configuration of QT to code in book "opencv2 computer vision application programming cookbook"

Question

I am teaching my self the book "opencv2 computer vision application programming cookbook" using QT creator 3.2.1 / QT 5.3.2 (clang5.0 (apple)). When I tried to build one program I had warnings as follows:

Undefined symbols for architecture x86_64:
  "cv::imread(std::string const&, int)", referenced from:
      MainWindow::on_pushButton_clicked() in mainwindow.o
      ColorDetectController::setInputImage(std::string) in mainwindow.o
  "cv::imshow(std::string const&, cv::_InputArray const&)", referenced from:
      MainWindow::on_pushButton_clicked() in mainwindow.o
      MainWindow::on_pushButton_2_clicked() in mainwindow.o
  "std::allocator<char>::allocator()", referenced from:
      MainWindow::on_pushButton_clicked() in mainwindow.o
      MainWindow::on_pushButton_2_clicked() in mainwindow.o
  "std::allocator<char>::~allocator()", referenced from:
      MainWindow::on_pushButton_clicked() in mainwindow.o
      MainWindow::on_pushButton_2_clicked() in mainwindow.o
  "std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&)", referenced from:
      MainWindow::on_pushButton_clicked() in mainwindow.o
      MainWindow::on_pushButton_2_clicked() in mainwindow.o
  "std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()", referenced from:
      MainWindow::on_pushButton_clicked() in mainwindow.o
      MainWindow::on_pushButton_2_clicked() 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)
make: *** [controller.app/Contents/MacOS/controller] Error 1
14:49:33: **The process "/usr/bin/make" exited with code 2**.
Error while building/deploying project controller (kit: Desktop Qt 5.3 clang 64bit)
When executing step "Make"

And here's my .pro file:

QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = controller
TEMPLATE = app


SOURCES += main.cpp\
           mainwindow.cpp \
           ../colorDetection/colorDetector.cpp \
           colorDetectController.cpp

HEADERS  += mainwindow.h \
            ../colorDetection/colorDetector.h \
            colorDetectController.h

FORMS    += mainwindow.ui

CONFIG += c++11
QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.10


INCLUDEPATH += "/usr/local/opt/opencv/include" \

LIBS += -L"/usr/local/opt/opencv/lib" \
        -lopencv_core \
        -lopencv_highgui \
        -lopencv_imgproc

I met similar warnings before, but I solved that by adding these two lines into .pro file:

CONFIG += c++11

QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.10

However, this time it doesn't work. Can anyone explain to me how to solve this ? Thanks a lot.

Answer 1

It aren't warnings, but errors. Linker errors. In particular, the line

"std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&)", referenced from:

Means that it couldn't even link a standard std::string, which is weird.

It couldn't find libraries for x64 architecture. I doesn't see in your config a switch to compile for this architecture, hence I am assumiing it is a default behavior, which in turn means that an according library (at least with std::string) should have been shipped with the compiler.

It led me to this question (though it was for gcc), where the problem was in libstdc++ being too old, and thus it didn't contained some definitions for C++11 standard.

Then I looked at the page with description for std::basic_string, and found that since c++11 in members was used std::allocator, which is what I see inside the braces of the undefined reference about a string.

Hence the problem with your c++ library being too old. You need to update your compiler. Also another interesting thing I noticed from your post: you probably saying a wrong version of clang. You told it is clang-5.0, so I looked at the version in my Kubuntu, and it was just 3.4. I wondered: did I so long not updated the system? Then I decided to peek at the clang site, and found that the latest version (which is written as being «in progress») is just 3.7.

The last thing I need to mention is the two undefined references at the beginning — you just need to add a path to a library that contains functions cv::imread, and cv::imshow.