c++ 二進制文件的讀寫，二進制文件的長度

讀：

std::ifstream in_database_file(database_path, std::ios::binary);
if (!in_database_file.is_open())
{
     std::cout << "Error: Fail to open database file." << std::endl;
     //throw std::exception("Error!");
     std::exception();
}
//int one_item_size = sizeof(FeatureItem);
char one_item[2052];  // 2052 == one_item_size
in_database_file.read(one_item, sizeof(FeatureItem));
FeatureItem *feature;
feature = reinterpret_cast<FeatureItem *>(one_item);
std::cout << feature->feature.size() << std::endl << feature->user_id << std::endl;
in_database_file.close();

寫：

// write to database
{
	//作用域內先把要寫的二進制文件加鎖，{}後自動解鎖
	std::lock_guard<std::mutex> lock(database_mutex);
	std::ofstream out_database_file(database_path, std::ios::binary | std::ios::app);
	if (!out_database_file.is_open())
	{
        std::cout << "Fail to open database file." << std::endl;
		//throw std::exception("Error!");
     	std::exception();
	}
    out_database_file.write(reinterpret_cast<char *>(&lfeature), sizeof(FeatureItem));
	out_database_file.close();
}

判斷二進制文件大小：

std::ifstream in_database_file(database_path, std::ios::binary);
if (!in_database_file.is_open())
{
	std::cout << "Fail to open database file." << std::endl;
	//throw std::exception("Error!");
	throw std::exception();
}
in_database_file.seekg(0, std::ios::end);
std::streamoff database_length = in_database_file.tellg();
in_database_file.close();
//減去插入的一個自己版本信息（uchar）,再除以Item大小，就可得出多少個Item
uint database_id_length = uint((static_cast<uint>(database_length) - sizeof(uchar)) / sizeof(FeatureItem));