Clyne 2be778931d | 4 years ago | |
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LICENSE | 4 years ago | |
README.md | 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 - 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;
}())];