example_difference_of_normals does not compile

[VictorLamoine]

BUILD_examples ON

CMakeFiles/pcl_example_difference_of_normals.dir/example_difference_of_normals.cpp.o: In function `main':
/home/victor/pcl/src/examples/features/example_difference_of_normals.cpp:212: undefined reference to `pcl::EuclideanClusterExtraction<pcl::PointNormal>::extract(std::vector<pcl::PointIndices, std::allocator<pcl::PointIndices> >&)'
CMakeFiles/pcl_example_difference_of_normals.dir/example_difference_of_normals.cpp.o:(.rodata._ZTVN3pcl19NormalEstimationOMPINS_11PointXYZRGBENS_11PointNormalEEE[_ZTVN3pcl19NormalEstimationOMPINS_11PointXYZRGBENS_11PointNormalEEE]+0x58): undefined reference to `pcl::NormalEstimationOMP<pcl::PointXYZRGB, pcl::PointNormal>::computeFeature(pcl::PointCloud<pcl::PointNormal>&)'
collect2: error: ld returned 1 exit status
make[2]: *** [bin/pcl_example_difference_of_normals] Error 1
make[1]: *** [examples/features/CMakeFiles/pcl_example_difference_of_normals.dir/all] Error 2
make: *** [all] Error 2

Not sure why.. it seems to be be correctly linked.

[taketwo]

Has something to do with OMP. I have it disabled, and the thing is built without problems.

[SergioRAgostinho]

@UnaNancyOwen Can you try to verify this please? With and without OpenMP.

[stefanbuettner]

Hey there,

could #1642 solve the problem? 'Cause it seems as the example should link against NormalEstimationOMP<..., PointNormal> which is not precompiled.

Cheers,
Stefan

[UnaNancyOwen]

@SergioRAgostinho I tried PCL 1.8.0 examples (set BUILD_examples option to ON) on Visual Studio.
I was set OpenMP_CXX_FLAGS (and OpenMP_C_FLAGS) option to /openmp when to enable OpenMP, and set to /openmp- when to disable OpenMP.
It can be built successfully in both cases that enable and disable OpenMP.
Please tell me if I was testing by wrong way. I will try again the more test. :)

[SergioRAgostinho]

Just noticed there was nothing on Victor's initial post mentioning that it was specific to Windows. I enabled the examples on my build and it got through with no errors.

@VictorLamoine Can you still reproduce this? The issue dates do Jan 2015 :)

@UnaNancyOwen That was it. Thanks.

@stefanbuettner That's also true... that should have yielded the typical linker error here. At first sight I'm not totally sure why the linker is not complaining when I tried to build it. To be honest I also don't understand why do we instantiate different types between NormalEstimation and NormalEstimationOMP.

[VictorLamoine]

I confirm that the latest trunk allows to build the examples without any compilation error on Linux.
Closing as this seems to be solved for every platform we support.