How do I convert between big-endian and little-endian values in C++?

Question

How do I convert between big-endian and little-endian values in C++?

EDIT: For clarity, I have to translate binary data (double-precision floating point values and 3

Answer 1

From The Byte Order Fallacy by Rob Pike:

Let's say your data stream has a little-endian-encoded 32-bit integer. Here's how to extract it (assuming unsigned bytes):

i = (data[0]<<0) | (data[1]<<8) | (data[2]<<16) | (data[3]<<24);

If it's big-endian, here's how to extract it:

i = (data[3]<<0) | (data[2]<<8) | (data[1]<<16) | (data[0]<<24);

TL;DR: don't worry about your platform native order, all that counts is the byte order of the stream your are reading from, and you better hope it's well defined.

Note: it was remarked in the comment that absent explicit type conversion, it was important that data be an array of unsigned char or uint8_t. Using signed char or char (if signed) will result in data[x] being promoted to an integer and data[x] << 24 potentially shifting a 1 into the sign bit which is UB.