Automatically pick a variable type big enough to hold a specified number

Question

Is there any way in C++ define a type that is big enough to hold at most a specific number, presumably using some clever template code. For example I want to be able to writ

Answer 1

Sure, it's possible. Here are the ingredients. Let's start with my two favorite meta-functions:

template
struct constant
{
    enum { value = N };
};

template
struct return_
{
    typedef T type;
};

Then, a meta-function that counts the bits required to store a number:

template
struct bitcount : constant<1 + bitcount<(N>>1)>::value> {};

template<>
struct bitcount<0> : constant<1> {};

template<>
struct bitcount<1> : constant<1> {};

Then, a meta-function that counts the bytes:

template
struct bytecount : constant<((bitcount::value + 7) >> 3)> {};

Then, a meta-function that returns the smallest type for a given number of bytes:

template
struct bytetype : return_ {};

template<>
struct bytetype<4> : return_ {};

template<>
struct bytetype<3> : return_ {};

template<>
struct bytetype<2> : return_ {};

template<>
struct bytetype<1> : return_ {};

And finally, the meta-function that you asked for:

template
struct Integer : bytetype::value> {};