使用constexpr的编译时数组

Question

    // codenz.cpp
    constexpr uint32_t Hashes[] =
    {
        // ntdll
        crc32::generate("memcpy"),

        // kernel32
        crc32::generate("MessageBoxA")
    };

// hash.hpp
#include <cstring>
#include <cstdint>

namespace crc32
{
    // Generate CRC lookup table
    template <unsigned c, int k = 8>
    struct f : f<((c & 1) ? 0xedb88320 : 0) ^ (c >> 1), k - 1> {};
    template <unsigned c> struct f<c, 0> { enum { value = c }; };

    #define A(x) B(x) B(x + 128)
    #define B(x) C(x) C(x +  64)
    #define C(x) D(x) D(x +  32)
    #define D(x) E(x) E(x +  16)
    #define E(x) F(x) F(x +   8)
    #define F(x) G(x) G(x +   4)
    #define G(x) H(x) H(x +   2)
    #define H(x) I(x) I(x +   1)
    #define I(x) f<x>::value ,

    constexpr unsigned crc_table[] = { A(0) };

    // Constexpr implementation and helpers
    constexpr uint32_t crc32_impl(const uint8_t* p, size_t len, uint32_t crc) {
        return len ?
            crc32_impl(p + 1, len - 1, (crc >> 8) ^ crc_table[(crc & 0xFF) ^ *p])
            : crc;
    }

    constexpr uint32_t crc32(const uint8_t* data, size_t length) {
        return ~crc32_impl(data, length, ~0);
    }

    constexpr size_t strlen_c(const char* str) {
        return *str ? 1 + strlen_c(str + 1) : 0;
    }

    constexpr uint32_t generate(const char* str) {
        return crc32((uint8_t*)str, strlen_c(str));
    }
}

As you can see the function itself and the array is constexpr and thus should be evaluated at compile time. The MSVC compiler spits out an error that "expression did not evaluate to a constant". Why is that?

Answer 1

You should remove redundant casts:

// now we only need one static cast
constexpr uint32_t crc32_impl(const char * p, size_t len, uint32_t crc) {
    return len ?
        crc32_impl(p + 1, len - 1, (crc >> 8) ^ crc_table[(crc & 0xFF) ^ static_cast< unsigned char >(*p)])
        : crc;
}

constexpr uint32_t crc32(const char * data, size_t length)
{
    return ~crc32_impl(data, length, ~0);
}

// we can obtain string literal array size at one go
template<size_t V_array_items_count> constexpr uint32_t
generate(const char ( & str )[V_array_items_count])
{
    return crc32(str, V_array_items_count - 1);
}

online compiler

Alternatively if you want to keep crc32 interface to accept uint8_t or byte then you may need to build a corresponding copy array at compile time.