(.text+0x20): undefined reference to `main' When trying to Compile .cpp file

Question

when trying to compile a .cpp file called rat with a .h file called normalize which is located in a different folder (rat.h is in the same folder as rat.cpp) , I'm typing

g++ -I/opt/cis/include/ -I/opt/cis/lib/ rat.cpp

I have included an int main () in the .cpp file itself, and have completely commented out the inner workings of the .h file and the .cpp file just to try to compile correctly.

The .h file (commented items not included)

  1 #ifndef RAT_H
  2 #define RAT_H
  3 #include <iostream>
  4 #include <string>
  5 
  6 class Rat {
  7         friend std::ostream &operator<<(std::ostream &, const Rat &);
  8         friend std::istream &operator>>(std::istream &, Rat &);
  9         private:
 10                 double n,d;
 11         public: 
 12                 Rat(): n(0), d(1) {}
 13 };
 14. #endif

The .cpp file

  1 #include <iostream>
  2 #include <cstdlib>
  3 #include <string>
  4 #include <sstream>
  5 #include "rat.h"
  6 #include "normalize.h"
  7 using namespace std;
  8 #ifdef TEST_RAT
  9 int main() {};
  10 #endif

What am I doing incorrectly? Sorry if it is obvious.

Bruno Ferreira · Answer 1 · 2014-04-24T16:13:35.410

4

Your main function exists only if the macro TEST_RAT is defined. Either:

add -DTEST_RAT to your compiler CFLAGS
add the line #define TEST_RAT before the #ifdef
remove the #ifdef TEST_RAT and #endif pair in your .cpp

edited Apr 24 '14 at 16:13

answered Apr 22 '14 at 17:10

Bruno Ferreira

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There are two allowed signatures of main: `int main()` and `int main(int argc, char **argv)` – HolyBlackCat Apr 22 '14 at 17:13
It is allowed, but is not the standard. – Bruno Ferreira Apr 22 '14 at 17:14
I'm not fully sure, but I think it's standart too. GCC with `-Wall -Wextra -pedantic` don't complain about it. – HolyBlackCat Apr 22 '14 at 17:19
Sorry, you are right. What is against the standard is `void main` – Bruno Ferreira Apr 22 '14 at 17:24
[`int main()` is standard.](http://stackoverflow.com/questions/22239/why-does-int-main-compile/22262#22262) – cf- Apr 22 '14 at 17:25

score 0 · Answer 2 · answered Apr 22 '14 at 17:09

0

you forgot :

#define TEST_RAT

This is not defined in your code so this should work now.

answered Apr 22 '14 at 17:09

user2076694

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score 0 · Answer 3 · answered Apr 22 '14 at 17:11

0

Since you put your main() in an #ifdef it is left out if TEST_RAT is not defined. Since it isn't you don't have a main so you cannot create an executable (there is no point of entry) and g++ fails with the error seen.

answered Apr 22 '14 at 17:11

clcto

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score 0 · Answer 4 · answered Apr 22 '14 at 17:12

0

Most likely you wanted to put this in your main (to make sure the Rat class is defined):

    #ifdef RAT_H
    int main() {};
    #endif

answered Apr 22 '14 at 17:12

IonutG

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score 0 · Answer 5 · answered Apr 22 '14 at 17:14

0

Another option, since no-one's mentioned it yet, is the -c flag:

g++ -c -I/opt/cis/include/ -I/opt/cis/lib/ rat.cpp

This will compile rat.cpp to an object file rat.o without attempting to link it into a program (and so require a main).

If you're testing for correctness you might also want to try -W or -Wall for warnings.

answered Apr 22 '14 at 17:14

Rup

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-c compiles and outputs the file to an object file. If you want only to test if it compiles, you may also add the `-o /dev/null` option – Bruno Ferreira Apr 22 '14 at 17:17

(.text+0x20): undefined reference to `main' When trying to Compile .cpp file

5 Answers5