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Constexpr string processing and dropping to runtime via std::array
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#include <array> | |
#include <iostream> | |
#include <string> | |
#include <utility> | |
#if __cpp_lib_constexpr_string >= 201907L | |
using string = std::string; | |
#else | |
struct string { | |
using value_type = char; | |
char* ptr = nullptr; | |
unsigned z = 0, cap = 0; | |
constexpr string(string&& r) | |
: ptr(std::exchange(r.ptr, nullptr)) | |
, z(std::exchange(r.z, 0)) | |
, cap(std::exchange(r.cap, 0)) {} | |
constexpr ~string() { delete[] ptr; } | |
constexpr string(char const* s) { | |
while (s[cap++]); | |
for (ptr = new char[cap]; z != cap; ++z) ptr[z] = s[z]; | |
} | |
constexpr string(char const* c, char const* d) { | |
while (c + cap++ != d); | |
for (ptr = new char[cap]; z != cap; ++z) ptr[z] = c + z == d ? 0 : c[z]; | |
} | |
constexpr char& operator[](unsigned n) { return ptr[n]; } | |
constexpr char operator[](unsigned n) const { return ptr[n]; } | |
constexpr unsigned size() const { return z; } | |
constexpr char const* data() const { return ptr; } | |
constexpr char const* begin() const { return ptr; } | |
constexpr char const* end() const { return ptr + z; } | |
}; | |
inline std::ostream& operator<<(std::ostream& os, string const& s) { | |
return os << std::string(s.begin(), s.end()); | |
} | |
#endif | |
template<class T, unsigned N> | |
struct flat : std::array<typename T::value_type, N> { | |
constexpr operator T() const { return T(this->begin(), this->end()); } | |
constexpr T value() const { return *this; } | |
template<class It, std::size_t... I> | |
constexpr flat(It it, std::index_sequence<I...>) : flat::array{(I, *it++)...} {} | |
}; | |
template<auto f> | |
constexpr auto flatten() { | |
constexpr unsigned N = f().size(); | |
return flat<decltype(f()), N>(f().begin(), std::make_index_sequence<N>()); | |
} | |
constexpr string f() { | |
// string processing at compile time! | |
auto s = string("hello"); | |
++s[0]; | |
return s; | |
} | |
template<flat a> | |
void g() { | |
auto s = a.value(); | |
++s[0]; | |
std::cout << s << std::endl; | |
} | |
int main() { | |
g<flatten<[] { return f(); }>()>(); | |
} |
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