Add custom function to ifstream

Question

I would like my own class to work just like ifstream, but I can easily get the file size.

here is the title:

#include <fstream> using namespace std; class ifile: public ifstream { size_t _file_size = 0; size_t calculate_file_size(); public: ifile(): ifstream(), _file_size(0) {} ifile(const char *filename, ios_base::open_mode mode = ios_base::in): ifstream(filename, mode) { _file_size = cal_file_size(); } size_t get_file_size(); virtual ~ifile(); }; 

I found a lot of information that I should not inherit from ifstream. How then can I easily solve my problem?

Edit:

calculate_file_size:

 size_t ifile::calculate_file_size() { auto present_pos = tellg(); seekg(0, ifstream::end); auto file_size = tellg(); seekg(present_pos); return file_size; } 

• It would be nice to see the right example (if I inherit from ifstream ).
• The reason is to calculate once and read many times.
• Why not get_file_size(ifstream &ifs) ? My ifstream obj is static, so it is evaluated many times.

Answer 1

It’s perfectly fine to inherit from ifstream to add new methods, here is an example of code that never failed me on any platform, However, as I understand it, std classes usually do not have virtual destructors that you need to know about when processing derived objects. Therefore, the conclusion from them creates new opportunities for making mistakes.