Calculate MD5 of a string in C++

You are passing a final newline to the md5sum program, but not to your code.

You can see that the bash <<< operator adds a newline:

$ od -ta <<<Hello0000000   H   e   l   l   o  nl0000006

To avoid this, use printf:

$ printf '%s' Hello | od -ta0000000   H   e   l   l   o0000005$ printf '%s' Hello | md5sum8b1a9953c4611296a827abf8c47804d7  -

Alternatively, you could include a newline in your program version:

std::string str("Hello\n");

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