Facing benchmark.h:1050:41: error: variable 'fn' has function type. Even with v1.3.1

[digital10111]

make
Scanning dependencies of target example
[  4%] Building CXX object example/CMakeFiles/example.dir/example.cpp.o
[  8%] Linking CXX executable example
[  8%] Built target example
Scanning dependencies of target multisink
[ 12%] Building CXX object example/CMakeFiles/multisink.dir/multisink.cpp.o
[ 16%] Linking CXX executable multisink
[ 16%] Built target multisink
Scanning dependencies of target spdlog-utests
[ 20%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_errors.cpp.o
[ 24%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_file_helper.cpp.o
[ 28%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_file_logging.cpp.o
[ 32%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_misc.cpp.o
[ 36%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_pattern_formatter.cpp.o
[ 40%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_async.cpp.o
[ 44%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_registry.cpp.o
[ 48%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_macros.cpp.o
[ 52%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/utils.cpp.o
[ 56%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/main.cpp.o
[ 60%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_mpmc_q.cpp.o
[ 64%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_fmt_helper.cpp.o
[ 68%] Linking CXX executable spdlog-utests
[ 68%] Built target spdlog-utests
Scanning dependencies of target formatter-bench
[ 72%] Building CXX object bench/CMakeFiles/formatter-bench.dir/formatter-bench.cpp.o
In file included from /tmp/tmp.CP8z4tf0ew/vendor/spdlog/bench/formatter-bench.cpp:6:0:
/usr/local/include/benchmark/benchmark.h: In instantiation of 'benchmark::RegisterBenchmark(const char*, Lambda&&, Args&& ...)::<lambda(benchmark::State&)> [with Lambda = void (&)(benchmark::State&, std::__cxx11::basic_string<char>); Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]':
/usr/local/include/benchmark/benchmark.h:1050:48:   required from 'struct benchmark::RegisterBenchmark(const char*, Lambda&&, Args&& ...) [with Lambda = void (&)(benchmark::State&, std::__cxx11::basic_string<char>); Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]::<lambda(class benchmark::State&)>'
/usr/local/include/benchmark/benchmark.h:1049:38:   required from 'benchmark::internal::Benchmark* benchmark::RegisterBenchmark(const char*, Lambda&&, Args&& ...) [with Lambda = void (&)(benchmark::State&, std::__cxx11::basic_string<char>); Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]'
/tmp/tmp.CP8z4tf0ew/vendor/spdlog/bench/formatter-bench.cpp:49:79:   required from here
/usr/local/include/benchmark/benchmark.h:1050:41: error: variable 'fn' has function type
       name, [=](benchmark::State& st) { fn(st, args...); });
                                         ^
compilation terminated due to -Wfatal-errors.
bench/CMakeFiles/formatter-bench.dir/build.make:62: recipe for target 'bench/CMakeFiles/formatter-bench.dir/formatter-bench.cpp.o' failed
make[2]: *** [bench/CMakeFiles/formatter-bench.dir/formatter-bench.cpp.o] Error 1
CMakeFiles/Makefile2:1168: recipe for target 'bench/CMakeFiles/formatter-bench.dir/all' failed
make[1]: *** [bench/CMakeFiles/formatter-bench.dir/all] Error 2
Makefile:138: recipe for target 'all' failed
make: *** [all] Error 2

[digital10111]

cmake --build _builds --target install
Scanning dependencies of target example
[  4%] Building CXX object example/CMakeFiles/example.dir/example.cpp.o
[  8%] Linking CXX executable example
[  8%] Built target example
Scanning dependencies of target multisink
[ 12%] Building CXX object example/CMakeFiles/multisink.dir/multisink.cpp.o
[ 16%] Linking CXX executable multisink
[ 16%] Built target multisink
Scanning dependencies of target spdlog-utests
[ 20%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_errors.cpp.o
[ 24%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_file_helper.cpp.o
[ 28%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_file_logging.cpp.o
[ 32%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_misc.cpp.o
[ 36%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_pattern_formatter.cpp.o
[ 40%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_async.cpp.o
[ 44%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_registry.cpp.o
[ 48%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_macros.cpp.o
[ 52%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/utils.cpp.o
[ 56%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/main.cpp.o
[ 60%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_mpmc_q.cpp.o
[ 64%] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_fmt_helper.cpp.o
[ 68%] Linking CXX executable spdlog-utests
[ 68%] Built target spdlog-utests
Scanning dependencies of target formatter-bench
[ 72%] Building CXX object bench/CMakeFiles/formatter-bench.dir/formatter-bench.cpp.o
In file included from /tmp/tmp.CP8z4tf0ew/vendor/spdlog/bench/formatter-bench.cpp:6:0:
/usr/local/include/benchmark/benchmark.h: In instantiation of 'benchmark::RegisterBenchmark(const char*, Lambda&&, Args&& ...)::<lambda(benchmark::State&)> [with Lambda = void (&)(benchmark::State&, std::__cxx11::basic_string<char>); Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]':
/usr/local/include/benchmark/benchmark.h:1050:48:   required from 'struct benchmark::RegisterBenchmark(const char*, Lambda&&, Args&& ...) [with Lambda = void (&)(benchmark::State&, std::__cxx11::basic_string<char>); Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]::<lambda(class benchmark::State&)>'
/usr/local/include/benchmark/benchmark.h:1049:38:   required from 'benchmark::internal::Benchmark* benchmark::RegisterBenchmark(const char*, Lambda&&, Args&& ...) [with Lambda = void (&)(benchmark::State&, std::__cxx11::basic_string<char>); Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]'
/tmp/tmp.CP8z4tf0ew/vendor/spdlog/bench/formatter-bench.cpp:49:79:   required from here
/usr/local/include/benchmark/benchmark.h:1050:41: error: variable 'fn' has function type
       name, [=](benchmark::State& st) { fn(st, args...); });
                                         ^
compilation terminated due to -Wfatal-errors.
bench/CMakeFiles/formatter-bench.dir/build.make:62: recipe for target 'bench/CMakeFiles/formatter-bench.dir/formatter-bench.cpp.o' failed
make[2]: *** [bench/CMakeFiles/formatter-bench.dir/formatter-bench.cpp.o] Error 1
CMakeFiles/Makefile2:1168: recipe for target 'bench/CMakeFiles/formatter-bench.dir/all' failed
make[1]: *** [bench/CMakeFiles/formatter-bench.dir/all] Error 2
Makefile:138: recipe for target 'all' failed
make: *** [all] Error 2

Followed the documentation and got this.

[gabime]

cant reproduce. probably wrong version of google benchmarks is used

[emmenlau]

I'm facing this same issue with google benchmark v1.4.1. Also, this issue has been reported twice in the last year, see also #931 . I can find a reference of a similar error in https://stackoverflow.com/questions/34815698/c11-passing-function-as-lambda-parameter

I think it may be worthwhile to re-open this issue and check of wrapping in a function pointer helps?

[emmenlau]

I also see this issue with google benchmark 1.5.0.

Could this be re-opened, or should I create a new issue?

[gabime]

Opened. @emmenlau If you have an idea how to fix, a PR would be much appreciated as I don't have time to get to it.

[emmenlau]

Thanks @gabime . Currently I have no clue what may go wrong. The only related link is the stackoverflow discussion I linked above. It seems to say that the method should be encapsulated in a function pointer. However it partially contradicts itself later, so that I'm also at a loss here.
Any help would be appreciated!

[jafar75]

@gabime I have the same problem. which version of google benchamrks have you tested with no errors?

[gabime]

latest commit always worked for me

[xnervwang]

I have the same issue. I'm using the latest google benchmark at https://github.com/google/benchmark.

[MathiasMagnus]

For those stumbling upon this error, it's a bug likely caused by compilers not properly decaying the supplied benchmarked function name into its pointer. Detailed answer here. Putting the address-of operator in front of the supplied function name (or in the context of GBench, the project could help by not simply capturing fn, but invoking std::decay on it as outlined in the linked answer) should solve the issue (at least it did for us, where it popped up while compiling the code using nvcc).

[MathiasMagnus]

@gabime The solution is to provide benchmark cases not through a naked function name, but through their pointer. Put an ampersand in front if all supplied function names, for eg. here. (Function names aren't strictly speaking part of the language (they don't have a type). Language rules dictate in what contexts the names of functions (and other stuff) "decay" into things with types. I'm not a language lawyer, so I can't tell where some compiler is buggy or in which case it's actually correct not to decay, but the fix is simple nontheless: don't rely on decay mechanics.

[gabime]

@MathiasMagnus Thanks. Can you confirm if this actually solves it for spdlog? I cant reproduce so I cant confirm myself.

[MathiasMagnus]

@gabime Yes, I verified and it works. Built in WSL using Ubuntu 18.04, installed the oldest GCC in the repo (g++-5) and tried building, get the same error. Add 3 &s on lines 37,55,76 and it compiles.

[MathiasMagnus]

cmake -D CMAKE_CXX_COMPILER=g++-5 -D SPDLOG_BUILD_BENCH=ON -D CMAKE_PREFIX_PATH=/home/mate/benchmark/install ..
-- The CXX compiler identification is GNU 5.5.0
-- Check for working CXX compiler: /usr/bin/g++-5
-- Check for working CXX compiler: /usr/bin/g++-5 -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Build spdlog: 1.8.2
-- Looking for C++ include pthread.h
-- Looking for C++ include pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - not found
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE
-- Build type: Release
-- Generating example(s)
-- Generating benchmarks
-- Generating install
-- Configuring done
-- Generating done
-- Build files have been written to: /home/mate/spdlog/build
mate@MATTY-GL702ZC:~/spdlog/build$ grep RegisterBenchmark ../bench/formatter-bench.cpp
        benchmark::RegisterBenchmark(pattern.c_str(), bench_formatter, pattern);
        //        benchmark::RegisterBenchmark(pattern.c_str(), bench_formatter, pattern);
        //        benchmark::RegisterBenchmark(pattern.c_str(), bench_formatter, pattern);
        benchmark::RegisterBenchmark(pattern.c_str(), bench_formatter, pattern)->Iterations(2500000);
        benchmark::RegisterBenchmark(pattern.c_str(), bench_formatter, pattern);
mate@MATTY-GL702ZC:~/spdlog/build$ cmake --build . --target formatter-bench | grep error
In file included from /home/mate/spdlog/bench/formatter-bench.cpp:6:0:
/home/mate/benchmark/install/include/benchmark/benchmark.h: In instantiation of ‘benchmark::RegisterBenchmark(const char*, Lambda&&, Args&& ...)::<lambda(benchmark::State&)> [with Lambda = void (&)(benchmark::State&, std::__cxx11::basic_string<char>); Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]’:
/home/mate/benchmark/install/include/benchmark/benchmark.h:1062:48:   required from ‘struct benchmark::RegisterBenchmark(const char*, Lambda&&, Args&& ...) [with Lambda = void (&)(benchmark::State&, std::__cxx11::basic_string<char>); Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]::<lambda(class benchmark::State&)>’
/home/mate/benchmark/install/include/benchmark/benchmark.h:1061:38:   required from ‘benchmark::internal::Benchmark* benchmark::RegisterBenchmark(const char*, Lambda&&, Args&& ...) [with Lambda = void (&)(benchmark::State&, std::__cxx11::basic_string<char>); Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]’
/home/mate/spdlog/bench/formatter-bench.cpp:37:79:   required from here
/home/mate/benchmark/install/include/benchmark/benchmark.h:1062:41: error: variable ‘fn’ has function type
       name, [=](benchmark::State& st) { fn(st, args...); });
mate@MATTY-GL702ZC:~/spdlog/build$ grep RegisterBenchmark ../bench/formatter-bench.cpp
        benchmark::RegisterBenchmark(pattern.c_str(), &bench_formatter, pattern);
        //        benchmark::RegisterBenchmark(pattern.c_str(), bench_formatter, pattern);
        //        benchmark::RegisterBenchmark(pattern.c_str(), bench_formatter, pattern);
        benchmark::RegisterBenchmark(pattern.c_str(), &bench_formatter, pattern)->Iterations(2500000);
        benchmark::RegisterBenchmark(pattern.c_str(), &bench_formatter, pattern);
mate@MATTY-GL702ZC:~/spdlog/build$ cmake --build . --target formatter-bench
[ 80%] Built target spdlog
Scanning dependencies of target formatter-bench
[ 90%] Building CXX object bench/CMakeFiles/formatter-bench.dir/formatter-bench.cpp.o
[100%] Linking CXX executable formatter-bench
[100%] Built target formatter-bench