c++ - Aligning static string literals

Question

I have a static array of structures:

struct CommandStruct
{
    char* data;
    unsigned ans_size;
};

static const CommandStruct commands[] =
{
    { "Some literal", 28 },
    { "Some other literal", 29 },
    { "Yet another literal", 8 },
};

And I want the strings to be 16-byte aligned. Is it possible to achieve it directly? I might get away with defining each literal separately, like __declspec(align(16)) static const char some_command_id[] = "my literal", but that's a mess. I need all initialization in a single block of code.

Answer 1

With C++11, following may help: https://ideone.com/IDEdY0

#include <cstdint>

// Sequence of char
template <char...Cs> struct char_sequence
{
    template <char C> using push_back = char_sequence<Cs..., C>;
};

// Remove all chars from char_sequence from ''
template <typename, char...> struct strip_sequence;

template <char...Cs>
struct strip_sequence<char_sequence<>, Cs...>
{
    using type = char_sequence<Cs...>;
};

template <char...Cs, char...Cs2>
struct strip_sequence<char_sequence<'', Cs...>, Cs2...>
{
    using type = char_sequence<Cs2...>;
};

template <char...Cs, char C, char...Cs2>
struct strip_sequence<char_sequence<C, Cs...>, Cs2...>
{
    using type = typename strip_sequence<char_sequence<Cs...>, Cs2..., C>::type;
};

// struct to create a aligned char array
template <std::size_t Alignment, typename chars> struct aligned_string;

template <std::size_t Alignment, char...Cs>
struct aligned_string<Alignment, char_sequence<Cs...>>
{
    alignas(Alignment) static constexpr char str[sizeof...(Cs)] = {Cs...};
};

template <std::size_t Alignment, char...Cs>
alignas(Alignment) constexpr
char aligned_string<Alignment, char_sequence<Cs...>>::str[sizeof...(Cs)];

// helper to get the i_th character (`` for out of bound)
template <std::size_t I, std::size_t N>
constexpr char at(const char (&a)[N]) { return I < N ? a[I] : ''; }

// helper to check if the c-string will not be truncated
template <std::size_t max_size, std::size_t N>
constexpr bool check_size(const char (&)[N])
{
    static_assert(N <= max_size, "string too long");
    return N <= max_size;
}

// Helper macros to build char_sequence from c-string
#define PUSH_BACK_8(S, I) 
    ::push_back<at<(I) + 0>(S)>::push_back<at<(I) + 1>(S)> 
    ::push_back<at<(I) + 2>(S)>::push_back<at<(I) + 3>(S)> 
    ::push_back<at<(I) + 4>(S)>::push_back<at<(I) + 5>(S)> 
    ::push_back<at<(I) + 6>(S)>::push_back<at<(I) + 7>(S)>

#define PUSH_BACK_32(S, I) 
        PUSH_BACK_8(S, (I) + 0) PUSH_BACK_8(S, (I) + 8) 
        PUSH_BACK_8(S, (I) + 16) PUSH_BACK_8(S, (I) + 24)

#define PUSH_BACK_128(S, I) 
    PUSH_BACK_32(S, (I) + 0) PUSH_BACK_32(S, (I) + 32) 
    PUSH_BACK_32(S, (I) + 64) PUSH_BACK_32(S, (I) + 96)

// Macro to create char_sequence from c-string (limited to 128 chars)
#define MAKE_CHAR_SEQUENCE(S) 
    strip_sequence<char_sequence<> 
    PUSH_BACK_128(S, 0) 
    >::type::template push_back<check_size<128>(S) ? '' : ''>

// Macro to return an aligned c-string
#define MAKE_ALIGNED_STRING(ALIGNMENT, S) 
    aligned_string<ALIGNMENT, MAKE_CHAR_SEQUENCE(S)>::str

And so you have:

static const CommandStruct commands[] =
{
    { MAKE_ALIGNED_STRING(16, "Some literal"), 28 },
    { MAKE_ALIGNED_STRING(16, "Some other literal"), 29 },
    { MAKE_ALIGNED_STRING(16, "Yet another literal"), 8 },
};