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I've been struggling with a simple hello-world style opencv application in c++, and while it compiles fine, it wouldn't link.

From what I've read,

 http://answers.opencv.org/question/25708/undefined-symbols-cv_outputarray_outputarraystd__1vectorcvmat-std__1allocatorcvmat/

the problem seemed to be a difference between how OpenCV was originally built on the VM, and how my application is being built

(
The VM I'm using is
https://github.com/razius/ppy-opencv-vagrant  (Ubuntu 14.04.5 / OpenCV 2.4.13))
https://github.com/dvreed77/vagrant-opencv
)

I WAS trying to build the application this way,

g++ -O2 `pkg-config --cflags --libs opencv` test.cpp -o test.out

but changing it to this, the linker suddenly builds fine:

g++ -O2 -o test.out test.cpp `pkg-config opencv --cflags --libs`

What is happening differently that makes it work?

I've been struggling with a simple hello-world style opencv application in c++, and while it compiles fine, it wouldn't link. link due to unresolved references to any cv function (OutputArray, waitKey, etc)

From what I've read,

 http://answers.opencv.org/question/25708/undefined-symbols-cv_outputarray_outputarraystd__1vectorcvmat-std__1allocatorcvmat/

the problem seemed to be a difference between how OpenCV was originally built on the VM, and how my application is being built

(
The VM I'm using is
https://github.com/razius/ppy-opencv-vagrant  (Ubuntu 14.04.5 / OpenCV 2.4.13))
https://github.com/dvreed77/vagrant-opencv
)

I WAS trying to build the application this way,

g++ -O2 `pkg-config --cflags --libs opencv` test.cpp -o test.out

but changing it to this, the linker suddenly builds fine:

g++ -O2 -o test.out test.cpp `pkg-config opencv --cflags --libs`

What is happening differently that makes it work?