创建编译时常量并设置前 N 位

Question

In C++17, is there are way to generate at compile-time a constant with the first N bits set?

In pseudocode, I am looking for something like:

constexpr uint32_t MY_CONSTANT = setBits<2>();

Would be equivalent to:

constexpr uint32_t MY_CONSTANT = 0b11;

In other words, given a compile-time constant N, return a compile-time constant M where bits 0 to (N-1) are 1 (set).

Answer 1

I don't think there's a ready made function for it in the standard library (although std::bitset::set is constexpr since C++23). You could make your own though:

template<class T, std::size_t N>
constexpr T setBits() {
    if constexpr (N == sizeof(unsigned long long) * CHAR_BIT) return ~T{};
    else return static_cast<T>((1ull << N) - 1);
}

constexpr auto MY_CONSTANT = setBits<std::uint32_t, 2>();

Example for setBits<std::uint8_t, 2>() :

   0b00000001
<<          2
-------------
=  0b00000100

   0b00000100
-           1
-------------
=  0b00000011

Or negate 0 to get all bits set and right shift away all but N bits:

template<class T, std::size_t N>
constexpr T setBits() {
    if constexpr (N == 0) return 0;
    else return ~T{} >> (sizeof(T) * CHAR_BIT - N);
}