Write to file with temporary buffer using ofstream

Question

How can I write a series of numbers (in binary) preceded by its length, without knowing the length beforehand?

Can I write the number series to a temporary buffer (eg buf.write() ), determine the buffer length, and copy the buffer to an ofstream afterwards (eg ofs << buf )?

Answer 1

Since you are writing in binary mode to the file, reserve a location for the quantity.

For example, let's have an 32-bit value for the quantity. We could write that at the beginning of the file:

uint32_t number_size = 0U;

// Open output file for read & write.
// Reserve space for the quantity by writing dummy value.
output_file.write((char *)&number_size, sizeof(number_size));

// Perform calculations & writing to file, incrementing "number_size"  

// Write the number size to the top of the file:
output_file.seekp(0, SEEK_BEG);
output_file.write((char *)&number_size, sizeof(number_size));

You can place quantity variable anywhere, just remember its file position. When finished, seek to that position and write it in.