コード:

#include <windows.h>
#include <locale>
#include <fstream>
#include <string>
#include <codecvt>

using namespace std;

std::wstring GetString_UTF16BE(const char* filePath)
{
	wifstream ifs(filePath);
	if (!ifs) {
		// 読み込み失敗
		return L"";
	}
	ifs.imbue(locale(locale::empty(), new codecvt_utf16<wchar_t, 0x10ffff, consume_header>));
	return wstring(istreambuf_iterator<wchar_t>(ifs), istreambuf_iterator<wchar_t>());
}

int WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, int)
{
	// ロケールをデフォルトに設定
	locale::global(locale(""));
	setlocale(LC_ALL, locale().c_str());

	// utf16be表示
	MessageBoxW(nullptr, GetString_UTF16BE("test-utf16be.txt").c_str(), L"", MB_OK);

	return 0;
}

コード:

#include <Windows.h>
#include <stdio.h>
#include <Shlwapi.h>
#include <io.h>
#include <fcntl.h>  
#include <sys/stat.h>
#pragma comment(lib,"Shlwapi.lib")

//BOMを内部で識別するためのマクロ
#define BOM_UTF_8    (0x01)      //utf-8
#define BOM_UTF_16   (0x02)      //utf-16
#define BOM_UTF_32   (0x04)      //utf-32
#define BOM_UTF_7    (0x08) //utf-7

#define BOM_BE       (0x10)       //be
#define BOM_LE       (0x20)       //le

//BOM
//[0]...BOMのサイズ、[1]...BOM識別用のマクロ、[2]-[6]BOM
const unsigned char BOM[6][6] = {
	{ 3,BOM_UTF_8, 0xef,0xbb,0xbf },
	{ 2,BOM_UTF_16|BOM_BE, 0xfe,0xff },
	{ 2,BOM_UTF_16|BOM_LE, 0xff,0xfe },
	{ 4,BOM_UTF_32|BOM_BE, 0x00,0x00,0xfe,0xff },
	{ 4,BOM_UTF_32|BOM_LE,0xff, 0xfe, 0x00, 0x00 },
	{ 3,BOM_UTF_7, 0x2b,0x2f,0x76 }
};

int getBOM(LPCWSTR lpFile,int *bomsize)
{
	FILE *fp;
	char Buf[4];

	_wfopen_s(&fp, lpFile, L"rb");
	if (!fp)
		return 0;
	

	fread(Buf, 4, 1, fp);

	for (int i = 0; i < 6; i++)
	{
		if (!memcmp(BOM[i] + 2, Buf, BOM[i][0]))
		{
			*bomsize = BOM[i][0];
			return BOM[i][1];
		}
	}

	//BOMなし
	*bomsize = 0;
	return 0;
}

size_t GetFileSizeEx(LPCWSTR lpFileName)
{
	struct _stat stbuf;
	int fd;

	_wsopen_s(&fd, lpFileName, _O_BINARY,
		_SH_DENYWR, _S_IREAD);
	if (fd == -1) {
		return INVALID_VALUE; //0xfffffff...
	}


	if (_fstat(fd, &stbuf) == -1) {
		/* エラー処理 */
		return INVALID_VALUE;
	}

	_close(fd);

	return stbuf.st_size;
}

size_t EDMultiByteToWideChar(LPCSTR lpszMultiByte, LPWSTR *WideCharTemp)
{
	size_t len;
	WCHAR *Buffer;

	len = MultiByteToWideChar(
		CP_ACP,
		MB_PRECOMPOSED,
		lpszMultiByte,
		-1,
		NULL,
		0
	);

	Buffer = (WCHAR*)malloc(len * sizeof(WCHAR));
	if (!Buffer)
	{
		*WideCharTemp = NULL;
		return FALSE;
	}

	MultiByteToWideChar(
		CP_ACP,
		MB_PRECOMPOSED,
		lpszMultiByte,
		-1,
		Buffer,
		(int)(len)
	);

	*WideCharTemp = Buffer;

	return len;
}

int edLoadPlainText(LPCWSTR lpFile,WCHAR **loadedText,const int forceBOM = 0)
{
	FILE *fp;
	int bomsize;
	size_t size;
	void *loaded;
	int filebom;
	WCHAR mode[32] = { 0 };

	wcscpy_s(mode, L"rb");

	filebom = getBOM(lpFile, &bomsize);
	if(forceBOM)
		filebom = forceBOM;

	if ((size = GetFileSizeEx(lpFile)) == INVALID_VALUE)
		return FALSE;

	if ((filebom & BOM_UTF_8))
		wcscat_s(mode, L", ccs=UTF-8");
	else if ((filebom & BOM_UTF_16) && (filebom & BOM_LE))
		wcscat_s(mode, L", ccs=UTF-16LE");
/*	else if ((filebom & BOM_UTF_16) && (filebom & BOM_BE))
		wcscat_s(mode, L", ccs=UNICODE");*/

	_wfopen_s(&fp, lpFile, mode);

	if (!fp)
		return FALSE;

	if ((loaded = calloc(1,size + 2/*NULL*/)) == NULL)
	{
		fclose(fp);
		return FALSE;
	}

	fread(loaded, size, 1, fp);

	//もしBOMがUTF-16を指している場合は
	if ((filebom & BOM_UTF_16))
	{
		*loadedText = (WCHAR*)loaded + bomsize / sizeof(WCHAR);
		fclose(fp);

		return TRUE;
	}

	//utf-8なら変換して返す
	EDMultiByteToWideChar((LPCSTR)loaded + bomsize, loadedText);

	fclose(fp);

	return TRUE;
}

int WINAPI WinMain(HINSTANCE,HINSTANCE,LPSTR,int)
{
       //テスト
	WCHAR *t1, *t2, *t3, *t4;
	edLoadPlainText(L"test-utf-8.txt", &t1);
	edLoadPlainText(L"test-utf16le.txt", &t2);
	edLoadPlainText(L"test-.txt", &t3);
	edLoadPlainText(L"test-utf16be.txt", &t4);
}