How to convert a number to string and vice versa in C++

Question

Update for C++11

As of the C++11 standard, string-to-number conversion and vice-versa are built in into the standard library. All the following functions are present in <string> (as per paragraph 21.5).

string to numeric

float              stof(const string& str, size_t *idx = 0);
double             stod(const string& str, size_t *idx = 0);
long double        stold(const string& str, size_t *idx = 0);
int                stoi(const string& str, size_t *idx = 0, int base = 10);
long               stol(const string& str, size_t *idx = 0, int base = 10);
unsigned long      stoul(const string& str, size_t *idx = 0, int base = 10);
long long          stoll(const string& str, size_t *idx = 0, int base = 10);
unsigned long long stoull(const string& str, size_t *idx = 0, int base = 10);

Each of these take a string as input and will try to convert it to a number. If no valid number could be constructed, for example because there is no numeric data or the number is out-of-range for the type, an exception is thrown (std::invalid_argument or std::out_of_range).

If conversion succeeded and idx is not 0, idx will contain the index of the first character that was not used for decoding. This could be an index behind the last character.

Finally, the integral types allow to specify a base, for digits larger than 9, the alphabet is assumed (a=10 until z=35). You can find more information about the exact formatting that can parsed here for floating-point numbers, signed integers and unsigned integers.

Finally, for each function there is also an overload that accepts a std::wstring as it's first parameter.

numeric to string

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

These are more straightforward, you pass the appropriate numeric type and you get a string back. For formatting options you should go back to the C++03 stringsream option and use stream manipulators, as explained in an other answer here.

As noted in the comments these functions fall back to a default mantissa precision that is likely not the maximum precision. If more precision is required for your application it's also best to go back to other string formatting procedures.

There are also similar functions defined that are named to_wstring, these will return a std::wstring.

How to convert a number to a string in C++03

1. Do not use the itoa or itof functions because they are non-standard and therefore not portable.
   

2. Use string streams
   

    #include <sstream>  //include this to use string streams
    #include <string> 
   
   int main()
   {    
       int number = 1234;
   
       std::ostringstream ostr; //output string stream
       ostr << number; //use the string stream just like cout,
       //except the stream prints not to stdout but to a string.
   
       std::string theNumberString = ostr.str(); //the str() function of the stream 
       //returns the string.
   
       //now  theNumberString is "1234"  
   }
   

   Note that you can use string streams also to convert floating-point numbers to string, and also to format the string as you wish, just like with cout

   std::ostringstream ostr;
   float f = 1.2;
   int i = 3;
   ostr << f << " + " i << " = " << f + i;   
   std::string s = ostr.str();
   //now s is "1.2 + 3 = 4.2" 
   

   You can use stream manipulators, such as std::endl, std::hex and functions std::setw(), std::setprecision() etc. with string streams in exactly the same manner as with cout

   Do not confuse std::ostringstream with std::ostrstream. The latter is deprecated

3. Use boost lexical cast. If you are not familiar with boost, it is a good idea to start with a small library like this lexical_cast. To download and install boost and its documentation go here. Although boost isn't in C++ standard many libraries of boost get standardized eventually and boost is widely considered of the best C++ libraries.

   Lexical cast uses streams underneath, so basically this option is the same as the previous one, just less verbose.

   #include <boost/lexical_cast.hpp>
   #include <string>
   
   int main()
   {
      float f = 1.2;
      int i = 42;
      std::string sf = boost::lexical_cast<std::string>(f); //sf is "1.2"
      std::string si = boost::lexical_cast<std::string>(i); //sf is "42"
   }
   

How to convert a string to a number in C++03

1. The most lightweight option, inherited from C, is the functions atoi (for integers (alphabetical to integer)) and atof (for floating-point values (alphabetical to float)). These functions take a C-style string as an argument (const char *) and therefore their usage may be considered a not exactly good C++ practice. cplusplus.com has easy-to-understand documentation on both atoi and atof including how they behave in case of bad input. However the link contains an error in that according to the standard if the input number is too large to fit in the target type, the behavior is undefined.

   #include <cstdlib> //the standard C library header
   #include <string>
   int main()
   {
       std::string si = "12";
       std::string sf = "1.2";
       int i = atoi(si.c_str()); //the c_str() function "converts" 
       double f = atof(sf.c_str()); //std::string to const char*
   }
   

2. Use string streams (this time input string stream, istringstream). Again, istringstream is used just like cin. Again, do not confuse istringstream with istrstream. The latter is deprecated.

   #include <sstream>
   #include <string>
   int main()
   {
      std::string inputString = "1234 12.3 44";
      std::istringstream istr(inputString);
      int i1, i2;
      float f;
      istr >> i1 >> f >> i2;
      //i1 is 1234, f is 12.3, i2 is 44  
   }
   

3. Use boost lexical cast.

   #include <boost/lexical_cast.hpp>
   #include <string>
   
   int main()
   {
      std::string sf = "42.2"; 
      std::string si = "42";
      float f = boost::lexical_cast<float>(sf); //f is 42.2
      int i = boost::lexical_cast<int>(si);  //i is 42
   }       
   

   In case of a bad input, lexical_cast throws an exception of type boost::bad_lexical_cast

In C++17, new functions std::to_chars and std::from_chars are introduced in header charconv.

std::to_chars is locale-independent, non-allocating, and non-throwing.

Only a small subset of formatting policies used by other libraries (such as std::sprintf) is provided.

From std::to_chars, same for std::from_chars.

The guarantee that std::from_chars can recover every floating-point value formatted by to_chars exactly is only provided if both functions are from the same implementation

 // See en.cppreference.com for more information, including format control.
#include <cstdio>
#include <cstddef>
#include <cstdlib>
#include <cassert>
#include <charconv>

using Type =  /* Any fundamental type */ ;
std::size_t buffer_size = /* ... */ ;

[[noreturn]] void report_and_exit(int ret, const char *output) noexcept 
{
    std::printf("%s\n", output);
    std::exit(ret);
}
void check(const std::errc &ec) noexcept
{
    if (ec ==  std::errc::value_too_large)
        report_and_exit(1, "Failed");
}
int main() {
    char buffer[buffer_size];        
    Type val_to_be_converted, result_of_converted_back;

    auto result1 = std::to_chars(buffer, buffer + buffer_size,  val_to_be_converted);
    check(result1.ec);
    *result1.ptr = '\0';

    auto result2 = std::from_chars(buffer, result1.ptr, result_of_converted_back);
    check(result2.ec);

    assert(val_to_be_converted == result_of_converted_back);
    report_and_exit(0, buffer);
}

Although it's not fully implemented by compilers, it definitely will be implemented.