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

i have created test class has public variable double x. have overloaded ostream << operater able print out variable x. have written external save(filename, object) function save content of object in particular way. use << operator save content of x file. header file (testh.hpp) looks 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 file defining 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 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 saved! ======\npathname: " << pname << std::endl; }  #endif /* testc_cpp_ */ 

and here code test 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 long list of errors, sais in testc.cpp code compiler cannot myfile << data; command:

in file included ../test.cpp:3:0: ../testc.cpp:33:9: note:   ‘const test’ not derived ‘const std::basic_string<_chart, _traits, _alloc>’   myfile << data; 

could please me resolving issue. thank time.

you streaming test non-const reference:

friend std::ostream& operator << (std::ostream& os, test& p); 

you want stream const reference:

friend std::ostream& operator << (std::ostream& os, const test& p);                                                     ^^^^^^ 

the error comes fact when call save(), passing in const reference:

template <class v> void save(const std::string& pname, const v& data) {     ...     myfile << data; // <== data const test&     ... }