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I am trying to install the MMMTools https://mmm.humanoids.kit.edu/installation.html. My cmake version is 3.16.3. I went through every step without any errors until this section

cd ~/MMMCore
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release ..
make

The make command returns me the following error.

(base) kong@kong-Standard:~/MMMCore/build$ make
[  1%] Building CXX object CMakeFiles/MMMCore.dir/MMM/XMLTools.cpp.o
/home/kong/MMMCore/MMM/XMLTools.cpp: In function ‘void MMM::XML::makeAbsolutePath(const string&, std::string&)’:
/home/kong/MMMCore/MMM/XMLTools.cpp:650:64: error: ‘operator/’ is not a member of ‘std::filesystem’; did you mean ‘operator~’?
  650 |   std::filesystem::path filenameNewComplete = std::filesystem::operator/(filenameBasePath, filenameNew);
      |                                                                ^~~~~~~~~
      |                                                                operator~
make[2]: *** [CMakeFiles/MMMCore.dir/build.make:76: CMakeFiles/MMMCore.dir/MMM/XMLTools.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:295: CMakeFiles/MMMCore.dir/all] Error 2
make: *** [Makefile:163: all] Error 2

But I googled the function and saw that it is a member of std::filesystem https://en.cppreference.com/w/cpp/filesystem/path/operator_slash. What went wrong?

I had a look through the CMakeLists.txt and saw this.

###########################################################
#### Compiler configuration                            ####
###########################################################

set(CMAKE_POSITION_INDEPENDENT_CODE ON) # enable -fPIC

if(MSVC)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /W4 /MP")
elseif(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_GNUCXX)
    # cmake 3.10 does not understand c++2a, so we tell it we will handle the standard flag
    set(CMAKE_CXX_STANDARD_DEFAULT)
    add_definitions(-std=c++2a)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wno-long-long -pedantic")
endif()

add_compile_options(-Werror)

I am compiling it on Ubuntu 20.04 so I guess it enters the elseif section. Does it mean that it is not using C++17? Do I need to edit the line set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wno-long-long -pedantic") to have it use C++17?

The guide indicates that it is for 18.04 but since the issue is with C++17 I don't think the fault lies in using 20.04?

Alan Birtles
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Kong
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    `operator/` is a member of `std::filesystem` since C++17. What standard is the makefile specifying? – Chris Sep 24 '21 at 04:18
  • @Chris I have updated my post to include the section from CMakeLists.txt that specifcies the compiler. Do I need to change it to C++17? I am new to CMake. – Kong Sep 24 '21 at 04:46
  • Spelling out `std::filesystem::operator/(filenameBasePath, filenameNew)` defeats the purpose of `operator/`. One is supposed to use `filenameBasePath/filenameNew`. Just sayin'. – n. m. could be an AI Sep 24 '21 at 06:40

1 Answers1

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Due to LWG 3065 the operator is now hidden and shouldn't be called directly.

std::filesystem::path filenameNewComplete = std::filesystem::operator/(filenameBasePath, filenameNew);

Should just be:

std::filesystem::path filenameNewComplete = filenameBasePath / filenameNew;

I'm guessing the code has only been tested against an older implementation (it looks like this was implemented in gcc/libstdc++ 9) than the one you are using, I don't know why it was written in such an overcomplicated way initially though.

Alan Birtles
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