Convert double to string using boost::lexical_cast in C++?

Question

I' d like to use lexical_cast to convert a float to a string. Usually it works fine, but I have some problems with numbers without decimal. How can I fix number of decimal shown in the string?

Example:

double n=5;
string number;
number = boost::lexical_cast<string>(n);

Result number is 5, I need number 5.00.

Answer 1

From the documentation for boost lexical_cast:

For more involved conversions, such as where precision or formatting need tighter control than is offered by the default behavior of lexical_cast, the conventional stringstream approach is recommended. Where the conversions are numeric to numeric, numeric_cast may offer more reasonable behavior than lexical_cast.

Example:

#include <sstream>
#include <iomanip>

int main() {
    std::ostringstream ss;
    double x = 5;
    ss << std::fixed << std::setprecision(2);
    ss << x;
    std::string s = ss.str();
    return 0;
}

Answer 2

Have a look at boost::format library. It merges the usability of printf with type safety of streams. For speed, I do not know, but I doubt it really matters nowadays.

#include <boost/format.hpp>
#include <iostream>

int main()
{
   double x = 5.0;
   std::cout << boost::str(boost::format("%.2f") % x) << '\n';
}

Answer 3

If you need complex formatting, use std::ostringstream instead. boost::lexical_cast is meant for "simple formatting".

std::string
get_formatted_value(double d) {
    std::ostringstream oss;
    oss.setprecision(3);
    oss.setf(std::ostringstream::showpoint);
    oss << d;
    return oss.str();
}

Answer 4

you can also use sprintf, which is faster then ostringstream

#include <cstdio>
#include <string>

using namespace std;

int main()
{
    double n = 5.0;

    char str_tmp[50];
    sprintf(str_tmp, "%.2f", n); 
    string number(str_tmp);
}