I read that cout and cin are objects of classes std::istream and std::ostream. Can there be user defined objects like cout and cin ?
eg.
How can I do something like this :
ostream obj;
obj<<"string"<<endl;
EDIT 1 : I want to define an object that can replicate cout and cin without messing with their in-built definitions.
To handle strings I'd recommend using std::stringstream.
The std::stringstream class is derived from istream.
std::stringstream obj;
obj << "Hello World" << endl;
// You can convert it to a string afterwards
std::string myString = obj.str();
Yes, you can create any stream you want.
Since a stream is a flow of data with a source and a sink, you typically want to use either of the following:
std::stringstream - add data yourself, access it in stream form
std::{i,o}fstream - data comes from / goes to a file
std::cout and std::cin are particular instances of streams that happen to be connected to STDOUT and STDIN respectively, but there's no reason you can't make your own streams.
You just have to be precise and knowledgeable about what you actually want them to do.
Related
I am looking for a way to use an ostream with my custom string class and overload the operator << to stream to the buffer which i can then flush to anywhere i want (in my case its just gonna be printed in a window)
I'm reasonably new to the inner workings of iostream's but from my understanding the method i've seen of making std::stringbuf a base of my custom stringstream would not work because the stringbuf deals with the std::string.
in essence i want to able to do this (or similar):
MyStringClass string
MyOutput << "hello" << string << "World" << std::endl;
Where MyOutput can be changed to print to anywhere i want.
Thank you.
Not a problem. Define your class, and within it's definition add an ostream& operator<<(const String&);.
Inside that operator, you can code whatever handling you want (look at std::string for inspiration)
I'm working on a C++ beginner level project (not absolute beginner like "what's a loop" but I wouldn't say it's intermediate level either).
In this project I need to save into a file some data stored in memory in struct variables (this is plain imperative programming, with no OOP involved).
I've read a bit about options like serialization, using some non-standard libraries and such. But I need to keep it as simple and clean as possible.
So far I have 2 structs, pretty much like these:
struct client {
string name;
string address;
double phone;
};
struct invoice {
string client_name;
double total;
};
I'm looking for something like this example provided at http://www.cplusplus.com/doc/tutorial/files:
#include <iostream>
#include <fstream>
using namespace std;
int main () {
ofstream myfile;
myfile.open ("example.txt");
myfile << "Writing this to a file.\n";
myfile.close();
return 0;
}
Is there a way to do something like that, but to write (and then be able to read) struct variables to a file, keeping it simple?
Some years ago I remember handling this in a very simple way in Pascal, when writing records to files. It was something like: open file, write record field 1, write field separador, write record field 2, write field separator, write record separator. Then when reading I would search for separators. Is this not recommended in C++?
Thanks.
You could check serialization for C++, eg. Boost::serialization in the Boost library.
Is it possible to serialize and deserialize a class in C++?
You have to associate 2 functions to this class : inputStruct and OutputStruct. Input should be able to read what Output generate. The easy way consist in writing each struct element in the same order that they are implemented.
The clear way to do that is implementing a serialize and deserialize function for every structure or class that you want to write to a file. You give the serialize function the reference of the output stream, and it writes each of the fields that you want to write. The deserialize do the opposite: it reads all the properties in the same order, and sets them in the current class or structure. If you use the stream operators for serialization, the output file will be a text file.
With c++ you can overload stream operator, so it will look pretty in your code, but in other languages you must use functions for that.
You can also use binary serialization, but it is more problematic, because you need to check the endianness of the platform that you use currently. If you will just use it on one platform, you can try write, and read functions. They need the pointer of the variable, and the size, and they copy them into/from the file. Use these for every property separate, never copy whole structures, because it can lead to errors easily.
UPDATE:
I made serialize and deserialize functions. I haven't tested them, so im not 100% sure they will work.
Static functions:
void serialize_client( ofstream& out, client& cl )
{
out << cl.name << endl;
out << cl.address << endl;
out << cl.phone << endl;
}
void deserialize_client( ifstream& in, client& cl )
{
getline( in, cl.name );
getline( in, cl.address );
in >> cl.phone;
}
Usage:
client client_instance;
deserialize_client( cin, client_instance );
serialize_client( cout, client_instance );
With operator overload:
ostream& operator<<( ostream& os, const client& cl )
{
os << cl.name << endl;
os << cl.address << endl;
os << cl.phone << endl;
return os;
}
istream& operator>>( istream& is, client& cl )
{
getlise( is, cl.name );
getlise( is, cl.address );
is >> cl.phone;
return is;
}
client client_instance;
cin >> client_instance;
cout << client_instance;
I've created an fstream object to write info to files.
I write strings to the new file like
fStreamObject << "New message.\n";
because I want each << to print a string to the next line.
I want to be able to set a property and make a call like
fstreamObject << "New message.";
which will write the string to the next line.
Are there flags/settings for fstream objects that allows this to be done?
I've seen the different file modes (i.e. ofstream::in, ofstream::out, etc.), but I couldn't find one that auto writes to a new line. Also, I'm not looking to write my own solution. I want to be able to use a built in feature.
No, there are no readily configurable capabilities of that sort within the standard streams.
You may have to subclass the stream type and fiddle with operator<< to get this to work the way you want, or do it with a helper function of some description:
fstreamObject << nl("New message.");
(but that's hardly easier than just having the \n in there (for a string, anyway).
It depends on what you mean by "setting the stream". If we consider this to be fairly broad then the answer happens to be "yes"!
Here is how:
Create a stream buffer which inserts a newline every time it is flushed, i.e., when sync() is called. Otherwise it just forwards characters.
Change the file stream's stream buffer to use this stream buffer filtering to the file stream's stream buffer.
Set the flag std::ios_base::unitbuf which causes a flush after every [properly written] output operation.
Here are is the example code to do just that:
#include <iostream>
class newlinebuf
: public std::streambuf {
std::ostream* stream;
std::streambuf* sbuf;
int overflow(int c) { return this->sbuf->sputc(c); }
int sync() {
return (this->sbuf->sputc('\n') == std::char_traits::eof()
|| this->sbuf->pubsync() == -1)? -1: 0;
}
public:
newlinebuf(std::ostream& stream)
: stream(&stream)
, sbuf(stream.rdbuf(this)) {
stream << std::unitbuf;
}
~newlinebuf() { this->stream->rdbuf(this->sbuf); }
};
int main() {
newlinebuf sbuf(std::cout);
std::cout << "hello" << "world";
}
Although this approach work, I would recommend against using it! On problem is that all composite output operators, i.e., those using multiple output operators to do their work, will cause multiple newlines. I'm not aware of anything which can be done to prevent this behavior. There isn't anything in the standard library which enables just configuring the stream to do this: you'll need to insert the newline somehow.
No, the C++ streams do not allow that.
There is no way to decide where one insertion stops and the next starts.
For example for custom types, their stream-inserters are often implemented as calls to other stream-inserters and member-functions.
The only things you can do, is write your own class, which delegates to a stream of your choosing, and does that.
That's of strictly limited utiliy though.
struct alwaysenter {
std::ostream& o;
template<class X> alwaysenter& operator<<(X&& x) {
o<<std::forward<X>(x);
return *this;
}
};
I'm adding some new functionality to some legacy code. The existing code reads some data from a text file. In the new version I'm going to be reading in much more data and want to use binary files, and on top of that the program could be used on Linux or Windows with the same (external) data file, so I want to enforce a big-endian sense when reading the binary data.
To that end I've created a new input file stream type - inherited from ifstream - with an overloaded ">>" operator that reads the binary data from file, interpreting it as big-endian. So far so good.
Now, when I'm reading data from file, I need to choose which type of input file stream object to create: regular ifstream when dealing with the old text files, or my new "iBinFile" type when dealing with the new binary files. The only solution I can come up with to this is to have two different pieces of code, one for the old type and one for the new type, which are identical apart from the input file stream type:
if (szFileName.compare(szFileName.size()-3,3,"bin")==0) {
iBinFile inFile(szFileName.c_str());
if (!inFile) {
cout << szFileName <<" file could not be opened" << endl;
exit (-1);
}
while(!inFile.eof())
inFile >> data;
}
else {
ifstream inFile(szFileName.c_str());
if (!inFile) {
cout << szFileName <<" file could not be opened" << endl;
exit (-1);
}
while(!inFile.eof())
inFile >> data;
}
But I feel like since iBinFile is derived from ifstream there should be a way to do it where the if statement only determines the file type and everything else is in common. If I was deriving iBinFile from my own class then I could make the ">>" operator virtual, but since it's not I don't know what the solution is, if there is one.
The abstraction for all of the current iostream classes is
formatted text. You do not want to derive from any of the
std::istream or std::ostream classes; you want to create
your own hierarchy. You probably do want to derive from
std::basic_ios<char>, for its error handling and streambuf
management. Similarly, do probably do want to use streambuf
and its derived classes.
I quite recently learned about the C++ classes friend keyword and the uses in serialization and now I need some help in getting it to work.
I have no problem serializing my class to a file, it's working great, however i'm having a hard time trying to read this file into a vector container. I'm sure I need a loop in my code that reads line by line, but since the class has different types I guess I can't use std::getline() and also maybe that approach wouldn't use the istream method i implemented?
A sample output file would be:
Person 1
2009
1
Person 2
2001
0
My code:
class SalesPeople {
friend ostream &operator<<(ostream &stream, SalesPeople salesppl);
friend istream &operator>>(istream &stream, SalesPeople &salesppl);
private:
string fullname;
int employeeID;
int startYear;
bool status;
};
ostream &operator<<(ostream &stream, SalesPeople salesppl)
{
stream << salesppl.fullname << endl;
stream << salesppl.startYear << endl;
stream << salesppl.status << endl;
stream << endl;
return stream;
}
istream &operator>>(istream &stream, SalesPeople &salesppl)
{
stream >> salesppl.fullname;
stream >> salesppl.startYear;
stream >> salesppl.status;
// not sure how to read that empty extra line here ?
return stream;
}
// need some help here trying to read the file into a vector<SalesPeople>
SalesPeople employee;
vector<SalesPeople> employees;
ifstream read("employees.dat", ios::in);
if (!read) {
cerr << "Unable to open input file.\n";
return 1;
}
// i am pretty sure i need a loop here and should go line by line
// to read all the records, however the class has different
// types and im not sure how to use the istream method here.
read >> employee;
employees.push_back(employee);
By the way, I know that the Boost library has a great serialization class, however I'm trying to learn how serialization would work using the STL library for now.
Thanks a lot in advance for any help that you can give me and for getting me in the right track!
It looks like you pretty much have all the code you need already! I copied your code and compiled it with some changes to read the SalesPeople in from a file in a loop. I will include the changes below, but since this is for your homework, you may just want to read and think about the following hints before looking at the code.
For reading the SalesPeople in a
loop, I would recommend that you take
a look at this FAQ. It has an
example of almost exactly what you
need. FAQ 15.4 will also help
you, I believe.
For your question on how to handle
the extra empty line when reading
from the file, check out this
link. You can very simply
extract whitespace this way.
As jfclavette suggested, I would
recommend looking into
std::getline for reading in the
SalesPerson's full name, since you
need everything on that line into one
string.
I have one question for you, though: what about the employeeID? I notice that it is being ignored in your sample code. Is that on purpose?
And now, if you still need help, you can check out the code I wrote to get this to work:
istream &operator>>(istream &stream, SalesPeople &salesppl)
{
//stream >> salesppl.fullname;
getline(stream, salesppl.fullname);
stream >> salesppl.startYear;
stream >> salesppl.status;
// not sure how to read that empty extra line here ?
stream >> ws;
return stream;
}
while(read >> employee)
{
// cout << employee; // to verify the input, uncomment this line
employees.push_back(employee);
}
Also, as jfclavette suggested, it may not be a bad idea to add some input validation (check the stream status after reading from it and verify that it is still good). Although I would recommend using the while() loop for the reasons stated in FAQ 15.5.
Not sure what your problem is. What exactly are you not understanding ? The fact that your names are composed of multiple tokens ? There's no magic way to do it, you might want to get the name trough getline(). Alternatively, you may want to specify the number of tokens when serializing and read the appropriate token count. ie, your file might look like.
2 Person 1
I assumed that Person was the first name and 1 the last name here. You might also enforce the notion that there's one first name, and one last name and just read each one separately.
You'll typically loop while (!ifstream.eof()) and read. Of course, you should always validate the inputs.
Also, why are you adding an extra endl between each record ? Serialized data need not be pretty. :)