diff options
| -rw-r--r-- | README.md | 51 | ||||
| -rw-r--r-- | to_string.hpp | 27 |
2 files changed, 24 insertions, 54 deletions
@@ -1,10 +1,12 @@ # constexpr-to-string +Requires C++14 or later. + **Features:** * Convert any integral type to a string at compile-time -* Supports converting to any base between 2 and 36 inclusive -* No external dependencies, only includes `type_traits` for template parameter checking +* Supports converting to bases 2 through 36 +* No external dependencies * Supports custom character types, e.g. `to_string<123, 10, wchar_t>` * C++20: Supports floating-point-to-string conversion with `f_to_string` @@ -32,48 +34,11 @@ puts(f_to_string<{3.1415926, 7}>); // Specify precision: "3.1415926" # How it works -The basic structure of `to_string` is shown below: - -```cpp -template<auto N, unsigned int base, typename char_type, /* N type-check and base bounds-check */> -struct to_string_t { - char_type buf[]; // Array size determination explained later. - constexpr to_string_t() {} // Converts the integer to a string stored in buf. - constexpr operator char_type *() {} // These allow for the object to be implicitly converted - constexpr operator const char_type *() {} // to a character pointer. - - // begin() and end() are supported too. -}; - -template<auto N, unsigned int base = 10, typename char_type = char> -constexpr to_string_t<N, base, char_type> to_string; // Simplifies usage, e.g. to_string_t<367>() becomes to_string<367>. -``` - -Since the number and base are template parameters, each differing `to_string` use will get its own character buffer. +C++14 greatly expanded the capabilities of compile-time code execution through `constexpr`. In particular, it allows for non-trivial constructors to be `constexpr`. -The integer/string conversion is done using a simple method I learned over the years, where the string is built in reverse using `n % base` to calculate the value of the lowest digit: +`to_string` takes advantage of this by providing an object that converts a template-parameter integer to a string using a basic `itoa` implementation in the constructor. Through an additional `constexpr` member function, we can calculate the length of the resulting string; this can be used to size the object's string buffer for a perfect fit. -(*Note: The below examples of code are not up-to-date, though they still give a general idea of how `to_string` works.*) +Beyond this, `to_string` simply provides familiar member functions that allow for iteration and data access. The expansion of the capabilities of `auto` in C++14 help make these definitions concise. -```cpp -constexpr char digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; - -constexpr to_string_t() { - auto ptr = buf + sizeof(buf) / sizeof(buf[0]); - *--ptr = '\0'; - for (auto n = N < 0 ? -N : N; n; n /= base) - *--ptr = digits[n % base]; - if (N < 0) - *--ptr = '-'; -} -``` - -As you may have noticed, `buf` needs to be given a size for all this to work; in fact, the above code relies on the buffer having a size equal to the generated string (or else `buf[0]` would still be uninitialized). This is actually the case: a lambda is used within `buf`'s declaration to count how many characters long the string will ultimately be. This counting is done in a manner similar to conversion loop shown above: +The floating-point implementation `f_to_string` takes a similar approach, but requires C++20 as it needs a `double_wrapper` object to capture the `double` value. `double` and `float` cannot directly be template parameters as of C++20, and a non-type template parameter like the `double_wrapper` structure was not allowed before C++20. -```cpp -char buf[([] { - unsigned int len = N >= 0 ? 1 : 2; // Need one byte for '\0', two if there'll be a minus - for (auto n = N < 0 ? -N : N; n; len++, n /= base); - return len; - }())]; -``` diff --git a/to_string.hpp b/to_string.hpp index 01a4c4d..d0a4dbe 100644 --- a/to_string.hpp +++ b/to_string.hpp @@ -7,11 +7,12 @@ #ifndef TCSULLIVAN_TO_STRING_HPP_ #define TCSULLIVAN_TO_STRING_HPP_ +#include <cstdint> #include <type_traits> namespace constexpr_to_string { -inline constexpr char digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; +constexpr char digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; /** * @struct to_string_t @@ -19,17 +20,17 @@ inline constexpr char digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; * @tparam N Number to convert * @tparam base Desired base, can be from 2 to 36 */ -template<auto N, int base, typename char_type, - std::enable_if_t<std::is_integral_v<decltype(N)>, int> = 0, +template<std::intmax_t N, int base, typename char_type, std::enable_if_t<(base > 1 && base < sizeof(digits)), int> = 0> class to_string_t { - // The lambda calculates what the string length of N will be, so that `buf` - // fits to the number perfectly. - char_type buf[([]() constexpr noexcept { - unsigned int len = N > 0 ? 1 : 2; - for (auto n = N; n; len++, n /= base); - return len; - }())] = {}; + + constexpr static auto buflen() noexcept { + unsigned int len = N > 0 ? 1 : 2; + for (auto n = N; n; len++, n /= base); + return len; + } + + char_type buf[buflen()] = {}; public: /** @@ -38,6 +39,7 @@ class to_string_t { constexpr to_string_t() noexcept { auto ptr = end(); *--ptr = '\0'; + if (N != 0) { for (auto n = N; n; n /= base) *--ptr = digits[(N < 0 ? -1 : 1) * (n % base)]; @@ -53,6 +55,7 @@ class to_string_t { constexpr operator const char_type *() const noexcept { return buf; } constexpr auto size() const noexcept { return sizeof(buf) / sizeof(buf[0]); } + // Element access constexpr auto data() noexcept { return buf; } constexpr const auto data() const noexcept { return buf; } @@ -62,6 +65,7 @@ class to_string_t { constexpr const auto& front() const noexcept { return buf[0]; } constexpr auto& back() noexcept { return buf[size() - 1]; } constexpr const auto& back() const noexcept { return buf[size() - 1]; } + // Iterators constexpr auto begin() noexcept { return buf; } constexpr const auto begin() const noexcept { return buf; } @@ -74,7 +78,8 @@ class to_string_t { /** * Simplifies use of `to_string_t` from `to_string_t<N>()` to `to_string<N>`. */ -template<auto N, int base = 10, typename char_type = char> +template<std::intmax_t N, int base = 10, typename char_type = char> constexpr constexpr_to_string::to_string_t<N, base, char_type> to_string; #endif // TCSULLIVAN_TO_STRING_HPP_ + |