Linking error : collect2: error: ld returned 1 exit status

Question

I created a header file named days_from_civil.hpp.

#ifndef BOOST_CHRONO_DATE_DAYS_FROM_CIVIL_HPP
#define BOOST_CHRONO_DATE_DAYS_FROM_CIVIL_HPP

namespace boost {

namespace chrono {

    template<class Int>
    Int
    days_from_civil(Int y,unsigned m,unsigned d) noexcept ;

            }
}

#endif

File days_from_civil.cpp is

 #include<type_traits>
 #include<limits>
 #include<stdexcept>
 #include"days_from_civil.hpp"

 namespace boost {

namespace chrono {

    template<class Int>

    Int
    days_from_civil(Int y,unsigned m,unsigned d) noexcept {
        static_assert(std::numeric_limits<unsigned>::digits >= 18,
            "This algorithm has not been ported to a 16 bit unsigned integer");
        static_assert(std::numeric_limits<Int>::digits >= 20,
            "This algorithm has not been ported to a 16 bit signed integer");
        y -= m <= 2;
        const Int era = (y >= 0 ? y : y-399) / 400;
        const unsigned yoe = static_cast<unsigned>(y - era * 400);      // [0, 399]
        const unsigned doy = (153*(m + (m > 2 ? -3 : 9)) + 2)/5 + d-1;  // [0, 365]
        const unsigned doe = yoe * 365 + yoe/4 - yoe/100 + doy;         // [0, 146096]
        return era * 146097 + static_cast<Int>(doe) - 719468;
        }

    }
}

Then I defined a file testalgo.cpp as

 #include <iostream>
 #include "days_from_civil.hpp"

 int main(int argc, char const *argv[])
 {
int y = 1981;
int m = 5;
int d = 30 ;
int x = boost::chrono::days_from_civil(y,m,d);
std::cout<<x<<std::endl;
return 0;
 }

Then I made a .o file using g++ -std=c++11 -c days_from_civil.cpp

Then I tried to do this : g++ -std=c++11 testalgo.cpp days_from_civil.o

But it is giving this error :


/tmp/ccwrTUOn.o: In function `main':
testalgo.cpp:(.text+0x32): undefined reference to `int boost::chrono::days_from_civil(int, unsigned int, unsigned int)'
collect2: error: ld returned 1 exit status

Please help me to solve this problem. I am doing everything correctly I suppose.

See [Why can templates only be implemented in the header file?](http://stackoverflow.com/q/495021/464709) — Frédéric Hamidi, Mar 04 '14 at 16:36

score 3 · Accepted Answer · answered Mar 04 '14 at 16:38

Note that days_from_civil is a template function, which usually means you need to provide a definition for it, and not just a declaration. Include the function's body in the header file and you're good to go, or provide an explicit instantiation like

template days_from_civil<int>(int y, unsigned m, unsigned d) noexcept;

Linking error : collect2: error: ld returned 1 exit status

1 Answers1