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Comment by pyrtsa | Hacker News Reader

And, by extending your second example, we can enforce the minimum array size with std::enable_if (C++11; boost::enable_if_c with C++03):

    template <size_t N>
    typename enable_if<(N >= 10), void>::type
    foo(int (&bar)[N]) { ... }

EDIT: Changed condition from N >= 0 into a more meaningful one. X-)

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... and just as I was going to make a smart-ass joke about Boost probably having another contrived contraption just for that - there it is, already sneaked into the standard too.

The more readable C++11 version would use static_assert:

    template <size_t N>
    foo(int (&bar)[N]) 
    { 
        static_assert(N >= 10, "Array needs >= 10 elements!"); 
        ...
    }

Correct. The key difference is that the enable_if version can be overloaded with mutually exclusive requirements:

    template <size_t N>
    typename enable_if<(1 < N && N < 10), void>::type
    foo(int (&bar)[N])
    {
        // called when 1 < N && N < 10
    }

    template <size_t N>
    typename enable_if<(N >= 10), void>::type
    foo(int (&bar)[N])
    {
        // called when N >= 10
    }

OTOH, the key advantage in static_assert is the meaningful error message.

This will generate separate function bodies for different values of N.

I don't think that is more readable. With enable_if the precondition is clearly visible at the function header, while the static_assert is inside the body and thus more easily accidentally ignored.

On the other hand, static_assert will have a more helpful compile error for users of the code. They will clearly see assertion condition that failed, rather than missing candidate error due to substitution failure (or worse, a different overload silently considered instead).

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