C++ constexpr:在编译时计算std数组

Question

我想将"数组"转换bool为整数序列.所以我需要std::array在编译时计算一个.

这是我的代码

#include <array>

template<typename InputIt, typename T >
inline constexpr typename std::iterator_traits<InputIt>::difference_type
count( InputIt first, InputIt last, const T &value ) {
    typename std::iterator_traits<InputIt>::difference_type ret = 0;
        for (; first != last; ++first) {
            if (*first == value) {
                ret++;
            }
        }
        return ret;
}

template<bool ..._values>
struct keep_value {
    static constexpr std::size_t numberOfValues = sizeof...(_values);
    static constexpr bool values[] = {_values...};
    static constexpr std::size_t numberToKeep = count(values, values + numberOfValues, true);

    static constexpr std::array<std::size_t, numberToKeep> computeIndices() {
        std::array<std::size_t, numberToKeep> array{};
        auto it = array.begin();
        for(std::size_t i{0}; i < numberOfValues; ++i)
            if(values[i] == true)
                *it++ = i;

        return array;
    }

    static constexpr std::array<std::size_t, numberToKeep> indices = computeIndices();

    template<typename Indices = std::make_index_sequence<numberToKeep>>
    struct as_index_sequence{};

    template<std::size_t ...Is>
    struct as_index_sequence<std::index_sequence<Is...>> : std::index_sequence<indices[Is]...>{};
};

int main() {
    keep_value<false, true, true>::template as_index_sequence<>{}; // Should return the sequence 1 2
}

我收到调用该computeIndices函数的行的错误.这段代码c ++ 14是否正确？是否可以这样做？我正在使用MSVC,我得到这个错误:表达式没有评估为常量

Answer 1

此代码看起来正确,并在编译为C++ 17时有效.它使用std::array::begin,只constexpr在C++ 17中制作.

使用clang时可以实现更好的编译错误,其中指出:

<source>:23:25: note: non-constexpr function 'begin' cannot be used in a constant expression
    auto it = array.begin();