# How to output label map in C++?

**URL:** https://discourse.itk.org/t/how-to-output-label-map-in-c/5459
**Category:** Uncategorized
**Created:** [November 2, 2022, 9:38am UTC](https://discourse.itk.org/t/how-to-output-label-map-in-c/5459 "2022-11-02T09:38:23Z")
**Posts on this page:** 6
**Page:** 1

<div class="post-metadata">

### Author: ![SummerZ2020](https://discourse.itk.org/user_avatar/discourse.itk.org/summerz2020/32/2614_2.png) [@SummerZ2020](https://discourse.itk.org/u/SummerZ2020)
#### Post date: [November 2, 2022, 9:38am UTC](https://discourse.itk.org/t/how-to-output-label-map-in-c/5459/1 "2022-11-02T09:38:23Z")

</div>

hello,  
I am using label map to remove some undesirable regions in binary image.  
first, call BinaryImageToShapeLabelMapFilter to get label map, then output the label map and all the objects’ centroids.  
I have checked the label map result’s m\_labelobject\_container, before output it to the main function, its object container is normal. and when I have got the label map in main function，all the elements in m\_labelobject\_container is null.  
Is there anything wrong with my code?  
how to output the label map normally?  
thanks a lot.

Here is my code segment:

```auto
	template< typename PixelType = bool, uint16_t dim = 3 >
	static void GetCenroidofObjects(itk::SmartPointer<itk::Image<PixelType, dim>>& image,
		std::map<int, itk::Point<double, dim>>& rmCen,
		itk::LabelMap<itk::ShapeLabelObject<uint64_t, dim>>*& outLabelMap)
	{
		// **calculate all the regions's priority** //
		using BinaryImageToShapeLabelMapFilterType = itk::BinaryImageToShapeLabelMapFilter<itk::Image<PixelType, dim>>;
		BinaryImageToShapeLabelMapFilterType::Pointer binaryImageToShapeLabelMapFilter =
			BinaryImageToShapeLabelMapFilterType::New();
		binaryImageToShapeLabelMapFilter->SetFullyConnected(true);
		binaryImageToShapeLabelMapFilter->SetInput(image);
		binaryImageToShapeLabelMapFilter->Update();
		/*auto*/ outLabelMap = binaryImageToShapeLabelMapFilter->GetOutput();
		auto num = outLabelMap->GetNumberOfLabelObjects();
		
		//uint16_t largestAreaIdx = 0;
		/*uint64_t largestArea = 0;*/
		mippVolumeEngineLog::GetInstance()->PrintDebugInfo(LogInfoType::Information,
			L"region num", std::to_wstring(num));
		
		//concurrency::parallel_for(0, int(num), [&](uint32_t i)
			for (unsigned int i = 0; i < num; i++)
			{
				BinaryImageToShapeLabelMapFilterType::OutputImageType::LabelObjectType* labelObject =
					outLabelMap->GetNthLabelObject(i);
			
				rmCen.emplace(std::make_pair(labelObject->GetLabel(),labelObject->GetCentroid()));

			}//);
	}
int main()
{
        using CentroidType = itk::Point<double, g_Dimension>;
	itk::LabelMap<itk::ShapeLabelObject<uint64_t, 3>>* outLabelMap;
	std::map<int, CentroidType> mCen;
	ITKWrapper::GetCenroidofObjects<bool>(binRes, mCen, outLabelMap);
	//outLabelMap->RemoveLabel(2000);
}

```

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<div class="post-metadata">

### Author: ![dzenanz](https://discourse.itk.org/user_avatar/discourse.itk.org/dzenanz/32/1093_2.png) [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [November 2, 2022, 2:47pm UTC](https://discourse.itk.org/t/how-to-output-label-map-in-c/5459/2 "2022-11-02T14:47:32Z")

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You should declare `outLabelMap` as a smart pointer, e.g. `itk::LabelMap<itk::ShapeLabelObject<uint64_t, 3>>::Pointer outLabelMap;`, then assign as a return value: `outLabelMap = GetCenroidofObjects(binRes, mCen);`.

Take a look at this [example](https://examples.itk.org/src/core/common/returnobjectfromfunction/documentation).

---

<div class="post-metadata">

### Author: ![SummerZ2020](https://discourse.itk.org/user_avatar/discourse.itk.org/summerz2020/32/2614_2.png) [@SummerZ2020](https://discourse.itk.org/u/SummerZ2020)
#### Post date: [November 3, 2022, 1:38am UTC](https://discourse.itk.org/t/how-to-output-label-map-in-c/5459/3 "2022-11-03T01:38:52Z")

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Thanks for your help. I learned again here.  
But it still cannot work in my case. I have learned the example how to return object from function.  
In my case, the function is template function. I tried to return a ITK smart pointer, there are some errors in the compilation. For simplicity’s sake, I add a simple function as below.  
Here is my code, the compilation is failed with several errors .

```auto
	template< typename PixelType = int >
	static itk::Image<PixelType, 2>::Pointer
		ReturnSmartPointer()
	{
		auto image = itk::Image<PixelType, 2>::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;
	}

```

Here are the failed output with several errors:

```auto
1>E:\GIT\repos\PostProcessingAlgorithm\Include\Segmentation\ITKWrapper.hpp(1004,2): warning C4346: 'Pointer': dependent name is not a type
1>E:\GIT\repos\PostProcessingAlgorithm\Include\Segmentation\ITKWrapper.hpp(1004,2): message : prefix with 'typename' to indicate a type
1>E:\GIT\repos\PostProcessingAlgorithm\Include\Segmentation\ITKWrapper.hpp(1004,2): error C2061: syntax error: identifier 'Pointer'
1>E:\GIT\repos\PostProcessingAlgorithm\Include\Segmentation\ITKWrapper.hpp(1006,2): error C2334: unexpected token(s) preceding '{'; skipping apparent function body

```

And then, I have tried the other two functions, no error in the compilation.  
first, it is a template function without return. it can be compiled successfully.

```auto
	template< typename PixelType = int >
	static void/*itk::Image<PixelType, 2>::Pointer*/
		ReturnSmartPointer()
	{
		auto image = itk::Image<PixelType, 2>::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;
	}

```

Second, it is a non-template function with smart pointer return type.it can be compiled successfully.

```auto
	//template< typename PixelType = int >
	static itk::Image<int, 2>::Pointer
		ReturnSmartPointer()
	{
		auto image = itk::Image<int, 2>::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;
	}

```

I am not good at this part, and looking forward to the help about it. How to write a template function with itk smart pointer return type?  
Thanks a lot.

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<div class="post-metadata">

### Author: ![SummerZ2020](https://discourse.itk.org/user_avatar/discourse.itk.org/summerz2020/32/2614_2.png) [@SummerZ2020](https://discourse.itk.org/u/SummerZ2020)
#### Post date: [November 3, 2022, 1:53am UTC](https://discourse.itk.org/t/how-to-output-label-map-in-c/5459/4 "2022-11-03T01:53:05Z")

</div>

I found the solution. it has nothing to do with ITK.  
" prepend `typename` before `itk::Image<int, 2>::Pointer` as it is a _dependent type_ ."  
from  
[https://stackoverflow.com/questions/16131838/error-dependent-name-is-not-a-type-when-use-typedef-type-in-class-as-return](https://stackoverflow.com/questions/16131838/error-dependent-name-is-not-a-type-when-use-typedef-type-in-class-as-return)  
In case to someone else encounter the same question, keep all the question and answer here.

---

<div class="post-metadata">

### Author: ![dzenanz](https://discourse.itk.org/user_avatar/discourse.itk.org/dzenanz/32/1093_2.png) [@dzenanz](https://discourse.itk.org/u/dzenanz)
#### Post date: [November 3, 2022, 1:42pm UTC](https://discourse.itk.org/t/how-to-output-label-map-in-c/5459/5 "2022-11-03T13:42:01Z")

</div>

> [@SummerZ2020](#):
>
> ```auto
> template< typename PixelType = int >
> static itk::Image<PixelType, 2>::Pointer
> ReturnSmartPointer()
> {
> 
> ```

I was going to suggest two options:

```cpp
template< typename PixelType = int >
	static auto
		ReturnSmartPointer() -> itk::Image<PixelType, 2>::Pointer
	{

```

and

```auto
template< typename PixelType = int >
	static itk::SmartPointer<itk::Image<PixelType, 2>>
		ReturnSmartPointer()
	{

```

I am more confident about the second suggestion.

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<div class="post-metadata">

### Author: ![SummerZ2020](https://discourse.itk.org/user_avatar/discourse.itk.org/summerz2020/32/2614_2.png) [@SummerZ2020](https://discourse.itk.org/u/SummerZ2020)
#### Post date: [November 4, 2022, 12:55am UTC](https://discourse.itk.org/t/how-to-output-label-map-in-c/5459/6 "2022-11-04T00:55:47Z")

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Thank you so so so much.  
I have tested just now, both of them can work. Thanks again. 😃