std::string tempPath;
FileSel file;

{...}

tempPath = file[i]; <<== error C2593: 'operator =' is ambiguous

tempPath = const_cast<String&>(file[i]);

	operator std::string() const;

     operator const char *() const   { return Begin(); }

std::string tempPath;
FileSel file;

String path;
if (file.ExecuteSelectDir())
	path = ~file;

// Conversion from UPP::String to const tchar * to std::string
if (!path.IsVoid())
	tempPath = ~path;

std::string tempPath;
FileSel file;
{...}
tempPath = ~file[i]; // converts the Upp::String to const char*
//OR
tempPath = file[i].Begin(); // basically the same as above
//OR
tempPath = std::string(~file[i],file[i].GetLength()); // create a new std::string with the same content and length
//OR
tempPath = std::string(~file[i],file[i].GetLength()); // again, more verbose variation on the previous line

tempFrame = imread( tempPath );
		if( findChessboardCorners( tempFrame, checkboardSize, foundCorners ))   <<=== Crashes here
		{
			frames.push_back( tempFrame );
			imagePoints.push_back( foundCorners );
		}

#include <CtrlLib/CtrlLib.h>

using namespace Upp;

GUI_APP_MAIN
{
	FileSel fs;

	std::string stdPath;

	if (fs.ExecuteOpen())
	{
		String path = ~fs;

		PromptOK("From UPP\1::\1String:&\1" + path);

		stdPath = ~path;

		PromptOK("From std\1::\1string:&\1" + String(stdPath));
	}
}