COMP345:
Advanced Program Design with C++
Lecture 3 Input/Output
Department of Computer Science and Software Engineering Concordia University
Contents
streams and stream operators
keyboard input console output
output formatting stream directives
file input/output MFC serialization
Advanced Program Design with C++ 3
Input and output: streams
C++ uses streams for input/output.
C++ programs can also use C-style input/output, which dont use streams.
Streams are objects that are defined to handle a device on which input or output operations are performed.
Streams allow C++ programs to interact with external entities controlled by the operating system, such as:
program execution console keyboard
files
Other external entities such as network sockets are not provided by basic C++ stream classes, but are available in various libraries (e.g. Boost).
Joey Paquet, 2007-2020
Advanced Program Design with C++ 4
Input and output: streams
I/Ostreamobjectscin,cout,cerr
Defined in the C++ library called
Must have these lines (pre-processor directives) :
#include
Tells C++ to use appropriate library so we can use the I/O objects cin, cout, cerr or #include
using std::cout;
To include only the cout object
cout is an object that is an instance of the ostream class, defined in the iostream library.
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Advanced Program Design with C++ 5
Input and output: streams
What can be outputted?
Any data can be outputted to a stream
Variables
Constants
Literals
Results of expressions
cout << numberOfGames + 1 << ” games played.”; 2 values are outputted: value of the expression numberOfGames + 1 literal string ” games played.” cout is a stream, << is the stream insertion operator that is used to output to the stream. Joey Paquet, 2007-2020 Advanced Program Design with C++ 6Input and output: streaming user-defined data types To enable a user-defined type to be outputted to a stream, the << operator must be overloaded to accept this type as an operand. May be overloaded as a free operator: Or as a friend operator to the class if it needs to access private members Operators and friends will be covered in detail later Joey Paquet, 2007-2020 Advanced Program Design with C++ 7End of line in output New lines in output Recall in C: ”
” is escape sequence for the char “newline” A second method: object endl Examples:cout << “Hello World
“; Sends string “Hello World” to display, and escape sequence ”
“, skipping to next linecout << “Hello World” << endl; Same result as above endl is a stream manipulator, which is a directive sent to a stream that instructs thestreams behavior. There are many stream manipulators, which we shall not cover here in detail. Joey Paquet, 2007-2020 Advanced Program Design with C++ 8Stream manipulators Formatting numeric values for outputcout << “The price is $” << price << endl; If price (declared double) has value 78.5, you might get The price is $78.500000or The price is $78.5 We must explicitly tell C++ how to output specially-formatted numbers in our programs using stream manipulators.Joey Paquet, 2007-2020 Advanced Program Design with C++ 9Stream manipulators Stream manipulators to force decimal sizes:cout << fixed << showpoint << setprecision(2); These directives force all future couted floating point values to have exactly two digits after the decimal point: fixed: set the stream to manipulate floating point numbers to be represented as fixed-point representation (as opposed to scientific notation, or integer notation) showpoint: show trailing zeroes in after the point, even if there are none. setprecision(n): set the number of digits of precision used for floating pointnumbers. cout << “The price is $” << price << endl; The price is $78.50Joey Paquet, 2007-2020 Advanced Program Design with C++ 10Keyboard input stream Use cin for input from the keyboard cin is an object that is an instance of the istream class. Uses “>> (stream extraction operator)
Its right-hand-side argument must be a variable.
cin >> num;
Waits on-screen for keyboard entry (reads keyboard until
Value entered at keyboard is assigned to the variable num
If the value is of wrong type, the operation fails and/or an exception is thrown.
Different C-style functions/operators can be used, e.g.: getline(istream, string)
getc(istream, char)
cin.get()
Joey Paquet, 2007-2020
Contents
streams and stream operators
keyboard input console output
output formatting stream directives
file input/output MFC serialization
Advanced Program Design with C++ 12
File input/output
Similar to cin/cout streams, there are streams for input/output from/to files File input : ifstream
File output : ofstream
Part of library
May use the same stream operators and formatting directives They are more complex to use compared to cin/cout:
Choice between different modes e.g text, binary, random-access
Different operators to use for different modes
May have to check for various stream states, which are set for example when operating on a non-existing file, or reaching the end of a file.
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Advanced Program Design with C++ 13
File input/output: Text files
Text file is the simplest file read/write mode Verysimilartocin/cout
Use<
Can also use C-style functions
get(): read a single character from a file in input text mode char ch;
ifstream input(myFile.txt);
ch = input.get();
put(): write a single character from a file in output text mode char ch {A};
ofstream output(myFile.txt);
output.put(ch);
getline(): read a string from a file in input text mode from the current position to a delimiter character
string str;
ifstream input(myFile.txt); getline(input, str, t);
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File input/output: open/close a file
In order to read/write to a file, it needs to be opened before and to attach a stream to it, and closed once the operation is over (1). This can be optionally done as the stream variable is declared (2).
Output
1. ofstream outputfilestream;
outputfilestream.open(scores.txt);
2. ofstream outputfilestream(scores.txt);
outputfilestream.close();
Input
1. ifstream inputfilestream;
infilestream.open(scores.txt);
2. ifstream inputfilestream(scores.txt);
inputfilestream.close();
Joey Paquet, 2007-2020
Advanced Program Design with C++ 15
File input/output: open/close a file
An fstream object can also be used, but in this case, file modes need to be specified:
fstream filestream; filestream.open(scores.txt, ios::out); //output operations filestream.close();
//do other things filestream.open(scores.txt, ios::in); //input operations
filestream.close();
Joey Paquet, 2007-2020
Advanced Program Design with C++ 16
File input/output: file modes
ios::in ios::out ios::app ios::ate
ios::trunc
the
ios::binary
Opens a file for input.
Opens a file for output.
Appends all output to the end of the file.
Opens a file for output. If the file already exists, move to the end of the file. Data can be written anywhere in the file.
Discards the files contents if the file already exists. (This is default action for ios:out).
Opens a file for binary input and output.
Joey Paquet, 2007-2020
Advanced Program Design with C++ 17
File input/output: text file output example
ofstream output;
// Create/open a file
output.open(scores.txt);
// Write two lines
output << “John” << ” ” << “T” << ” ” << “Smith” << ” ” << 90 << endl; output << “Eric” << ” ” << “K” << ” ” << “Jones” << ” ” << 85 << endl; // Close the fileoutput.close();…Joey Paquet, 2007-2020 Advanced Program Design with C++ 18File input/output: text file input exampleifstream input(“scores.txt”);string firstName, lastName; char mi; int score;input >> firstName >> mi >> lastName >> score;
cout << firstName << ” ” << mi << ” ” << lastName << ” ” << score << endl; input >> firstName >> mi >> lastName >> score;
cout << firstName << ” ” << mi << ” ” << lastName << ” ” << score << endl; input.close();…OR…string firstName, lastName; char mi; int score;ifstream input(“scores.txt”);input >> firstName >> mi >> lastName >> score;
while (!input.eof()) {
cout << firstName << ” ” << mi << ” ” << lastName << ” ” << score << endl;input >> firstName >> mi >> lastName >> score;
}
input.close();
Joey Paquet, 2007-2020
Advanced Program Design with C++ 19
File input/output: stream states and stream states functions
eof()
fail()
bad()
good()
clear()
ios::eofbit
ios::failbit
ios::hardfail
ios::badbit
ios::goodbit
returns true if the eofbit flag is set.
returns true if the failbit or hardfail flag is set returns true if the badbit flag is set
returns true is the goodbit flag is set
clear all stream state flags
set when the end of an input stream is reached set when an operation on the stream has failed set when an unrecovered error has occurred
set when an invalid operation has been attempted set if none of the preceding bits is set
Joey Paquet, 2007-2020
Advanced Program Design with C++ 20
File input/output: stream states and stream states functions example
#include
#include
#include
using namespace std;
int main() {
fstream inout;
inout.open(temp.txt, ios::out);
inout << “Dallas”;cout << “Normal operation (no errors)” << endl; showState(inout);inout.close();inout.open(“temp.txt”, ios::in);string city;inout >> city;
cout << “End of file (no errors)” << endl;showState(inout);inout.close();inout >> city;
cout << “Bad operation (errors)” << endl;showState(inout);return 0; }void showState(const fstream& stream){cout << “Stream status: ” << endl;cout << “eof(): ” << stream.eof() << endl;cout << “fail(): ” << stream.fail() << endl;cout << “bad(): ” << stream.bad() << endl;cout << “good(): ” << stream.good() << endl;} Joey Paquet, 2007-2020 Contentsstreams and stream operators keyboard input console output output formatting stream directives file input/outputMFC serialization Advanced Program Design with C++ 22File input/output: serialization Sometimes you may want to save objects to a file, or in general do what is called object serialization i.e. transform an object into a stream of bytes that can be transferred and/or saved/retrieved. Java provides object serialization though the java.io.Serializable interface. C++ does not provide such native solution, but there are two well-recognized ways toachieve that though libraries: MFCs CObject::Serialize() function Boosts Serialization The following serves as an example to demonstrate: How to use MFC serialization. That using libraries can be very peculiar. Joey Paquet, 2007-2020 Advanced Program Design with C++ 23File input/output: MFC serialization In each class that we want to serialize, in the cpp file: #include “DerivedRectangleFromAbstractGeometricObject.h”//Signify to MFC serialization that objects of this class are serializable //Parameter 1 : Name of the class//Parameter 2 : Name of the first non-abstract class up on the inheritance chain //Parameter 3 : Class schema version name. Must use same value across classes. IMPLEMENT_SERIAL(JRectangle, CObject, 1) implement the Serialize member function://! Serialize a Rectangle to/from a MFC CArchive//! @param ar : CArchive object to serialize to/from //! @return none//!void JRectangle::Serialize(CArchive& ar){ // Always call base class Serialize(). GeometricObject::Serialize(ar);// Serialize dynamic members and other raw dataif (ar.IsStoring()) // if the Carchive stream is open for output {}Joey Paquet, 2007-2020ar ar }else {<< width;<< height;// need to overload the << operator for your own classes // when you want to serialize them as part of an object// if the Carchive stream is open for inputar >> width;
ar >> height;
}
Advanced Program Design with C++ 24
File input/output: MFC serialization
In header file:
//! @file
//! @brief Header file for DerivedJRectangleFromAbstractGeometricObject.cpp //!
#ifndef JRectangle_H
#define JRectangle_H
#include AbstractGeometricObject.h
//! Rectangle class that is a subclass of the GeometricObject class
//! It needs to be a subclass of CObject in order to be serializable, and implement //! a Serialize() member function
class JRectangle : public GeometricObject
{
public:
JRectangle();
JRectangle(double width, double height);
JRectangle(double width, double height, const string& color, bool filled); double getWidth() const;
void setWidth(double);
double getHeight() const;
void setHeight(double);
double getArea() const;
double getPerimeter() const;
virtual void Serialize(CArchive& ar);
private:
double width; double height;
protected:
DECLARE_SERIAL(JRectangle);
}; #endif
Joey Paquet, 2007-2020
Advanced Program Design with C++ 25
File input/output: MFC serialization
To use the serialization-enabled class:
int main() {
CFile theFile;
//open a file in output mode
theFile.Open(_T(CArchiveTest.txt), CFile::modeCreate | CFile::modeWrite); //create a Carchive stream and connect it to the file
CArchive archive(&theFile, CArchive::store);
Circle *circle = new Circle(5, black, true); JRectangle *JRect = new JRectangle(5, 3, red, true);
//Serialize the objects into the file
circle->Serialize(archive); JRect->Serialize(archive);
delete circle; delete JRect; archive.Close(); theFile.Close();
CFile theOtherFile;
//open a file in input mode
theOtherFile.Open(_T(CArchiveTest.txt), CFile::modeRead);
//Create a CArchive and connect it to the file
CArchive otherArchive(&theOtherFile, CArchive::load); Circle *circle2 = new Circle();
JRectangle *JRect2 = new JRectangle();
//Serialize the objects out from the file
circle2->Serialize(otherArchive); JRect2->Serialize(otherArchive);
delete circle2; delete JRect2; otherArchive.Close(); theOtherFile.Close();
return 0;
Joey Paquet, 2007-2020
References
Y. Daniel Liang, Introduction to Programming with C++ (Chapter 1, 13), Pearson, 2014.
Bjarne Stroustrup, The C++ Programming Language (Chapter 30, 38), Addison-Wesley,
2013.
Microsoft Developer Network. Serialization: Making a Serializable Class.
http://msdn.microsoft.com/en-us/library/00hh13h0.aspx
Microsoft Developer Network. Serialization in MFC. http://msdn.microsoft.com/en- us/library/6bz744w8.aspx
Microsoft Developer Network. Storing and Loading CObjects via an Archive. http://msdn.microsoft.com/en-us/library/3bfsbt0t.aspx
Cplusplus.com. Input/output.
Barnette, McQuain. C++ Input/Output: Streams. Virginia Tech.
Kurt McMahon. Output Formatting. Northern Illinois University.
Joey Paquet COMP345 course Notes Concordia university.
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