How do I convert a double into a string in C++?

Question

I need to store a double as a string. I know I can use printf if I wanted to display it, but I just want to store it in a string variable so that I can store it in a map later (as the value, not the key).

Answer 1

You can convert any thing to anything using this function:

template<class T = std::string, class U>
T to(U a) {
    std::stringstream ss;
    T ret;
    ss << a;
    ss >> ret;
    return ret;
};

usage :

std::string str = to(2.5);
double d = to<double>("2.5");

Answer 2

You can use std::to_string in C++11

double d = 3.0;
std::string str = std::to_string(d);

Answer 3

If you use C++, avoid sprintf. It's un-C++y and has several problems. Stringstreams are the method of choice, preferably encapsulated as in Boost.LexicalCast which can be done quite easily:

template <typename T>
std::string to_string(T const& value) {
    stringstream sstr;
    sstr << value;
    return sstr.str();
}

Usage:

string s = to_string(42.5);

Answer 4

You may want to read my prior posting on SO. (Macro'ed version with a temporary ostringstream object.)

For the record: In my own code, I favor snprintf(). With a char array on the local stack, it's not that inefficient. (Well, maybe if you exceeded the array size and looped to do it twice...)

(I've also wrapped it via vsnprintf(). But that costs me some type checking. Yelp if you want the code...)

Answer 5

Use to_string().
example :

#include <iostream>   
#include <string>  

using namespace std;
int main ()
{
    string pi = "pi is " + to_string(3.1415926);
    cout<< "pi = "<< pi << endl;

  return 0;
}

run it yourself : http://ideone.com/7ejfaU
These are available as well :

string to_string (int val);
string to_string (long val);
string to_string (long long val);
string to_string (unsigned val);
string to_string (unsigned long val);
string to_string (unsigned long long val);
string to_string (float val);
string to_string (double val);
string to_string (long double val);