getline无法将参数1从“std::ifstream"转换为"_Elem*

Question

std::ifstream purchasefile;
purchasefile.open(cmmodity_file_name);
if (!purchasefile)  std::cout << "fail!";
else {
	std::string line;
	while (std::cin.getline(purchasefile, line)) 

Answer 1

purchasefile是什么对象