Return Object From Function

Synopsis

Return an object from a function.

Results

Output:

ImageRegion (0x7ff7f950ba70)
Dimension: 2
Index: [0, 0]
Size: [10, 10]

Code

C++

#include "itkImage.h"

namespace
{
using ImageType = itk::Image<unsigned char, 2>;
}

ImageType::Pointer
ReturnSmartPointer()
{
  ImageType::Pointer  image = ImageType::New();
  itk::Index<2>       corner = { { 0, 0 } };
  itk::Size<2>        size = { { 10, 10 } };
  itk::ImageRegion<2> region(corner, size);
  image->SetRegions(region);
  image->Allocate();

  return image;
}

ImageType *
ReturnPointer()
{
  ImageType::Pointer  image = ImageType::New();
  itk::Index<2>       corner = { { 0, 0 } };
  itk::Size<2>        size = { { 10, 10 } };
  itk::ImageRegion<2> region(corner, size);
  image->SetRegions(region);
  image->Allocate();

  return image;
}

int
main(int, char *[])
{
  {
    // This is how it should be done.
    ImageType::Pointer smartPointer = ReturnSmartPointer();
    std::cout << smartPointer->GetLargestPossibleRegion() << std::endl;
  }

  {
    ImageType * pointer = ReturnPointer();
    // This crashes the program because the smart pointer created in the function goes out of scope and gets deleted
    // because it is returned as a normal pointer.
    // std::cout << pointer->GetLargestPossibleRegion() << std::endl;
    pointer = nullptr; // Here to silence warning
  }

  {
    ImageType * pointer = ReturnSmartPointer();
    // This crashes the program because though the function returned a ::Pointer, it was not stored
    // anywhere so the reference count was not increased, so it got deleted.
    // std::cout << pointer->GetLargestPossibleRegion() << std::endl;
    pointer = nullptr; // Here to silence warning
  }

  {
    // I thought this might also work, but it does not (crash).
    // My reasoning was that even though you don't return a smart pointer, you assign the object to a smart
    // pointer at return time, so it still has a reference count of 1.
    // ImageType::Pointer smartPointer = ReturnPointer(); // this line causes a 'glibc error'
    // std::cout << smartPointer->GetLargestPossibleRegion() << std::endl;
  }

  return 0;
}

Classes demonstrated

template<typename TPixel, unsigned int VImageDimension = 2>
class Image : public itk::ImageBase<VImageDimension>

Templated n-dimensional image class.

Images are templated over a pixel type (modeling the dependent variables), and a dimension (number of independent variables). The container for the pixel data is the ImportImageContainer.

Within the pixel container, images are modelled as arrays, defined by a start index and a size.

The superclass of Image, ImageBase, defines the geometry of the image in terms of where the image sits in physical space, how the image is oriented in physical space, the size of a pixel, and the extent of the image itself. ImageBase provides the methods to convert between the index and physical space coordinate frames.

Pixels can be accessed directly using the SetPixel() and GetPixel() methods or can be accessed via iterators that define the region of the image they traverse.

The pixel type may be one of the native types; a Insight-defined class type such as Vector; or a user-defined type. Note that depending on the type of pixel that you use, the process objects (i.e., those filters processing data objects) may not operate on the image and/or pixel type. This becomes apparent at compile-time because operator overloading (for the pixel type) is not supported.

The data in an image is arranged in a 1D array as [][][][slice][row][col] with the column index varying most rapidly. The Index type reverses the order so that with Index[0] = col, Index[1] = row, Index[2] = slice, …

See

ImageBase

See

ImageContainerInterface

ITK Sphinx Examples:

Subclassed by itk::GPUImage< TPixel, VImageDimension >

See itk::Image for additional documentation.