QMake seems to classify .c++ files as C source code.

From this project file:

SOURCES += "main.c++"

it generates a makefile containing:

main.o: main.c++
   $(CC) -c $(CFLAGS) $(INCPATH) -o main.o main.c++

instead of the correct rule: (if you change main.c++ to main.cpp)

main.o: main.cpp
   $(CXX) -c $(CXXFLAGS) $(INCPATH) -o main.o main.cpp