C undefined reference to defined function

Question

I cannot figure out why this is not working   I will put up all three of my files and possibly someone can tell me why it is throwing this error   I am using g   to compile the program   Program    include  lt iostream gt   include  h8 h   using namespace std   int main       char sentence MAX SENTENCE LENGTH     char writeTo      output txt     int distanceTo likePosition  length  numWords    cout  lt  lt   ENTER A SENTENCE         cin getline sentence  299     length   strlen sentence     numWords   wordCount sentence  length     for int x   0  x  lt  3    x          likePosition   likePos numWords       distanceTo   lengthTo sentence  likePosition  length       insertLike sentence  distanceTo  length  writeTo         return 0        Function file   void insertLike const char sentence     const int lengthTo  const int length  char writeTo        char part1 MAX SENTENCE LENGTH   part2 MAX SENTENCE LENGTH     char like       like      for int y   0  y  lt  lengthTo    y      part1 y    sentence y     for int z   lengthTo 1  z  lt  length - lengthTo    z      part2 z    sentence z     strcat part1  like     strcat part1  part2     writeToFile sentence  writeTo     return      Header file   void insertLike const char sentence    const int lengthTo  const int length  const char writeTo       The error exactly is   undefined reference to  insertLike char const   int  int  char const    collect2  ld returned 1 exit status

User · Answer

This could also happen if you are using CMake. If you have created a new class and you want to instantiate it, at the constructor call you will receive this error -even when the header and the cpp files are correct- if you have not modified CMakeLists.txt accordingly.

With CMake, every time you create a new class, before using it the header, the cpp files and any other compilable files (like Qt ui files) must be added to CMakeLists.txt and then re-run cmake . where CMakeLists.txt is stored.

For example, in this CMakeLists.txt file:

cmake_minimum_required(VERSION 2.8.11)

project(yourProject)

file(GLOB ImageFeatureDetector_SRC *.h *.cpp)

### Add your new files here ###
add_executable(yourProject YourNewClass.h YourNewClass.cpp otherNewFile.ui})

target_link_libraries(imagefeaturedetector ${SomeLibs})

If you are using the command file(GLOB yourProject_SRC *.h *.cpp) then you just need to re-run cmake . without modifying CMakeLists.txt.

User · Answer

The declaration and definition of insertLike are different  In your header file   void insertLike const char sentence    const int lengthTo  const int length  const char writeTo      In your  function file    void insertLike const char sentence     const int lengthTo  const int length char writeTo      C   allows function overloading  where you can have multiple functions methods with the same name  as long as they have different arguments  The argument types are part of the function s signature   In this case  insertLike which takes const char  as its fourth parameter and insertLike which takes char   as its fourth parameter are different functions

User · Answer

If you are including a library which depends on another library  then the order of inclusion is also important   g   -o MyApp MyMain o -lMyLib1 -lMyLib2   In this case  it is okay if MyLib1 depends on MyLib2  However  if there reverse is true  you will get undefined references

User · Answer

You need to compile and link all your source files together   g   main c function file c

User · Answer

Though previous posters covered your particular error  you can get  Undefined reference  linker errors when attempting to compile C code with g    if you don t tell the compiler to use C linkage   For example you should do this in your C header files   extern  C          void myfunc int param             To make  myfunc  available in C   programs   If you still also want to use this from C  wrap the extern  C    and   in  ifdef   cplusplus preprocessor conditionals  like   ifdef   cplusplus extern  C     endif   This way  the extern block will just be    skipped    when using a C compiler

User · Answer

As Paul said  this can be a linker complaint  rather than a compiler error   If you read your build output logs carefully  may need to look in a separate IDE window to see the full details  you can dell if the problem is from the compiler  needs to be fixed in code  or from the linker  and need to be fixed in the make cmake project level to include a missing lib

[c++] C++ undefined reference to defined function

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