c++ - Why is my integer math with std::pow giving the wrong answer? -

consider following piece of code:

#include <iostream> #include <cmath>  int main() {     int = 23;     int j = 1;     int base = 10;     int k = 2;     += j * pow(base, k);     std::cout << << std::endl; } 

it outputs "122" instead of "123". bug in g++ 4.7.2 (mingw, windows xp)?

std::pow() works floating point numbers, not have infinite precision, , implementation of standard library using implements pow() in (poor) way makes lack of infinite precision become relevant.

however, define own version works integers. in c++11, can make constexpr (so result computed @ compile-time when possible):

constexpr int int_pow(int b, int e) {     return (e == 0) ? 1 : b * int_pow(b, e - 1); } 

here live example.

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tail-recursive form (credits dan nissenbaum):

constexpr int int_pow(int b, int e, int res = 1) {     return (e == 0) ? res : int_pow(b, e - 1, b * res); }