ifstream::read doesn't tell how many bytes it really reads?

问题

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?

回答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.

来源：https://stackoverflow.com/questions/11720880/ifstreamread-doesnt-tell-how-many-bytes-it-really-reads