yury-fedorov / 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>

How to use:

This single header file 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.

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[];                          // Size selection 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;
          }())];