Remove template parameters from copy constructor

[hgminh95]

Contributes to #6033
Contributes to #6522

C++23 no longer allow template parameters in constructor

[hgminh95]

@microsoft-github-policy-service agree

[jameslamb]

Thanks for this! I've edited the description to link to #6033.

With this change, were you able to compile LightGBM using the C++20 (or even better, C++23 standard)? Or is this just one step towards that?

Either way, we appreciate the help!

[jameslamb]

Makes sense to me, based on my read of e.g. https://stackoverflow.com/a/63514123/3986677 and the bits of the C++20 standard linked from it.

Thanks for the help!

[hgminh95]

With this change, were you able to compile LightGBM using the C++20 (or even better, C++23 standard)? Or is this just one step towards that?

Can confirm I can compile with C++23, gcc-12. However, it requires me to remove the __mm_malloc macros - #4331 (but this seem be a GCC thing)

[jameslamb]

Ok interesting, thank you for that! I don't recall the history for these _mm_malloc macros:

LightGBM/include/LightGBM/utils/common.h

Lines 41 to 55 in 3d386be

	#if defined(_MSC_VER)
	#include <malloc.h>
	#elif MM_MALLOC
	#include <mm_malloc.h>
	// https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html
	// https://www.oreilly.com/library/view/mac-os-x/0596003560/ch05s01s02.html
	#elif defined(__GNUC__) && defined(HAVE_MALLOC_H)
	#include <malloc.h>
	#define _mm_malloc(a, b) memalign(b, a)
	#define _mm_free(a) free(a)
	#else
	#include <stdlib.h>
	#define _mm_malloc(a, b) malloc(a)
	#define _mm_free(a) free(a)
	#endif

Maybe @StrikerRUS or @guolinke will remember why they were added in #2699.

[guolinke]

sometimes we need the memory allocation with aligned address, like

LightGBM/include/LightGBM/cuda/vector_cudahost.h

Lines 41 to 60 in 4842832

	template <class U> CHAllocator(const CHAllocator<U>& other);
	T* allocate(std::size_t n) {
	T* ptr;
	if (n == 0) return NULL;
	n = SIZE_ALIGNED(n);
	#ifdef USE_CUDA
	if (LGBM_config_::current_device == lgbm_device_cuda) {
	cudaError_t ret = cudaHostAlloc(&ptr, n*sizeof(T), cudaHostAllocPortable);
	if (ret != cudaSuccess) {
	Log::Warning("Defaulting to malloc in CHAllocator!!!");
	ptr = reinterpret_cast<T*>(_mm_malloc(n*sizeof(T), 16));
	}
	} else {
	ptr = reinterpret_cast<T*>(_mm_malloc(n*sizeof(T), 16));
	}
	#else
	ptr = reinterpret_cast<T*>(_mm_malloc(n*sizeof(T), 16));
	#endif
	return ptr;
	}

[jameslamb]

Ah got it, thank you! Ok I'll investigate this further. Maybe some headers have changed in newer versions of compilers.