Compile-time (preprocessor) hashing of string

Question

Is there any way to create a hash of string at compile time using the C/C++ preprocessor (or even template-metaprogramming)?

e.g. UNIQUE_SALT("HelloWord", 3DES);

The idea is that HelloWorld will not be present in the compiled binary, just a hash.

Edit: There are many of these declarations spread over a large codebase.

Answer 1

This can be done with Boost.MPL but it might not be the type of hash you are after.

http://arcticinteractive.com/2009/04/18/compile-time-string-hashing-boost-mpl/

Answer 2

Why not make generating the hash part of your build process? You can write a simple batch file to generate the hash (assuming you have a program to do such a thing - if not, write one) and have it output a preprocessor directive something like:

#define MY_HASH 123456789 

to a .h file that then gets #included in your application.

Answer 3

There is no compile time string processing in C++. With the preprocessor you can't seperate the string into tokens, with templates you can't access single characters. There was however a discussion on the speculated approach using C++0x.

What you can do now is to pass the string character-wise (possible using multi-character literals):

foo = hash<3DES, str<'a','b','c'> >::result;
// or:
foo = hash<3DES, str<'abc','def'> >::result;

... or simply do it as a pre-build step.

Answer 4

Even if this can't be (reasonably) done with the preprocessor, if you used a string literal or declared it as static const and did not create any lasting references to it the compiler will likely go ahead and do all of the math to generate the result and omit the string in the object file if you compile with optimizations. The hardest part of this is that you can't make the code to initialize a global or static variable too complicated or the compiler will say "hey, you! Don't you know you can't use a for loop outside of a function?".