As @aaronman pointed out, if you are working with integers only the fastest way to do this is to have all powers of 2 in table as there are not that many. By construction, in an unsigned 32 bit integer there are 32 powers of 2 (including the number 1), in a 64 bit integer there are 64 and so on.

But if you want to do it on the fly for a generic case you can easily calculate the surrounding powers of 2 of any number. In c/c++:

#include <math.h>

(...)

double bottom, top, number, exponent;

number = 1234;    // Set the value for number

exponent = int(log(number) / log(2.0));  // int(10.2691) = 10
bottom = pow(2, exponent);               // 2^10 = 1024
top = bottom * 2;                        // 2048

// Calculate the difference between number, top and bottom and add or subtract
// 1 accordingly
number = (top - number) <  (number - bottom) ? number + 1 : number - 1;