Question

Even by explicitly mentioning that the parameter in the function pointer is const, it doesn’t seem to be able to convert the function to this type:

#include <iostream>

template <typename T>
class Image{};

template <typename TPixel>
static void
FillImage(const Image<TPixel>* const image){}
//FillImage(Image<TPixel>* const image){} // Replacing the above line with this one compiles fine

int main()
{
  typedef Image<float> ImageType;
  ImageType* image = new ImageType;
  void (*autoFunctionPointer)(const decltype(image)) = FillImage;
  autoFunctionPointer(image);
}

Can anyone explain how to make it do that conversion?

Answer 1

The const applies to the pointer.

So, const decltype(image) is equivalent to ImageType* const and not const ImageType*

If you change image to

const ImageType* image = new ImageType;

the first version of FillImage() works as expected.

To get a const ImageType* you can use std::remove_pointer

#include <type_traits>
...
void (*autoFunctionPointer)(const std::remove_pointer<decltype(image)>::type *) = FillImage;