find the “string length” of an int

Question

basically I want to return the number of digits in the int -> values like this:

(int)1 => 1
(int)123 => 3
(int)12345678 => 8

I kno

Answer 1

If your integer value (e.g. 12345678u) is a compile-time constant, you can let the compiler determine the length for you:

template
constexpr unsigned int_decimal_digits(T value)
{
    return (    value / 10
                    ?   int_decimal_digits(value/10) + 1
                    :   1 );
}

Usage:

unsigned n = int_decimal_digits(1234); 
// n = 4

#include 
unsigned m = int_decimal_digits(ULLONG_MAX);
// m = maximum length of a "long long unsigned" on your platform

This way, the compiler will compute the number of decimal places automatically, and fill in the value as a constant. It should be the fastest possible solution, because there is no run-time computation involved and integer constants are usually put into the instruction opcodes. (This means that they travel by instruction pipeline, not by data memory/cache.) However, this requires a compiler that supports C++11.