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
• Works best in C++20 GCC or C++17/20 Clang

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.

Known issues:

• With C++17 GCC, to_string must be used to initialize variables; otherwise, the integer-string conversion is done at run-time.

How it works

The basic structure of to_string is shown below:

template<auto N, unsigned int base, /* N type-check and base bounds-check */>
struct to_string_t {
    char buf[];                          // Size selection explained later.
    constexpr to_string_t() {}           // Converts the integer to a string stored in buf.
    constexpr operator char *() {}       // These allow for the object to be implicitly converted
    constexpr operator const char *() {} // to a character pointer.
};

template<auto N, unsigned int base = 10>
to_string_t<N, base> to_string;          // Simplifies usage: to_string_t<N, base>() becomes to_string<N, base>.

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:

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;
          }())];