c++ - Automatically extend file size when seeking / writing to a location on a read/write fstream -

i'm working on legacy code uses win32 writefile() write random location in binary file. offset of write can past end of file in case writefile() seems automatically extend file size offset , write data file.

i'd use std::fstream same, when try seekp() appropriate location, past end of file, seekp() fails , subsequent write() fails well.

so seems me have 'manually' fill in space between current eof , location want write to.

the code looks this:

void save(size_t offset, const element& element) {     m_file.seekp(offset, std::ios_base::beg);     m_file.write(reinterpret_cast<const char*>(&element), sizeof(element));     if (m_file.fail()) {         // ... error handling     } } 

so option 'manually' write 0s current eof offset?

here example picked verbatim msdn:

// basic_ostream_seekp.cpp // compile with: /ehsc #include <fstream> #include <iostream>  int main() {     using namespace std;     ofstream x("basic_ostream_seekp.txt");     streamoff = x.tellp();     cout << << endl;     x << "testing";     = x.tellp();     cout << << endl;     x.seekp(2);   // put char in third char position in file     x << " ";      x.seekp(2, ios::end);   // put char 2 after end of file     x << "z"; } 

the file "basic_ostream_seekp.txt" has te ting\0\0z @ end of program, i.e., allowed seek past end of file.

in case, if write fail you, check , see if seekp too. if does, can detect failure earlier.