#include <fstream>
std::ifstream::pos_type filesize(const char* filename)
{
std::ifstream in(filename, std::ifstream::ate | std::ifstream::binary);
return in.tellg();
}
See http://www.cplusplus.com/doc/tutorial/files/ for more information on files in C++.
edit: this answer is not correct since tellg() does not necessarily return the right value. See http://stackoverflow.com/a/22986486/1835769