Qt and Mongocxx linker error - c++

Question

I'm working on my project which uses mongoDB. I've installed mongocxx according to the official guide and it works when I'm compiling the code from command line. Now I'd like to use it in my Qt project, I've added all necessary libraries in .pro file but I can't compile my code because of linker error. The code is the simplest:
#include <mongocxx/client.hpp>
#include <mongocxx/instance.hpp>
void connectToMongo() {
mongocxx::instance abc{};
}
.pro:
CONFIG += c++11
QMAKE_CXXFLAGS += -I/usr/local/include/mongocxx/v_noabi -I/usr/local/include/libmongoc-1.0 \
-I/usr/local/include/bsoncxx/v_noabi -I/usr/local/include/libbson-1.0 \
-L/usr/local/lib -lmongocxx -lbsoncxx
SOURCES += main.cpp
...
But, unfortunately, there is a linker's error:
Undefined symbols for architecture x86_64:
"mongocxx::v_noabi::instance::instance()", referenced from:
connectToMongo() in main.o
"mongocxx::v_noabi::instance::~instance()", referenced from:
connectToMongo() in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [hw.app/Contents/MacOS/hw] Error 1
18:38:01: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project hw (kit: Desktop Qt 5.8.0 clang 64bit)
When executing step "Make"
the make's string:
Starting: "/usr/bin/make"
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -c -pipe -stdlib=libc++ -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk -mmacosx-version-min=10.12 -I/usr/local/include/mongocxx/v_noabi -I/usr/local/include/libmongoc-1.0 -I/usr/local/include/bsoncxx/v_noabi -I/usr/local/include/libbson-1.0 -L/usr/local/lib -lmongocxx -lbsoncxx -g -std=gnu++1z -Wall -W -fPIC -DQT_QML_DEBUG -DQT_WEBENGINEWIDGETS_LIB -DQT_WEBENGINECORE_LIB -DQT_QUICKCONTROLS2_LIB -DQT_QUICK_LIB -DQT_PRINTSUPPORT_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_WEBCHANNEL_LIB -DQT_QML_LIB -DQT_NETWORK_LIB -DQT_POSITIONING_LIB -DQT_CORE_LIB -I../hw -I. -I../Qt/5.8/clang_64/lib/QtWebEngineWidgets.framework/Headers -I../Qt/5.8/clang_64/lib/QtWebEngineCore.framework/Headers -I../Qt/5.8/clang_64/lib/QtQuickControls2.framework/Headers -I../Qt/5.8/clang_64/lib/QtQuick.framework/Headers -I../Qt/5.8/clang_64/lib/QtPrintSupport.framework/Headers -I../Qt/5.8/clang_64/lib/QtWidgets.framework/Headers -I../Qt/5.8/clang_64/lib/QtGui.framework/Headers -I../Qt/5.8/clang_64/lib/QtWebChannel.framework/Headers -I../Qt/5.8/clang_64/lib/QtQml.framework/Headers -I../Qt/5.8/clang_64/lib/QtNetwork.framework/Headers -I../Qt/5.8/clang_64/lib/QtPositioning.framework/Headers -I../Qt/5.8/clang_64/lib/QtCore.framework/Headers -I. -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/System/Library/Frameworks/OpenGL.framework/Headers -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/System/Library/Frameworks/AGL.framework/Headers -I../Qt/5.8/clang_64/mkspecs/macx-clang -F/Users/alexey/Qt/5.8/clang_64/lib -o main.o ../hw/main.cpp
clang: warning: -lmongocxx: 'linker' input unused
clang: warning: -lbsoncxx: 'linker' input unused
clang: warning: argument unused during compilation: '-L/usr/local/lib'
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -headerpad_max_install_names -stdlib=libc++ -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk -mmacosx-version-min=10.12 -Wl,-rpath,/Users/alexey/Qt/5.8/clang_64/lib -o hw.app/Contents/MacOS/hw main.o qrc_qml.o moc_filldatacollections.o -F/Users/alexey/Qt/5.8/clang_64/lib -framework QtWebEngineWidgets -framework QtWebEngineCore -framework QtQuick -framework QtQml -framework QtNetwork -framework QtCore -framework DiskArbitration -framework IOKit -framework QtGui -framework QtWebChannel -framework QtPositioning -framework QtPrintSupport -framework QtWidgets -framework QtQuickControls2 -framework OpenGL -framework AGL
I've already found a decision for such problem in different topics, e.g. http://widequestion.com/question/msgasserted-linker-errors-on-mongo-c-drivers-on-macosx-10-10-3-yosemite/
but it'n't working in my case. My OS X is 10.11.
Help me, please, to resolve this problem.

Answer 1

clang: warning: -lmongocxx: 'linker' input unused
clang: warning: -lbsoncxx: 'linker' input unused
clang: warning: argument unused during compilation: '-L/usr/local/lib'
You seem to have set your linker flags as cxxflags so they were applied building main.o (when they were unused) and not applied during linking to build the executable.
I haven't worked with Qt, but a quick search suggests maybe those should have been set on LIBS. See docs for declaring other libraries.