support c++14

README.md

@@ -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;
-          }())];
-```

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 {
+
+    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_
+