Printing private variables in Class via overloaded '<<' - c++

Question

I'm switching from C to C++ and i have some basic but annoying problem with Class hermetization. I did some research here, but I'm probably really dumb and I can't use yor methods properly, could you help me?
The problem is printing private member of a class via overloaded '<<' operator.
Let's get to the code:
In file Vector.hh
#include <iostream>
class Vector {
double Val1, Val2, Val3;
public:
void PrintVector(Vector);
//There is also a constructor but it doesn't really matter
}
In File Vector.cpp
#include "Vector.hh"
#include <iostream>
void Vector::PrintVector(Vector Wek)
{
std::cout << Wek.Val1 << Wek.Val2 << Wek.Val3 << std::endl;
}
std::istream& operator >> (std::istream &Strm, Vector &Wek)
{
//It actually works with no problem so i'll pass it
}
std::ostream& operator << (std::ostream &Strm, Vector Wek)
{
Vector::PrintVector(Wek);
return Strm;
}
and in main.cpp
#include Vector.hh
#include <iostream>
using namespace std;
int main()
{
Vector Vector1;
cout << endl << " Start " << endl << endl;
cin >> Vector1;
cout << Vector1;
}
When I try to compile g++ says:
g++ -c -g -Iinc -Wall -pedantic -o obj/Vector.o src/Vector.cpp
src/Vector.cpp:12:37: error: no 'void Vector::PrintVector(Vector)' member function declared in class 'Vector'
void Vector::PrintVector(Vector Wek)
^
src/Vector.cpp: In function 'std::ostream& operator<<(std::ostream&, Vector)':
src/Vector.cpp:29:1: error: 'PrintVector' is not a member of 'Vector'
Vector::PrintVector(Wek);
make: Fatal error: Command failed for target `obj/Vector.o'
Could you please give me a hand with it so I learn basics properly?

Answer 1

Either a class function is static and not called on any particular class object, or it is non-static and must be called on an instance of the class, which can be used inside the function. You have mixed the approach. You need to select one of the two:
class Vector
{
static void PrintVector(const Vector& Wek)
{
std::cout << Wek.Val1 << Wek.Val2 << Wek.Val3 << std::endl;
}
};
// to call
Vector::PrintVector(Wek);
or
class Vector
{
void PrintVector() const
{
std::cout << Wek.Val1 << Wek.Val2 << Wek.Val3 << std::endl;
}
};
// to call
Wek.PrintVector();
Either is fine, but I would prefer the latter.
The other problem is how you have implemented the stream operator. This operator takes a stream to output the object to. You have called this stream Strm, but it is never used in your operator. Your print function is hardcoded to the cout stream.
I suggest you remove the PrintVector function and implement your operator as follows:
class Vector
{
friend std::ostream& operator << (std::ostream &Strm, const Vector& Wek);
};
std::ostream& operator << (std::ostream &Strm, const Vector& Wek)
{
Strm << Wek.Val1 << Wek.Val2 << Wek.Val3 << "\n";
return Strm;
}
Now you can output to any type of stream, such as a file stream. You also needed to declare the operator as a friend to allow it to access the private members.
I changed some parameters to reference to increase speed and also replaced endl with "\n" which is faster when writing to files (you can google why).
You may want to consider adding spaces between the output members, as this makes it easer to read back.
Hope this helps.

Answer 2

Your error is simple. You cannot use an instance method without an object. Try:
Wek.PrintVector();
Also, pass it by constant reference, don't copy.

Answer 3

General way to do this is to make operator<<() a friend function of the class. Friend function can access private members of the class. So operator<<() can directly access private members for printing them. Refer sample.
Defining a public function PrintVector for printing is a bad idea. Since it is public, user (developer using your class) might start using it instead of << operator. You don't want user to use it.