Question

I’m using ifstream::read to read a file,

ifstream ifs("a.txt");
char buf[1024];
ifs.read(buf, 1024);

But a.txt’s size might be less than 1000 bytes, so how am I supposed to know how many bytes have been read from ifs?

Answer 1

You can get the amount of characters extracted by the last operation with std::ifstream::gcount:

ifstream ifs("a.txt");
char buf[1024];
ifs.read(buf, 1024);
size_t extracted = ifs.gcount();

or

ifstream ifs("a.txt");
char buf[1024];
size_t extracted = ifs.read(buf, 1024).gcount();

since read(...) returns *this.