Multiple definitions of same function in C++

Question

I am writing a library for neural nets. There are some necessary functions I needed so I separated them in a separate header file. I also provided the definition guards. I also included the header file in only one file but then also linker claims that there are multiple definitions of all the functions in the program. The library structure is as follows:

namespace maya:
     class neuron [neuron.hpp, neuron.cpp]
     class ffnet [ffnet.hpp, ffnet.cpp]
     struct connection [connection.hpp]
     functions [functions.hpp]

the functions header file is written something like this:

#ifndef FUNCTIONS_HPP
#define FUNCTIONS_HPP
// some functions here
double random_double(){//some code}
#endif

this functions.hpp file is included only one in neuron.hpp and since ffnet is dependent on neuron, I included neuron.hpp in ffnet only once. This ffnet.hpp is included in main.cpp only once. main.cpp is file that I use for testing my library.

this linker throws error something like this:
/usr/bin/ld: /tmp/ccN7ywby.o: in function `maya::random_double()': neuron.cpp:(.text+0x0): multiple definition of maya::random_double()'; /tmp/ccvDr1aG.o:main.cpp:(.text+0x0): first defined here

/usr/bin/ld: /tmp/cc66mBIr.o: in function `maya::random_double()':`` ffnet.cpp:(.text+0x0): multiple definition of `maya::random_double()'; /tmp/ccvDr1aG.o:main.cpp:(.text+0x0): first defined here

Also I compiled my program using :
g++ main.cpp neuron.cpp ffnet.cpp -o net

I don't think this will be needed but just in case :
$ uname -a
Linux brightprogrammer 4.19.0-kali3-amd64 #1 SMP Debian 4.19.20-1kali1 (2019-02-14) x86_64 GNU/Linux

Answer 1

You must write code of random_double() in .cpp file other than .hpp or .h file. Or, add inline before double random_double() { //some code } if you keep your code in your .hpp file.