it should still fix some issue Literal strings in C++ are really constant arrays of the character type, that's why L"test.png" is mentioned as the typeconst wchar_t  (the size is 9 to fit the terminator). As any other array it can decay to a pointer to its first element, and this pointer have the type const wchar_t*.
SpriteSheet(const wchar_t* filename, Graphics*gfx);
// Note the const here
'default argument' : cannot convert from 'const char ' to 'const wchar_t *'