why can't I access imagedata in main?

Question

I am trying to write a programme to read MRI data by using VTK and C++. But I can't get spacing of MRI raw data in main. The "GetSpacing" only works in "ReadImageData" function. I think I made some mistake in C++ programming. But I don't know where it is.

vtkImageData* ReadImageData(string mri_imagedata_file)
{
    vtkSmartPointer<vtkMetaImageReader> reader =
        vtkSmartPointer<vtkMetaImageReader>::New();
    reader->SetFileName(mri_imagedata_file.c_str());
    reader->Update();
    vtkImageData* metaimage = reader->GetOutput();
    double sp[3];
    metaimage->GetSpacing(sp);
    cout << sp[0] << " " << sp[1] << " " << sp[2] <<endl; //<----------It works here.
    return metaimage;
}

int main (int argc, char *argv[])
{
    if(argc != 2)
    {
        cerr << "Usage: " << argv[0] << " MRI image data" <<endl;
        return EXIT_FAILURE;
    }
    string mri_imagedata_file = argv[1];// Input "prost00.mhd"

    vtkImageData* metaimage = ReadImageData(mri_imagedata_file);
    double sp2[3];
    metaimage->GetSpacing(sp2);
    cout << sp2[0] << " " << sp2[1] << " " << sp2[2] << endl; //<-----It doesn't work here

}

Thank you for your attention.

Answer 1

我的假设是， vtkMetaImageReader::GetOutput()返回指针一些vtkMetaImageReader内部数据，所以当你退出ReadImageData您的reader被破坏，返回指针变为无效。

Answer 2

Assuming vtkSmartPointer<vtkMetaImageReader> is a kind of smart pointer whatever, reader points to in the function is destructed when ReadImageData returns. That includes what metaimage is pointing to.

    return metaimage;
    // reader->~();  // Destructed here. Including what is pointed to by metaimage. 
}

A good solution would be to return the smart pointer instead of metaimage . That way what is pointed to by reader will not be destructed when ReadImageData returns and will be available in main.

Answer 3

It looks like you forgot to pass the array as a parameter to GetSpacing in main , so it calls the overload that returns a double* , leaving your array untouched.

You're also printing an array called spacing although it looks like you want the spacing in sp2 .