Clyne
6710f1e699
Removing magic constant from template param check |
4 years ago | |
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LICENSE | 4 years ago | |
README.md | 4 years ago | |
f_to_string.hpp | 4 years ago | |
to_string.hpp | 4 years ago |
README.md
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 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:
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;
}())];