Opencv functions can only be invoked in C code fashion but not in C++ fashion

Question

I am really new to Opencv. After downloading and installing Opencv 2.4 according to the instruction, I began writing my first Opencv program, which was basically a copy of the tutorial on the web.

#include <stdio.h>
#include <iostream>
#include <vector>

#include "cv.h"
#include "highgui.h"
#include <stdio.h>
#include <stdlib.h>
#include <opencv2/opencv.hpp>
using namespace std;
using namespace cv;

int main( int argc, char** argv )
{
    char* filename = "C:\\Research\abc.pgm";  
     IplImage *img0;

    if( (img0 = cvLoadImage(filename,-1)) == 0 )
        return 0;

    cvNamedWindow( "image", 0 );
    cvShowImage( "image", img0 );
    cvWaitKey(0);  
    cvDestroyWindow("image");
    cvReleaseImage(&img0);



    return 0;
}

The codes work very well, but you may notice that in the above code invoking Opencv function is in a C code fashion. I therefore decide to proceed with C++ code fashion with the following codes:

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>

using namespace cv;
using namespace std; 

int main( int argc, char** argv )
{ 
    if( argc != 2) 
    {
     cout <<" Usage: display_image ImageToLoadAndDisplay" << endl;
     return -1;
    }

    Mat image;
    image = imread(argv[1], CV_LOAD_IMAGE_COLOR);   // Read the file

    if(! image.data )                              // Check for invalid input
    {
        cout <<  "Could not open or find the image" << std::endl ;
        return -1;
    }

    namedWindow( "Display window", CV_WINDOW_AUTOSIZE );// Create a window for display.
    imshow( "Display window", image );                   // Show our image inside it.

    waitKey(0);                                          // Wait for a keystroke in the window
    return 0;
}

However, in this case the program has several link errors although compiling seems fine. The link errors I have received are as follows:

Error   2   error LNK2019: unresolved external symbol "void __cdecl cv::namedWindow(class stlp_std::basic_string<char,class stlp_std::char_traits<char>,class stlp_std::allocator<char> > const &,int)" (?namedWindow@cv@@YAXABV?$basic_string@DV?$char_traits@D@stlp_std@@V?$allocator@D@2@@stlp_std@@H@Z) referenced in function _main    C:\Research\OpencvTest\OpencvTest.obj
Error   1   error LNK2019: unresolved external symbol "void __cdecl cv::imshow(class stlp_std::basic_string<char,class stlp_std::char_traits<char>,class stlp_std::allocator<char> > const &,class cv::_InputArray const &)" (?imshow@cv@@YAXABV?$basic_string@DV?$char_traits@D@stlp_std@@V?$allocator@D@2@@stlp_std@@ABV_InputArray@1@@Z) referenced in function _main    C:\Research\OpencvTest\OpencvTest.obj

I am quite sure that I have added necessary Opencv libraries in my program (I use VC10), and the additional libraries I have added are as follows:

stl_port.lib
opencv_highgui242d.lib
opencv_core242d.lib

I was wondering what's wrong with my setting. Why does it work for the first program but not the second one? Any ideas will be appreciated. Thanks!

You are using `opencv_highgui242d.lib` and `opencv_core242d.lib` so these are suitable for debug mode build. Perhaps you are building in release? Moreover, try avoiding the `using` statements and write `std::` and `cv::`. `using` statements are thought (at least by some people) to be bad practice. See http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-a-bad-practice-in-c — Digital Da, Aug 02 '12 at 16:25
@DigitalDa Thanks, and I am sure that I am building in debug. I will follow the suggestion on using statement. — feelfree, Aug 02 '12 at 16:34

score 1 · Accepted Answer · answered Aug 02 '12 at 18:37

1

It has something to do with mixing STLPort and MSVC STL. You probably didn't build OpenCV libraries yourself, so they're using VC10 STL. With C interface there's just char* but with C++ interface linker gets confused with std::string in methods. You should see the same result with imread if you cast it's input to string too.

Can I mix STL implementations in my project?

answered Aug 02 '12 at 18:37

Mohammad

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Thanks, and it is true that I am using compiled OpenCV libraries downloaded from the website. In fact I have problems in building OpenCV with my local machine because of the confusion between STLPort and MSVC STL. – feelfree Aug 03 '12 at 07:30

Opencv functions can only be invoked in C code fashion but not in C++ fashion

1 Answers1

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