knowledge of other programming languages. Of course any knowledge of other programming languages or any general computer skill can be useful to better understand this tutorial, although it is not essential.

Input/Output with files

C++ provides the following classes to perform output and input of characters to/from files:

Home

Implicit conversion

Function templates

Defined data types (typedef)

Friend functions

Overloading operators

See also

See also

Arguments passed by value and by reference.

Literals

To whom is this tutorial directed?

Introduction

Basics of C++

Control Structures

Compound Data Types

Object Oriented Programming

Advanced Concepts

C++ Standard Library