LinuxQuestions.org - C++ Template For Generic Function

I'm trying to write something roughly equivalent to the get-from (>>) and put-to (<<) operators from the istream and ostream STL classes. My class, however, is really just a wrapper for an fstream; it bundles two fstream objects into one interface, providing both << and >> operators. So my problem is, I want to use operator<< to write whatever the argument is to an "input" instance variable, and operator>> to read from an "output" instance variable into the argument. In other words, I want to pass the call to the << and >> operators down to whatever stream they apply to. But it's not working like I expected, and I get linker errors saying that specific functions (for different-typed arguments) are not defined.

The code will probably illustrate it best. Here's the header file for my class, Text_IO:

Code:

#ifndef _TEXT_IO_HH

#define _TEXT_IO_HH



#include <fstream>

#include <sstream>

#include <string>



#include "taylor/string.hh"



class Text_IO {

public:

    Text_IO(std::string* in, std::string* out);

    Text_IO(const char* in, const char* out);

    std::string getline();

    template <typename DATA_TYPE> Text_IO* operator>>(DATA_TYPE& dest);

    template <typename DATA_TYPE> Text_IO* operator<<(DATA_TYPE src);

    std::string getSentence();

private:

    std::ifstream* input;

    std::ofstream* output;

    const char* SENTENCE_DELIM;

    const char* WORD_DELIM;

};



#endif

And here's the actual implementation:

Code:

#include "text_io.hh"



Text_IO :: Text_IO(std::string* in, std::string* out) : SENTENCE_DELIM(".;:?!"), WORD_DELIM(" \t\r\n") {

    this->input = new std::ifstream(in->c_str());

    this->output = new std::ofstream(out->c_str());

}



Text_IO :: Text_IO(const char* in, const char* out) : SENTENCE_DELIM(".;:?!"), WORD_DELIM(" \t\r\n") {

    this->input = new std::ifstream(in);

    this->output = new std::ofstream(out);

}



std::string Text_IO :: getline(void) {

    std::ostringstream oss;

    char c;

    while ((c = this->input->get()) != std::char_traits<char>::eof() && \

          taylor :: string :: is_member_of(c, "\n")) oss << c;

    return oss.str();

}



template <typename DATA_TYPE>

Text_IO* Text_IO :: operator>>(DATA_TYPE& dest) {

    input >> dest;

    return this;

}



template <typename DATA_TYPE>

Text_IO* Text_IO :: operator<<(DATA_TYPE data) {

    this->output << data;

    return this;

}



std::string Text_IO :: getSentence(void) {

    std::ostringstream oss;

    char c;



    while ((c = this->input->get()) != std::char_traits<char>::eof() && \

          taylor :: string :: is_member_of(c, SENTENCE_DELIM));

    while ((c = this->input->get()) != std::char_traits<char>::eof() && \

          ! taylor :: string :: is_member_of(c, SENTENCE_DELIM)) oss << c;



    return oss.str();

}

Lastly, here's my testing code:

Code:

#include <iostream>



#include "text_io.hh"



int main(int argc, char** argv) {

    Text_IO file_io("input.txt", "output.txt");

    std::string s;

    s = file_io.getline();

    std::cerr << "Read line from file: " << s << std::endl;

    int i;

    file_io >> i;

    std::cerr << "Read integer from file: " << i << std::endl;

    file_io << i;

    file_io << "w00t!\n";

    return 0;

}

And finally, here's the compilation command:

Code:

$ gmake text_io_test

g++ -g -Wall -Wabi -ansi -pedantic -o text_io_test text_io.o text_io_test.cc taylor/string.o

/var/tmp//ccQ993Ei.o(.text+0x1d4): In function `main':

/home/taylor/programs/testing/text_io_test.cc:21: undefined reference to `Text_IO* Text_IO::operator>><int>(int&)'

/var/tmp//ccQ993Ei.o(.text+0x21b):/home/taylor/programs/testing/text_io_test.cc:23: undefined reference to `Text_IO* Text_IO::operator<< <int>(int)'

/var/tmp//ccQ993Ei.o(.text+0x22f):/home/taylor/programs/testing/text_io_test.cc:24: undefined reference to `Text_IO* Text_IO::operator<< <char const*>(char const*)'

gmake: *** [text_io_test] Error 1

I thought that what I was trying to do was possible in C++, but maybe I was wrong. Do you have to define the function differently for every single type? Or can you use a template to automatically discern what type is being used in the call? Maybe it's just a syntax mistake on my part. Whatever the case, thanks in advance for any advice; this problem's had me stumped all day.