struct foo
{
    size_t a;
    size_t b;
    unsigned char c[32];

    constexpr foo(size_t a_, size_t b_, ??? s) :
        a(a_),
        b(b_),
        c(s)
    {
    }

}

struct foo
{
    size_t a;
    size_t b;
    unsigned char c[32];

    template<typename ... UnsignedChar>
    constexpr foo(size_t a_, size_t b_, UnsignedChar ... s) :
        a(a_),
        b(b_),
        c{s...} //use curly braces here
    {
    }
};

foo f(10, 20, 1,2,3,4,5, .....,32);
     //a,  b, -------s-----------