C++ 将字节写入文件

Question

I am trying to write one byte to a file in C++. When I save it, is is 8 byte large, instead of 1 byte. How can I save exactly one byte?

ofstream binFile("compressed.bin", ios::out | ios::binary);
bitset<8> a("10010010");
binFile << a;

Output of ls -la:

.rw-r--r-- name staff   8 B  Sat Dec  4 23:26:18 2021  compressed.bin

How can I small it down to one byte?

Answer 1

operator << is designed for formatted output.

When writing strict binary, you should focus on member functions put (for one byte) or write (for variable number of bytes).

This will write your bitset as a single byte.

binFile.put( a.to_ulong() );

Answer 2

I'm not sure what class bitset<size_t> is, but it seems to be creating a uint64_t underneath. Maybe the template parameter is for number of bytes instead of bits?

I can achieve a single byte binary file with

std::ofstream binFile("onebyte.bin", std::ios::out | std::ios::binary);
uint8_t aByte = 0x77; // this is w in ASCII. Easy to see in a bin file
binFile << aByte;

Perhaps you need to cast bitset to a uint8_t?