ezhangle / constexpr-to-string

C++17 code to convert integers to strings at compile-time

constexpr-to-string

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 custom character types, e.g. to_string<123, 10, wchar_t>
• C++20: Supports floating-point-to-string conversion with f_to_string

How to use:

The file to_string.hpp provides a to_string utility, which may be used as below:

const char *number = to_string<2147483648999954564, 16>; // produces "1DCD65003B9A1884"
puts(number);
puts(to_string<-42>); // produces "-42"
puts(to_string<30, 2>); // produces "11110"

With to_string, all that will be found in program disassembly are the resulting string literals, as if you wrote the strings yourself.

Try it on Compiler Explorer.

f_to_string.hpp, requiring C++20, provides an f_to_string utility for floating-point conversion:

puts(f_to_string<3.1415926>); // Defaults to 5-point precision: "3.14159"
puts(f_to_string<{3.1415926, 7}>); // Specify precision: "3.1415926"

How it works

The basic structure of to_string is shown below:

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.

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:

(Note: The below examples of code are not up-to-date, though they still give a general idea of how to_string works.)

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 = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"[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:

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;
          }())];