Initialise static const int with static function

Question

I have a template class with some integers as arguments. One static const integer (call it Length) of this class needs to be calculated based on the arguments. The calculation does need a loop (as far as I know) so a simple expression won't help.

static int setLength()
{
    int length = 1;
    while (length <= someTemplateArgument)
    {
        length = length << 1;
    }
    return length;
}

The returned length should be used to init Length. Lengthis used as a fixed length of an array so I need it to be constant.

Is there a solution for this issue? I know that constexp could help but I can't use C11 or later.

Of course, you can do some template-metaprogramming to get your constant right. I'm personally not really into the details of that because I think that template-metaprogramming is an atrocity that's best avoided, but it's certainly doable. — cmaster - reinstate monica
– cmaster - reinstate monica, Commented Dec 18, 2015 at 15:29

Revolver_Ocelot · Accepted Answer · 2015-12-18 15:46:53Z

2

Using metaprogramming. Implementation of C++11 enable_if taken from cppreference.com

#include <iostream>

template<bool B, class T = void>
struct enable_if {};

template<class T>
struct enable_if<true, T> { typedef T type; };

template <int length, int arg, typename = void>
struct length_impl
{
    static const int value = length_impl<(length << 1), arg>::value;
};

template <int length, int arg>
struct length_impl<length, arg, typename enable_if<(length > arg)>::type>
{
    static const int value = length ;
};

template <int arg>
struct length_holder
{
    static const int value = length_impl<1, arg>::value;
};

template<int n>
struct constexpr_checker
{
    static const int value = n;
};

int main()
{
    std::cout << constexpr_checker< length_holder<20>::value >::value;
}

answered Dec 18, 2015 at 15:46

Revolver_Ocelot

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2 Comments

ruhig brauner Over a year ago

Hey, that seems to be the correct approach here. Thanks!

cmaster - reinstate monica Over a year ago

Seems to be the correct solution, so +1 for that. Still, it only goes to prove what I said in the comments above: Template metaprogramming is an atrocity...

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