Creating a function for stream objects
Author
Question: How do you create a function/class that can be called using a stream?
Something like this:
class Cat
{
public:
function operator<<(); // <- I know this is wrong
private:
string s1;
int i1;
};
function operator<<()
{
output the varibles to a ostream
}
main()
{
cout << Cat;
}
(yes, I know the code wont compile and doesn't initialize data, etc... I just someone to explain how to use << so I can have my classes save there data to a ostream using something like this)
I have looked everywhere, this site and others, and can find no clear cut example (maybe Im looking in the wrong places...)
#include <iostream>#include <string>class Cat{ public: // member functions private: std::string descriptiveName; int anotherDescriptiveName;};std::ostream& operator<<(std::ostream& stream, const Cat& cat){ // output the varibles to a ostream return stream;}int main(){ Cat c; std::cout << c;}Enigma
EDIT: forgot to make the cat parameter of operator<< const.
Since all the variables are private, you'll also want to have public get methods or else declare the function in the class as
friend std::ostream& operator<<(std::ostream& stream, const Cat& cat);
So that the private members are visible to the function.
friend std::ostream& operator<<(std::ostream& stream, const Cat& cat);
So that the private members are visible to the function.
That or have the operator<< delegate to a virtual print(std::ostream&) member function to do the stream insertions.
Author
I figured it out...
Here I have my class...
class WackyCat
{
data and functions and things, oh my!
}
Then, I do this:
ostream& operator<<(ostream& os, WackyCat &uWCat)
{
os << uWCat.whatevermemberfunction() << " ";
os << uWCat.whateverelsememberfunction() << " ";
return os;
}
It works pretty good! Anyone see any problems with it?
Here I have my class...
class WackyCat
{
data and functions and things, oh my!
}
Then, I do this:
ostream& operator<<(ostream& os, WackyCat &uWCat)
{
os << uWCat.whatevermemberfunction() << " ";
os << uWCat.whateverelsememberfunction() << " ";
return os;
}
It works pretty good! Anyone see any problems with it?
Looks OK to me. I'd make it take a const reference to the WackyCat instance though so the function isn't allowed to modify the data whilst outputting it.
Could also add some localization support, and using a virtual print member function could help remove the get functions from the class interface, and also allows you to output to the stream polymorphically to a greater extent than you can with your current implementation.
Author
//////////////////////////////////////////////////////
Looks OK to me. I'd make it take a const reference to the WackyCat instance though so the function isn't allowed to modify the data whilst outputting it.
//////////////////////////////////////////////////////
Sounds good, I'll add the const thingy so the data wont change.
/////////////////////////////////////////////////////
Could also add some localization support, and using a virtual print member function could help remove the get functions from the class interface, and also allows you to output to the stream polymorphically to a greater extent than you can with your current implementation.
//////////////////////////////////////////////////////
I haven't the faintest idea what your talking about, but thanks for helping... (I'm a beginner).
Looks OK to me. I'd make it take a const reference to the WackyCat instance though so the function isn't allowed to modify the data whilst outputting it.
//////////////////////////////////////////////////////
Sounds good, I'll add the const thingy so the data wont change.
/////////////////////////////////////////////////////
Could also add some localization support, and using a virtual print member function could help remove the get functions from the class interface, and also allows you to output to the stream polymorphically to a greater extent than you can with your current implementation.
//////////////////////////////////////////////////////
I haven't the faintest idea what your talking about, but thanks for helping... (I'm a beginner).
What the other guy is saying with the virtual print member functions is something like:
In Java, the paradigm is that every Object can provide a toString() method, which is called when needed for outputting. I suppose you could do something like this in C++, by implementing "operator std::string()", but then you get into some of C++'s weirder implicit-conversion idiosyncrasies eventually, and also will probably need a strstream when you write your implementation (so that it's kind of self-defeating).
class WackyCat { public: virtual ostream& printTo(ostream& os) { // do stuff for printing a 'normal' WackyCat: os << foo() << " " << bar() << " "; // and you can print data here too, even private stuff. // When I was in university, they taught me to handle // this by making ostream a friend class so that it could // access the needed public data directly. IMHO that's an // awful way of doing things though. return os; }}class WackierCat: public WackyCat { public: ostream& printTo(ostream& os) { // print different stuff because this cat is wackier! os << bar() << " " << baz() << " "; return os; }}ostream& operator<<(ostream& os, const WackyCat &uWCat) { uWCat.printTo(os); // at runtime, this will check which kind of WackyCat you have // and call the appropriate printTo() method. return os;}In Java, the paradigm is that every Object can provide a toString() method, which is called when needed for outputting. I suppose you could do something like this in C++, by implementing "operator std::string()", but then you get into some of C++'s weirder implicit-conversion idiosyncrasies eventually, and also will probably need a strstream when you write your implementation (so that it's kind of self-defeating).
This topic is closed to new replies.
Advertisement
Popular Topics
Advertisement
