CMake & C++ : linker error : undefined reference to function - c++

Question

I am trying to compile a simple C++ program with CMake, but I am getting a linker error :
[2/2] Linking CXX executable bin/MY_PROGRAM
FAILED: bin/MY_PROGRAM
: && g++ -g CMakeFiles/MY_PROGRAM.dir/src/main.cpp.o -o bin/MY_PROGRAM && :
/usr/bin/ld: CMakeFiles/MY_PROGRAM.dir/src/main.cpp.o: in function `main':
/home/user/Code/root/src/main.cpp:27: undefined reference to `str_toupper(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&)'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.
I have tried looking at some questions with similar issues but couldn't find what I did wrong. There must be a problem with my directory structure and CMake files. I could change the directory structure to make things easier, but I may be missing something important and I'd like to figure out why. Also, I am new to C++ so I might be doing something wrong in the code itself, but my IDE doesn't find any issue.
My directory structure is :
root
|-- CMakeLists.txt
|-- src
|-- main.cpp
|-- utils
|-- CMakeLists.txt
|-- src
|-- strutils.cpp
|-- include
|-- strutils.h
The top-level CMakeLists.txt is :
// general stuff (standard, etc...)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY bin)
set(MY_PROGRAM_SRC src/main.cpp)
add_executable(MY_PROGRAM ${MY_PROGRAM_SRC})
include_directories(utils/include)
link_libraries(MY_LIBRARY)
and utils/CMakeLists.txt :
set(MY_LIBRARY_SRC include/strutils.h src/strutils.cpp)
add_library(MY_LIBRARY STATIC ${MY_LIBRARY_SRC})
// This is to include strutils.h in strutils.cpp
// (but is it needed ? I could just #include <string> in strutils.cpp, I guess)
target_include_directories(MY_LIBRARY PUBLIC include)
Finally, the source files :
// strutils.h
#include <string>
void str_toupper(const std::string &in, std::string &out);
// strutils.cpp
#include <algorithm>
#include <strutils.h>
void str_toupper(const std::string &in, std::string &out) {
std::transform(in.begin(), in.end(), out.begin(), [](char c){return std::toupper(c);});
}
// main.cpp
#include <strutils.h>
#include <cstdlib>
#include <cstdio>
#include <string>
int main(int argc, char *argv[]) {
// ...
std::string arg_in;
for (int i = 0; i < argc; i++) {
str_toupper(std::string(argv[i]), arg_in);
}
}
Does anyone have an idea of what's going on ? Thanks !

Answer 1

The first sentence in the manual link_libraries
Link libraries to all targets added later.
You use this directive after the target MY_PROGRAM is added - the target MY_PROGRAM is added prior link_libraries.
Prefer use target_link_libraries(MY_PROGRAM MY_LIBRARY) - other targets can require different dependencies and it's better not use a global set of dependencies for all targets.