c/c++ 浮点型处理

#include <stdio.h>
#include <iostream>
#include <string>
#include <string.h>
#include <sstream>
#include <iomanip>
using namespace std;

double round(double in, unsigned short decimal_presition)
{
    int n_i = in;
    double small = (double)(in - n_i + (double)(5*pow(10, -decimal_presition - 1));
    int n_f = small*(double)(pow(10, decimal_presition));
    return n_i + (double)n_f/pow(10, decimal_presition);
}

double round_price(double price, int dotnum)
{
    double out_price = 0.0;
    char buf[10];
    memset(buf, 0, 0);
    std::string str("%.");
    sprintf(buf, "%d", dotnum);
    str += buf;
    str += "lf";
    memset(buf, 0, 0);
    double cal_price = price + 0.0000001;

    sprintf(buf, str.c_str(), cal_price);
    cout << buf << endl;

    sscanf(buf, "%lf", &out_price);
    return out_price;
}

int main()
{
    double dd = 5.6666666666;
    double two = round_price(dd, 2);

    cout << two << endl;
    stringstream stream;
    stream <<setiosflags(ios::fixed);
    stream.precision(2);

    stream << 5.665111;

    double one;
    stream >> one;

    cout << one << endl;

    return 0;
}