Undefined reference to cv::Subdiv2D::getTriangleList - faceAverage.cpp about learnopencv

[Luca96]

Hi @alexander-zou, maybe your problem is about the standard c++ library.
What i mean is that OpenCV use the gnustl_shared standard library, so if you don't use this library in your android project, the compiler will complain with every methods that has string or vector as argument.

If you use CMake you can add this snippet in your build.gradle under android->defaultConfig:

externalNativeBuild {
            cmake {
                cppFlags "-std=c++11 -fexceptions"
                abiFilters 'x86', 'x86_64', 'armeabi', 'armeabi-v7a', 'arm64-v8a'  //keep only supported arch by your app
                arguments "-DANDROID_PLATFORM=android-15",  // change with your min api level
                        "-DANDROID_TOOLCHAIN=clang",
                        "-DANDROID_STL=gnustl_shared", // the standard lib
                        "-DANDROID_CPP_FEATURES=rtti exceptions"
            }
        }

and in your CMakeLists you have to add something like this:
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fexceptions -std=gnustl")

that is, i hope it works!

[calicratis19]

@Luca96 your previous comment worked for me finally. This was indeed an issue with gnu and libc++ stl. But I had to modify this
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fexceptions -std=gnustl")

to

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fexceptions -std=gnu++11")

[alexander-zou]

I am desperate with the same problem in my android project, may I ask how did you solve it? Thank you so much!

[calicratis19]

Hi, from the official docs (https://developer.android.com/ndk/guides/cpp-support.html) the gnustl is deprecated from ndk16 and will be removed.

gnustl

The GNU C++ Library is called gnustl on Android to differentiate it from the system runtime. This runtime is the libstdc++ available on a GNU/Linux system.

This runtime is tightly coupled to GCC, which is no longer supported in the NDK. As such, it has not received updates for several releases. The version in the NDK only supports C++11, and some portions of this library are incompatible with Clang.

Note: This library will be deprecated and removed in a future NDK release. Beginning with NDK r16, you should use libc++ instead.

I'm having the same issue for couple of days now.

[Luca96]

Maybe you can try to recompile OpenCV with libc++ instead of gnustl.