Some useful little functions to round, ceil, and floor to the nearest N. A little rough, but they work

rounding_fun.cpp

template<class T, class U>
T round_by( T const & value, U const & rnd_by ) {
	static_assert(std::is_arithmetic<T>::value, "First template parameter must be an arithmetic type");
	static_assert(std::is_floating_point<U>::value, "Second template parameter must be a floating point type");
	const auto rnd = round( static_cast<U>(value) / rnd_by );
	const auto ret = rnd*rnd_by;
	return static_cast<T>(ret);
}

template<class T, class U>
T floor_by( T const & value, U const & rnd_by ) {
	static_assert(std::is_arithmetic<T>::value, "First template parameter must be an arithmetic type");
	static_assert(std::is_floating_point<U>::value, "Second template parameter must be a floating point type");
	const auto rnd = floor( static_cast<U>(value) / rnd_by );
	const auto ret = rnd*rnd_by;
	assert( ret <= value );// , __func__": Error, return value should always be less than or equal to value supplied" );
	return static_cast<T>(ret);
}

template<class T, class U>
T ceil_by( T const & value, U const & rnd_by ) {
	static_assert(std::is_arithmetic<T>::value, "First template parameter must be an arithmetic type");
	static_assert(std::is_floating_point<U>::value, "Second template parameter must be a floating point type");
	const auto rnd = ceil( static_cast<U>(value) / rnd_by );
	const auto ret = rnd*rnd_by;
	assert( ret >= value ); // , __func__": Error, return value should always be greater than or equal to value supplied" );
	return static_cast<T>(ret);
}


int main( int, char** ) {
	std::cout << "36 rounded by 5 = " << round_by( 36, 5.0 ) << "\n";	// Will output 35
	std::cout << "36 ceil by 5 = " << ceil_by( 36, 5.0 ) << "\n";	// Will output 40
	std::cout << "36 floor by 5 = " << floor_by( 36, 5.0 ) << "\n";	// Will output 35
	std::cout << "36 rounded by 3.3 = " << round_by( 36, 3.3 ) << "\n"; // Will output 36.3
	std::cout << "86 2/3 rounded by 0.25 = " << round_by( 86.0+2.0*(1.0/3.0), 0.25 ) << "\n"; // Will output 86.75
	return 0;
}