cv::VideoCapture from string gives linker error in Qt5 on Mac - c++

Question

Problem
I am trying to get the OpenCV VideoCapture class running in a Qt project.
When I call it with an int (0 for the video camera on my mac), it works fine:
#include "opencv2/opencv.hpp"
#include <opencv2/highgui/highgui.hpp>
...
cv::VideoCapture cap(0);
The project compiles and when I run it, the light on my camera goes on.
BUT: when I try to compile it with a std::string as argument, I get a linker error:
#include "opencv2/opencv.hpp"
#include <opencv2/highgui/highgui.hpp>
...
cv::VideoCapture cap("/Users/xxxxx/small.mp4");
results in:
Undefined symbols for architecture x86_64:
"cv::VideoCapture::VideoCapture(std::string const&)", referenced from:
MainWindow::MainWindow(QWidget*) in mainwindow.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
althought the constructor from std::string should actually exist:
http://docs.opencv.org/modules/highgui/doc/reading_and_writing_images_and_video.html#videocapture-videocapture
Settings
My specifications:
Qt 5.4.0 (Clang 6.0 (Apple), 64 bit)
Mac OSX 10.10.1
OpenCV 2.4.9 installed from homebrew
My .pro file:
QT += core gui widgets
TARGET = VideoCaptureTest
TEMPLATE = app
SOURCES += main.cpp mainwindow.cpp
HEADERS += mainwindow.h
FORMS += mainwindow.ui
INCLUDEPATH += /usr/local/include
LIBS += -L/usr/local/lib
LIBS += -lopencv_core
LIBS += -lopencv_imgproc
LIBS += -lopencv_highgui
LIBS += -lopencv_ml
LIBS += -lopencv_video
LIBS += -lopencv_videostab
LIBS += -lopencv_features2d
LIBS += -lopencv_calib3d
LIBS += -lopencv_objdetect
LIBS += -lopencv_contrib
LIBS += -lopencv_legacy
LIBS += -lopencv_flann
What I tried
1.
I found this question on SO, which seems to be the exact same problem as mine:
Linking error: undefined reference to `cv::VideoCapture::open(std::string const&)`
It seem my problem could have something to do with clashing versions of OpenCV. But I checked my include directories and my lib directories and I seem to only have 2.4.9 installed. Is there maybe a different version that comes with Qt?
I tried including version 2.4.9 directly in my .pro file with
LIBS += -lopencv_core2.4.9
...
But how can I make sure I include this version as well in my source file?
2.
I though it might have something to do with Mac and x86_64 stuff. I included the following lines, which I found somewhere online, in my .pro file:
CONFIG += x86 ppc x86_64 ppc64
CONFIG += MAC_CONFIG
MAC_CONFIG {
QMAKE_CXXFLAGS = -std=c++11 -stdlib=libstdc++ -mmacosx-version-min=10.7
QMAKE_LFLAGS = -std=c++11 -stdlib=libstdc++ -mmacosx-version-min=10.7
}
But this also showed no effect.
3.
Here (http://answers.opencv.org/question/14772/solved-linking-error-undefined-reference-to-cvvideocaptureopenstdstring-const/) it is suggested to try cv::String instead of std::string. I also tried this:
#include "opencv2/opencv.hpp"
...
cv::String s("/Users/xxxxx/small.mp4");
cv::VideoCapture cap(s);
and strangely I still get the exact same error:
Undefined symbols for architecture x86_64:
"cv::VideoCapture::VideoCapture(std::string const&)", referenced from:
MainWindow::MainWindow(QWidget*) in mainwindow.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
which seems like the cv::String is actually converted to std::string, I guess because the compiler somewhere finds the definition of the constructor with std::string, but the linker does not?
4.
Apparently there was a similar problem with OSX 10.9, not with cv::VideoCapture, but in general resulting also in linking error for x86_64. The solution was to include
QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.9
in the .pro file. I tried this as well with 10.9 and 10.10 (which I am actually using). Still the same error. Could this be a problem with my version of OSX?
Help
I found no other suggestions on how to solve the problem. Does somebody have an idea what I am doing wrong? Any help appreciated!
Patrick

Answer 1

The answer has been given in the discussion(#wezside) of the link (item 1 of your description).
for OpenCV 3.0 (also 2.4.9 ?)
you need to add
LIBS += -lopencv_videoio
to the .pro, which is missing in yours.

Answer 2

After researching for several hours, I found that adding QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.11 to your .pro file and removing the previous .pro.user file and build directory can solve the problem. The second step is required.
I'm on El Capitan.