C++ linking issue: multiple definition - g++

Question

Unfortunately I cannot post all of the source code here. I can describe the structure though. All header files have #ifndef/#define/#endif guards. The structure is as follows:
node.h - included by tree.h
tree.h - included by tree.cpp and main.cpp
tree.cpp
main.cpp
In node.h in the global namespace, I declare the following free standing function:
bool char_sort_func(Path first, Path second)
{
return (first->label() < second->label());
}
(Note: as shown bellow, a Path is just a shared_ptr) When I try to build, I get a multiple definition error saying that the function is present in both tree.o and main.o:
> make
g++ -c -g -Wall main.cpp -I /usr/include
g++ -c -g -Wall tree.cpp -I /usr/include
g++ -Wall -o tool tree.o main.o -L /usr/lib -l boost_program_options
main.o: In function `char_sort_func(boost::shared_ptr<Edge>, boost::shared_ptr<Edge>)':
node.h:70: multiple definition of `char_sort_func(boost::shared_ptr<Edge>, boost::shared_ptr<Edge>)'
tree.o:node.h:70: first defined here
collect2: ld returned 1 exit status
make: *** [all] Error 1
I tried searching all of the source files to see if that was true, however, I don't see it in any wrong places:
> grep char_sort_func *
Binary file main.o matches
node.h:bool char_sort_func(Path first, Path second)
node.h: std::sort(edges_.begin(), edges_.end(), char_sort_func);
Binary file trie.o matches
Sure enough though, it is in the binary files. How can I restructure my code to prevent this linking issue?

Answer 1

This will happen if you declare normal functions in a .h file, because they will be generated in every file that #includes it. Perhaps you meant to declare it inline or static?