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## How it works

`BOOST_STATIC_ASSERT` works as follows. There is class `STATIC_ASSERTION_FAILURE` which is defined as:

    namespace boost{

    template <bool> struct STATIC_ASSERTION_FAILURE;

    template <> struct STATIC_ASSERTION_FAILURE<true>{};

    }

The key feature is that the error message triggered by the undefined expression `sizeof(STATIC_ASSERTION_FAILURE<0>)`, tends to be consistent across a wide variety of compilers. The rest of the machinery of `BOOST_STATIC_ASSERT` is just a way to feed the `sizeof` expression into a `typedef`. The use of a macro here is somewhat ugly; however boost members have spent considerable effort trying to invent a static assert that avoided macros, all to no avail. The general conclusion was that the good of a static assert working at namespace, function, and class scope outweighed the ugliness of a macro.

| Copyright © 2000, 2005 Steve Cleary and John Maddock