The header-only library seems broken

[1flei]

When I try to include "falconn/lsh_nn_table.h" from multiple cpp files, it will cause "multiple definition of ***" linking error for a lot of fht functions, whereas "normal" header-only library should not have this problem (e.g. Eigen).

A minimum example that triggers this problem may be like:
a.cpp
#include "falconn/lsh_nn_table.h"

b.cpp
#include "falconn/lsh_nn_table.h"

main.cpp
#include
int main()
{
std::cout << "hello world" << std::endl;
return 0;
}

CMakeLists.txt
PROJECT(TEST_FALCONN)

set (CMAKE_CXX_STANDARD 17)

include_directories("FALCONN/src/include")
include_directories("FALCONN/external/simple-serializer")
include_directories("FALCONN/external/eigen")

ADD_EXECUTABLE(test ${DIR_SRCS}
"a.cpp"
"b.cpp"
"main.cpp"
)

and when linking a.o and b.o together, it will report multiple definition of *** (e.g. helper_float_/helper_double_/...).

I tried cmake 3.12.0 and gcc/g++6.5/8.0 although I do not think it is the problem of the version of compiler/toolchain.

[OancaAndrei]

I was facing the same issue. After some digging, I've patched it by moving the last include line from lsh_nn_table.h:

#include "wrapper/cpp_wrapper_impl.h"

to a new file lsh_nn_table_impl.h:

#ifndef __WRAPPER_IMPL_H__
#define __WRAPPER_IMPL_H__

#include "lsh_nn_table.h"
#include "wrapper/cpp_wrapper_impl.h"

#endif

The trick is to include only once per "project" the lsh_nn_table_impl.h file, ideally in one of your cpp files (so the definitions lay in only one object file of your project) and then use lsh_nn_table.h where needed across your project.