Question

I’ve looked at several related posts but no luck with this error.
I receive this undefined reference error message below when my namespace exists across multiple files. If I compile only ConsoleTest.cpp with contents of Console.cpp dumped into it the source compiles.

I would appreciate any feedback on this issue, thanks in advance.

g++ Console.cpp ConsoleTest.cpp  -o ConsoleTest.o -Wall

/tmp/cc8KfSLh.o: In function `getValueTest()':
 ConsoleTest.cpp:(.text+0x132): undefined reference to `void Console::getValue<unsigned int>(unsigned int&)'
collect2: ld returned 1 exit status

Console.h

#include <iostream>
#include <sstream>
#include <string>

namespace Console
{
    std::string getLine();

    template <typename T>
    void getValue(T &value);
}

Console.cpp

#include "Console.h"

using namespace std;

namespace Console
{
    string getLine()
    {
        string str;
        while (true)
        {
            cin.clear();
            if (cin.eof()) {
                break;  // handle eof (Ctrl-D) gracefully
            }

            if (cin.good()) {
                char next = cin.get();
                if (next == '\n')
                    break;
                str += next;    // add character to string
            } else {
                cin.clear(); // clear error state
                string badToken;
                cin >> badToken;
                cerr << "Bad input encountered: " << badToken << endl;
            }
        }
        return str;
    }               


    template <typename T>
    void getValue(T &value)
    {
        string inputStr = Console::getLine();
        istringstream strStream(inputStr);

        strStream >> value;
    }
}

ConsoleTest.cpp

#include "Console.h"

void getLineTest()
{
    std::string str;

    std::cout << "getLinetest" << std::endl;

    while (str != "next")
    {
        str = Console::getLine();
        std::cout << "<string>" << str << "</string>"<< std::endl;
    }
}

void getValueTest()
{
    std::cout << "getValueTest" << std::endl;
    unsigned x = 0;
    while (x != 12345)
    {
        Console::getValue(x);
        std::cout << "x: " << x << std::endl;
    }
}

int main()
{
    getLineTest();
    getValueTest();
    return 0;
}

Answer 1

The template function needs to be defined, not only declared, in the header. The compiler needs to see the template implementation and the template argument to build the specialization you require in your main. So put the definition in the header directly:

namespace Console
{
    std::string getLine();

    template <typename T>
    inline void getValue(T &value) {
        string inputStr = Console::getLine();
        istringstream strStream(inputStr);
        strStream >> value;

    }
}