模板推演失败

Question

有没有人能解释一下为什么这个不起作用：

#include "itkCovariantVector.h"
#include "itkImage.h"

template <typename TComponent, int NumberOfComponents>
void FillImage(itk::Image<itk::CovariantVector<TComponent,
NumberOfComponents>, 2>* const image,
               itk::Image<itk::CovariantVector<TComponent,
NumberOfComponents>, 2>* const output)
{
  std::cout << "Works." << std::endl;
}

int main(int, char* [] )
{
  typedef itk::Image<itk::CovariantVector<float, 3u>, 2u> ImageType;

  ImageType::Pointer imageSmartPointer = ImageType::New();
  ImageType* image = imageSmartPointer.GetPointer();
  FillImage(image, image);
  return 0;
}
/*
no matching function for call to ‘FillImage(ImageType*&, ImageType*&)’
note: candidate is:
template<class TComponent, int NumberOfComponents> void
FillImage(itk::Image<itk::CovariantVector<TComponent,
NumberOfComponents>, 2u>*, itk::Image<itk::CovariantVector<TComponent,
NumberOfComponents>, 2u>*)
*/

这些类模板的定义是：http://www.itk.org/Doxygen/html/classitk_1_1CovariantVector.html http://www.itk.org/Doxygen/html/classitk_1_1Image.html

我用非ITK类创建了同样的情况，并且它可以正常工作：

#include <iostream>

template <typename TPixel, int Dimensions>
struct Image
{
};

template <typename TComponent, int NumberOfComponents>
struct Vector
{
};

template <typename TComponent, int NumberOfComponents>
void FillImage(Image<Vector<TComponent, NumberOfComponents>, 2 >* const image,
               Image<Vector<TComponent, NumberOfComponents>, 2 >* const output)
{
  std::cout << "Works." << std::endl;
}

int main(int, char* [] )
{
  typedef Image<Vector<float, 3>, 2 > ImageType;

  ImageType* image = new ImageType;
  FillImage(image, image);
  delete image;
  return 0;
}

有没有人能解释一下可能的区别？

Answer 1

问题是我必须将签名从<int NumberOfComponents>更改为<unsigned int NumberOfComponents>，这样它才能与ITK类的定义相匹配。

#include <iostream>

template <typename TPixel, int Dimensions>
struct Image
{
};

// The deduction works with this
// template <typename TComponent, int NumberOfComponents>
// struct Vector
// {
// };

// The deduction does NOT work with this (unsigned int vs int above)
template <typename TComponent, unsigned int NumberOfComponents>
struct Vector
{
};

template <typename TComponent, int NumberOfComponents>
void FillImage(Image<Vector<TComponent, NumberOfComponents>, 2 >* const image,
               Image<Vector<TComponent, NumberOfComponents>, 2 >* const output)
{
  std::cout << "Works." << std::endl;
}

int main(int, char* [] )
{
  typedef Image<Vector<float, 3>, 2 > ImageType;

  ImageType* image = new ImageType;
  FillImage(image, image);
  return 0;
}