Convert double to string C++? [duplicate]

Question

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

I want to combine a string and a double and g++ is throwing this error:

main.cpp: In function ‘int main()’:
main.cpp:40: error: invalid operands of types ‘const char [2]’ and ‘double’ to binary ‘operator+’

Here is the line of code which it is throwing the error on:

storedCorrect[count] = "("+c1+","+c2+")";

storedCorrect[] is a string array, and c1 and c2 are both doubles. Is there a way to convert c1 and c2 to strings to allow my program to compile correctly?

Answer 1

You can't do it directly. There are a number of ways to do it:

1. Use a std::stringstream:

   std::ostringstream s;
   s << "(" << c1 << ", " << c2 << ")";
   storedCorrect[count] = s.str()
   

2. Use boost::lexical_cast:

   storedCorrect[count] = "(" + boost::lexical_cast<std::string>(c1) + ", " + boost::lexical_cast<std::string>(c2) + ")";
   

3. Use std::snprintf:

   char buffer[256];  // make sure this is big enough!!!
   snprintf(buffer, sizeof(buffer), "(%g, %g)", c1, c2);
   storedCorrect[count] = buffer;
   

There are a number of other ways, using various double-to-string conversion functions, but these are the main ways you'll see it done.

Answer 2

In C++11, use std::to_string if you can accept the default format (%f).

storedCorrect[count]= "(" + std::to_string(c1) + ", " + std::to_string(c2) + ")";

Answer 3

Use std::stringstream. Its operator << is overloaded for all built-in types.

#include <sstream>    

std::stringstream s;
s << "(" << c1 << "," << c2 << ")";
storedCorrect[count] = s.str();

This works like you'd expect - the same way you print to the screen with std::cout. You're simply "printing" to a string instead. The internals of operator << take care of making sure there's enough space and doing any necessary conversions (e.g., double to string).

Also, if you have the Boost library available, you might consider looking into lexical_cast. The syntax looks much like the normal C++-style casts:

#include <string>
#include <boost/lexical_cast.hpp>
using namespace boost;

storedCorrect[count] = "(" + lexical_cast<std::string>(c1) +
                       "," + lexical_cast<std::string>(c2) + ")";

Under the hood, boost::lexical_cast is basically doing the same thing we did with std::stringstream. A key advantage to using the Boost library is you can go the other way (e.g., string to double) just as easily. No more messing with atof() or strtod() and raw C-style strings.

Answer 4

std::string stringify(double x)
 {
   std::ostringstream o;
   if (!(o << x))
     throw BadConversion("stringify(double)");
   return o.str();
 }

C++ FAQ: http://www.parashift.com/c++-faq-lite/misc-technical-issues.html#faq-39.1