Why is this file setup with the BigInt header generating a linker error?

Question

I implemented a function which generated the first n fibonacci numbers as a coding exercise. To increase the amount of numbers I could generate before overflowing any of the standard integer types I downloaded the BigInt header from github and put it in /usr/include (ubuntu 22.04.4).

No errors are given when everything is kept in a single .cpp file, but I encountered a problem when trying to move all the fibonacci related stuff into its own translation unit. The following code is enough to reproduce the error:

test.cpp:

#include "test_import.h"

int main() {}

test_import.h:

#ifndef TEST_IMPORT
#define TEST_IMPORT

#include <BigInt.hpp>

#endif

test_import.cpp:

#include "fibonacci.h"

This generates a multiple definition linker error for every function in <BigInt.hpp> which for example looks like this:

/usr/bin/ld: CMakeFiles/test.dir/test_import.cpp.o: in function bool std::__constant_string_p<char>(char const*)': /usr/include/BigInt.hpp:140: multiple definition of is_valid_number(std::__cxx11::basic_string<char, std::char_traits, std::allocator > const&)'; CMakeFiles/test.dir/test.cpp.o:/usr/include/BigInt.hpp:140: first defined here

But I dont understand why this gives a linker error.

The compiler is: gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0

And here is my CMakeLists.txt if that gives any useful information:

set(PROJECT_NAME chap8)
cmake_minimum_required(VERSION 3.22)
project(${PROJECT_NAME} VERSION 1.0)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

include_directories("/usr/include/boost_1_85_0") #adds bosst library headers (most does not need to be built)

set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)

set(TARGET_NAME test) 
add_executable(${TARGET_NAME} "${TARGET_NAME}.cpp" "test_import.cpp") 


target_compile_options(${TARGET_NAME} PRIVATE 
    -std=c++17 
    -fexceptions 
    -Wall
    -g
    -Wmissing-declarations
)

Answer 1

This is a library issue. Functions defined in a header file should be declared as inline. This stops the linker from trying to resolve multiple definitions across translation units. It would seem that the library has not been designed with multiple-source projects in mind.

To fix this locally, edit the released BigInt.hpp header, and add inline in front of every function definition.

Example:

/*
    is_valid_number
    ---------------
    Checks whether the given string is a valid integer.
*/

inline bool is_valid_number(const std::string& num) {
    for (char digit : num)
        if (digit < '0' or digit > '9')
            return false;

    return true;
}

You likely need to do that for construtor / member function definitions also. Basically everything.

Example:

/*
    Default constructor
    -------------------
*/

inline BigInt::BigInt() {
    value = "0";
    sign = '+';
}

So, put on some appropriate music, switch your brain off, copy the text inline onto your clipboard and spend a couple of minutes scrolling and pasting. The problem should go away.

Ideally this should be fixed in the library itself. You can do that yourself by following the guidelines for creating a pull request. If you're not confident to do that, log an issue instead.

The source has been split up into multiple headers in the following locations, which likely all require this fix:

• include/constructors/*.hpp
• include/functions/*.hpp
• include/operators/*.hpp