Overloading fstream << operator to save "any" kind of data - c++

Question

I have created a Test class that has a public variable double x. I have overloaded the ostream << operater to be able to print out the variable x. I have also written an external save(filename, object) function that would save the content of the object in a particular way. I would like to use the << operator to save the content of x into a file.
Header file (TestH.hpp) looks like this:
#ifndef TESTH_HPP_
#define TESTH_HPP_
#include <iostream>
#include <fstream>
#include <string>
class Test {
public:
double x;
friend std::ostream& operator << (std::ostream& os, Test& p);
inline Test();
inline virtual ~Test();
};
inline std::ostream& operator << (std::ostream& os, Test& p);
template <class V>
inline void save(const std::string& pname, const V& data);
#endif /* TESTH_HPP_ */
This is the file for defining the functions (TestC.cpp):
#ifndef TESTC_CPP_
#define TESTC_CPP_
#include "TestH.hpp"
Test::Test() {
x=10;
}
Test::~Test() {}
std::ostream& operator << (std::ostream& os, Test& p) {
// Output to screen
os << "Test:\t";
os << p.x;
return os;
}
template <class V>
void save(const std::string& pname, const V& data) {
std::ofstream myfile;
myfile.open(pname.c_str());
myfile << data;
myfile.close();
std::cout << "====== File is saved! ======\nPathname: " << pname << std::endl;
}
#endif /* TESTC_CPP_ */
And finally here is the code to test the save function (Test.cpp):
#include <iostream>
#include "TestC.cpp"
int main () {
std::string fn = "test.txt";
int i=1;
Test a;
// std::cout << a;
save(fn,a);
return 0;
}
I get a really long list of errors, but basically it sais that in the TestC.cpp code the compiler cannot do the myfile << data; command:
In file included from ../Test.cpp:3:0:
../TestC.cpp:33:9: note: ‘const Test’ is not derived from ‘const std::basic_string<_CharT, _Traits, _Alloc>’
myfile << data;
Could you please help me resolving this issue. Thank you for your time.

Answer 1

You are streaming Test by non-const reference:
friend std::ostream& operator << (std::ostream& os, Test& p);
You want to stream it by const reference:
friend std::ostream& operator << (std::ostream& os, const Test& p);
^^^^^^
The error comes from the fact that when you call it from save(), you are passing in a const reference:
template <class V>
void save(const std::string& pname, const V& data) {
...
myfile << data; // <== data is const Test&
...
}

Answer 2

You get the error because you are trying to save a const object and you have overloaded the operator for non const case.
Turn
std::ostream& operator << (std::ostream& os, Test& p) {
into
std::ostream& operator << (std::ostream& os, const Test& p) {