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

// The C way:
char buffer[32];
snprintf(buffer, sizeof(buffer), "%g", myDoubleVar);

// The C++03 way:
std::ostringstream sstream;
sstream << myDoubleVar;
std::string varAsString = sstream.str();

// The C++11 way:
std::string varAsString = std::to_string(myDoubleVar);

// The boost way:
std::string varAsString = boost::lexical_cast<std::string>(myDoubleVar);

Answer 2

The boost (tm) way:

std::string str = boost::lexical_cast<std::string>(dbl);

The Standard C++ way:

std::ostringstream strs;
strs << dbl;
std::string str = strs.str();

Note: Don't forget #include <sstream>

Answer 3

The Standard C++11 way (if you don't care about the output format):

#include <string>

auto str = std::to_string(42.5); 

to_string is a new library function introduced in N1803 (r0), N1982 (r1) and N2408 (r2) "Simple Numeric Access". There are also the stod function to perform the reverse operation.

If you do want to have a different output format than "%f", use the snprintf or ostringstream methods as illustrated in other answers.

Answer 4

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

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

Answer 5

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 6

sprintf is okay, but in C++, the better, safer, and also slightly slower way of doing the conversion is with stringstream:

#include <sstream>
#include <string>

// In some function:
double d = 453.23;
std::ostringstream os;
os << d;
std::string str = os.str();

You can also use Boost.LexicalCast:

#include <boost/lexical_cast.hpp>
#include <string>

// In some function:
double d = 453.23;
std::string str = boost::lexical_cast<string>(d);

In both instances, str should be "453.23" afterward. LexicalCast has some advantages in that it ensures the transformation is complete. It uses stringstreams internally.

Answer 7

I would look at the C++ String Toolkit Libary. Just posted a similar answer elsewhere. I have found it very fast and reliable.

#include <strtk.hpp>

double pi = M_PI;
std::string pi_as_string  = strtk::type_to_string<double>( pi );

Answer 8

The problem with lexical_cast is the inability to define precision. Normally if you are converting a double to a string, it is because you want to print it out. If the precision is too much or too little, it would affect your output.

Answer 9

You could also use stringstream.

Answer 10

Heh, I just wrote this (unrelated to this question):

string temp = "";
stringstream outStream;
double ratio = (currentImage->width*1.0f)/currentImage->height;
outStream << " R: " << ratio;
temp = outStream.str();

/* rest of the code */

Answer 11

Normaly for this operations you have to use the C default ecvt, fcvt or gcvt Functions:

/* gcvt example */
#include <stdio.h>
#include <stdlib.h>

main ()
{
  char buffer [20];
  gcvt (1365.249,6,buffer);
  puts (buffer);
  gcvt (1365.249,3,buffer);
  puts (buffer);
  return 0;
}

Output:
1365.25
1.37e+003   

As a Function:

void double_to_char(double f,char * buffer){
  gcvt(f,10,buffer);
}

Answer 12

Take a look at sprintf() and family.

Answer 13

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 14

Note that a string is just a representation of the double and converting it back to double may not result in the same value. Also note that the default string conversion may trim the conversion to a certain precision. In the standard C++ way, you can control the precision as follows:

#include <sstream>
#include <math.h>
#include <iostream>
#include <iomanip>

int main()
{
    std::ostringstream sout;
    sout << M_PI << '\n';
    sout << std::setprecision(99) << M_PI << '\n';
    sout << std::setprecision(3) << M_PI << '\n';
    sout << std::fixed; //now the setprecision() value will look at the decimal part only.
    sout << std::setprecision(3) << M_PI << '\n';
    std::cout << sout.str();
}

which will give you the output

3.14159                                                                                                                                                                            
3.141592653589793115997963468544185161590576171875                                                                                                                                 
3.14                                                                                                                                                                               
3.142  

Answer 15

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);

Answer 16

You could try a more compact style:

std::string number_in_string;

double number_in_double;

std::ostringstream output;

number_in_string = (dynamic_cast< std::ostringstream*>(&(output << number_in_double <<

std::endl)))->str(); 

Answer 17

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 18

C++17 has introduced: std::to_chars, std::to_chars_result - cppreference.com

std::to_chars_result to_chars( char* first, char* last, float       value,
                               std::chars_format fmt, int precision );
std::to_chars_result to_chars( char* first, char* last, double      value,
                               std::chars_format fmt, int precision );
std::to_chars_result to_chars( char* first, char* last, long double value,
                               std::chars_format fmt, int precision );

Which provide fast low level way to convert floating points into string with some level of format control. This should be fast since no allocation is done, only custom implementation for specific scenario should be faster.

C++20 has introduced high level easy to use format string (equivalent of fmt library):

std::format - cppreference.com

std::format

template< class... Args >
std::string format( /*format_string<Args...>*/ fmt, Args&&... args );

template< class... Args >
std::wstring format( /*wformat_string<Args...>*/ fmt, Args&&... args );

template< class... Args >
std::string format( const std::locale& loc,
                    /*format_string<Args...>*/ fmt, Args&&... args );

template< class... Args >
std::wstring format( const std::locale& loc,
                     /*wformat_string<Args...>*/ fmt, Args&&... args );

Which is quite nice and handy. Should be faster then sprintf.