I have a float value that needs to be put into a std::string. How do I convert from float to string?

float val = 2.5; std::string my_val = val; // error here
1

8 Answers

As of C++11, the standard C++ library provides the function std::to_string(arg) with various supported types for arg.

2

Unless you're worried about performance, use string streams:

#include <sstream> //.. std::ostringstream ss; ss << myFloat; std::string s(ss.str());

If you're okay with Boost, lexical_cast<> is a convenient alternative:

std::string s = boost::lexical_cast<std::string>(myFloat);

Efficient alternatives are e.g. FastFormat or simply the C-style functions.

4

Important:
Read the note at the end.

Quick answer :
Use to_string(). (available since c++11)
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 :
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);

Important Note:
As @Michael Konečný rightfully pointed out, using to_string() is risky at best that is its very likely to cause unexpected results.
From :

With floating point types std::to_string may yield unexpected results as the number of significant digits in the returned string can be zero, see the example.
The return value may differ significantly from what std::cout prints by default, see the example. std::to_string relies on the current locale for formatting purposes, and therefore concurrent calls to std::to_string from multiple threads may result in partial serialization of calls. C++17 provides std::to_chars as a higher-performance locale-independent alternative.

The best way would be to use stringstream as others such as @dcp demonstrated in his answer.:

This issue is demonstrated in the following example :
run the example yourself : 

#include <iostream> #include <sstream> #include <string> template < typename Type > std::string to_str (const Type & t) { std::ostringstream os; os << t; return os.str (); } int main () { // more info : double f = 23.43; double f2 = 1e-9; double f3 = 1e40; double f4 = 1e-40; double f5 = 123456789; std::string f_str = std::to_string (f); std::string f_str2 = std::to_string (f2); // Note: returns "0.000000" std::string f_str3 = std::to_string (f3); // Note: Does not return "1e+40". std::string f_str4 = std::to_string (f4); // Note: returns "0.000000" std::string f_str5 = std::to_string (f5); std::cout << "std::cout: " << f << '\n' << "to_string: " << f_str << '\n' << "ostringstream: " << to_str (f) << "\n\n" << "std::cout: " << f2 << '\n' << "to_string: " << f_str2 << '\n' << "ostringstream: " << to_str (f2) << "\n\n" << "std::cout: " << f3 << '\n' << "to_string: " << f_str3 << '\n' << "ostringstream: " << to_str (f3) << "\n\n" << "std::cout: " << f4 << '\n' << "to_string: " << f_str4 << '\n' << "ostringstream: " << to_str (f4) << "\n\n" << "std::cout: " << f5 << '\n' << "to_string: " << f_str5 << '\n' << "ostringstream: " << to_str (f5) << '\n'; return 0; }

output : 

std::cout: 23.43 to_string: 23.430000 ostringstream: 23.43 std::cout: 1e-09 to_string: 0.000000 ostringstream: 1e-09 std::cout: 1e+40 to_string: 10000000000000000303786028427003666890752.000000 ostringstream: 1e+40 std::cout: 1e-40 to_string: 0.000000 ostringstream: 1e-40 std::cout: 1.23457e+08 to_string: 123456789.000000 ostringstream: 1.23457e+08
1

You can define a template which will work not only just with doubles, but with other types as well.

template <typename T> string tostr(const T& t) { ostringstream os; os<<t; return os.str(); }

Then you can use it for other types.

double x = 14.4; int y = 21; string sx = tostr(x); string sy = tostr(y);
0

Use std::to_chars once your standard library provides it:

std::array<char, 32> buf; auto result = std::to_chars(buf.data(), buf.data() + buf.size(), val); if (result.ec == std::errc()) { auto str = std::string(buf.data(), result.ptr - buf.data()); // use the string } else { // handle the error }

The advantages of this method are:

  • It is locale-independent, preventing bugs when writing data into formats such as JSON that require '.' as a decimal point
  • It provides shortest decimal representation with round trip guarantees
  • It is potentially more efficient than other standard methods because it doesn't use the locale and doesn't require allocation

Unfortunately std::to_string is of limited utility with floating point because it uses the fixed representation, rounding small values to zero and producing long strings for large values, e.g.

auto s1 = std::to_string(1e+40); // s1 == 10000000000000000303786028427003666890752.000000 auto s2 = std::to_string(1e-40); // s2 == 0.000000

C++20 might get a more convenient std::format API with the same benefits as std::to_chars if the P0645 standards proposal gets approved.

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

float val = 2.5; std::string my_val = std::to_string(val);
3

If you're worried about performance, check out the Boost::lexical_cast library.

1

This tutorial gives a simple, yet elegant, solution, which i transcribe:

#include <sstream> #include <string> #include <stdexcept> class BadConversion : public std::runtime_error { public: BadConversion(std::string const& s) : std::runtime_error(s) { } }; inline std::string stringify(double x) { std::ostringstream o; if (!(o << x)) throw BadConversion("stringify(double)"); return o.str(); } ... std::string my_val = stringify(val);