compiling problems with flat_hash_map

[LinuxFre eBird]

Hello Abseil Representatives

Using either cmake or g++ I am able to compile my code with absl::btree_map with no issues, but including absl::flat_hash_map does not compile for me. The compiling error I get is shown below.

`absl::container_internal::HashtablezInfoHandle::~HashtablezInfoHandle()':
benchmark_containers.cpp:(.text._ZN4absl18container_internal20HashtablezInfoHandleD2Ev[_ZN4absl18container_internal20HashtablezInfoHandleD5Ev]+0x33): undefined reference to `absl::container_internal::UnsampleSlow(absl::container_internal::HashtablezInfo*)'
collect2: error: ld returned 1 exit status

My build command is shown below

g++ -I "~/Source/CMakeProject/install/include" -std=c++17 benchmark_containers.cpp -o benchmark_containers.out -ltcmalloc 2>&1 | tee out.txt

My cpp script is shown below

#include "~/Source/CMakeProject/install/include/absl/container/btree_map.h"

#include "~/Source/CMakeProject/install/include/absl/container/flat_hash_map.h"

int main()

{

  absl::btree_map<int,int> _btm;

  absl::flat_hash_map<int,int> _fhm;

  ... 

  return 0;

}

What should I do?

Sincerely,

Benjamin Commeau

[Andy Getzendanner]

Your compile command does not link in any Abseil libraries, so the linker is unable to find definitions for things declared in Abseil's headers but defined in its .cc files.  You should use either CMake or Bazel to specify your binary's dependence on Abseil's flat_hash_map library (the build system will take care of any transitive dependencies).  The portions of btree_map you used must be header-only (not uncommon for templated types).

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[LinuxFre eBird]

Thank you! I was able to compile flat_hash_map. May I ask, how do I generate a hash from a pair of ints?

[Andy Getzendanner]

Like this, I think:

absl::Hash<std::pair<int, int>>()({5, 7})

You should be able to use e.g. an absl::flat_hash_set<std::pair<int, int>> without passing a custom hasher.

[aditya sharma]

hi LinuxFre eBird,

what libraries you used to link , for successfull compilation of abs::flat_hash_map

I too am facing similar issue explained below.

aditya sharma

3:43 PM (1 hour ago) 

to Abseil.io

I am getting below error while compiling the absl::flat_hash_map sample program

hash.cc:(.text+0x60): undefined reference to `absl::lts_20230802::hash_internal::CityHash32(char const*, unsigned long)'

helloworld.cpp

#include <iostream>
#include "absl/container/flat_hash_map.h"
int main() {
absl::flat_hash_map<int, std::string> map = {{1, "huey"}, {2, "dewey"}, {3, "louie"}};
for (auto it = map.begin(); it != map.end();) {
std::cout<<"it->second[0]"<<it->second[0];it++;
}
return 0;
}

the compile command is as below:

g++ -std=c++17 hello_world.cpp -o newout -L{installDir}/lib64 -labsl_hash -labsl_raw_hash_set -labsl_hashtablez_sampler -labsl_exponential_biased -labsl_synchronization -labsl_raw_logging_internal -labsl_base -I {installDir}/include

[Rana Usman]

Lan put lao pyooo pyoo hi hunda😅

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[Sourabh Goel]

I am facing the same problem. Could you please share your command on how to compile the sample program? Earlier you were using " g++ -I "~/Source/CMakeProject/install/include" -std=c++17 benchmark_containers.cpp -o benchmark_containers.out -ltcmalloc 2>&1 | tee out.txt "
What changes did you make in this command to make your compilation successful?

On Tuesday, July 7, 2020 at 8:29:36 AM UTC+5:30 LinuxFre eBird wrote: