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Oct 9, 2009 (last update: Nov 22, 2012)

Converting numbers to strings and strings to numbers

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*****

Converting numbers to text and vice versa is a common issue as it can be useful in many different situations and C++98 doesn't provide a tool designed specifically to solve this problem.
Luckily C++ is a general purpose language so it allows to solve this quite easily and, as most things, you have many ways of accomplishing this task.
Here are listed some

Contents:



C++ - stringstreams

The C++ stream library is powerful and it allows easy formatted input output operations. With stringstreams you can perform this input/output to string, this allows you to convert numbers ( or any type with the << >> stream operators overloaded ) to and from strings.
With stringstreams you can use the same syntax to convert the different numeric types.
To use stringstreams you need to #include <sstream>

Number to String

Converting a number to a string takes two steps using stringstreams:
  1. Outputting the value of the number to the stream
  2. Getting the string with the contents of the stream
As with this conversion needs only output operation with the stream, an ostringstream ( output string stream ) can be used instead of the stream for both input and output ( stringstream )

Here is an example which shows each step:
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int Number = 123;       // number to be converted to a string

string Result;          // string which will contain the result

ostringstream convert;   // stream used for the conversion

convert << Number;      // insert the textual representation of 'Number' in the characters in the stream

Result = convert.str(); // set 'Result' to the contents of the stream

          // 'Result' now is equal to "123"

This operation can be shorten into a single line:
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int Number = 123;
string String = static_cast<ostringstream*>( &(ostringstream() << Number) )->str();
Here is constructed an unnamed stringstream object and performed the output ostringstream() << Number Then, since the << returns a reference to an ostream ( a base of ostringstream ) the result of the operation needs to be casted back to a stringstream static_cast<ostringstream*> Finally, we get the contents of the resulting stream as a string ->str() and we assign that value to the string string String =

Custom formatting


Stringstreams allow manipulators and locales to customize the result of these operations so you can easily change the format of the resulting string

Example: - This is not a complete program -
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// Headers needed:

#include <iomanip>
#include <locale>
#include <sstream>
#include <string> // this should be already included in <sstream>


// Defining own numeric facet:

class WithComma: public numpunct<char> // class for decimal numbers using comma instead of point
{
    protected:
        char do_decimal_point() const { return ','; } // change the decimal separator
};


// Conversion code:

double Number = 0.12;           // Number to convert to string

ostringstream Convert;

locale MyLocale(  locale(), new WithComma);// Crate customized locale

Convert.imbue(MyLocale);       // Imbue the custom locale to the stringstream

Convert << fixed << setprecision(3) << Number; // Use some manipulators

string Result = Convert.str(); // Give the result to the string

// Result is now equal to "0,120"

String to Number

Also converting a string to a number via stringstream takes two steps:
  1. Constructing the stream from the string
  2. Reading the value into the variable
For this ( as you need to read input from the stream ) an istringstream will be used
While a number can always be converted in a string, a string must be valid to be converted to a number ( eg: An attempt of converting "hello" to an integer would certainly fail ) so on this conversion, some checking must be done

Here is the code example:
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string Text = "456"; // string containing the number

int Result;          //number which will contain the result

istringstream convert(Text); // stringstream used for the conversion constructed with the contents of 'Text' 
                             // ie: the stream will start containing the characters of 'Text'

if ( !(convert >> Result) ) //give the value to 'Result' using the characters in the stream
    Result = 0;             //if that fails set 'Result' to 0

//'Result' now equal to 456

This conversion is even easier to reduce to a single line:
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string Text = "456";
int Number;
if ( ! (istringstream(Text) >> Number) ) Number = 0;
In the above code an object of istringstream gets constructed from 'Text' istringstream(Text) and its contents get read into the numeric variable >> Number.
If that operation fails if ( !, 'Number' is set to zero Number = 0;
Locales and manipulators can be used as well as with any stream

More complex cases


A generic stringstream ( which could be used both for input and for output ) can be useful in some more complex situations and in almost any situation you need to perform operations not provided by string

Simple Sample Functions


Here are listed some functions to perform these conversion using stringstreams:

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template <typename T>
  string NumberToString ( T Number )
  {
     ostringstream ss;
     ss << Number;
     return ss.str();
  }
Usage: NumberToString ( Number ); 
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template <typename T>
  T StringToNumber ( const string &Text )
  {
     istringstream ss(Text);
     T result;
     return ss >> result ? result : 0;
  }
Usage: StringToNumber<Type> ( String );


Notice: In the code examples std:: was omitted to make the code simpler
Using the last functions, there is no way of detecting whether the conversion succeded or failed


C++11

C++11 introduced some standard library functions that can directly convert
basic types to std::string objects and vice-versa.

std::to_string Converts basic numeric types to strings.

The set of functions
stoi, stol, stoll convert to integral types, the functions
stof, stod, stold to floating-point values.

These functions are declared in declared in <string>.

Note that since these functions were added in the latest version of the C++ standard,
they may not be available unless your implementation is very recent.

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int number = 123;
string text = to_string(number);

text = "456"
number = stoi(number);



C++ - Boost Library

Using stringstreams is the standard C++ way of doing these conversions but they usually need a few lines of code
Among the Boost libraries there is lexical_cast which allows to perform the stringstream conversions through simple function call
To make this library working, just include the header, it doesn't need to be linked

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// Boost header needed:
#include <boost/lexical_cast.hpp>

// Number to string conversion:
Text = boost::lexical_cast<string>(Number);

// String to number conversion:
Number = boost::lexical_cast<Type>(Text);

The above examples don't handle eventual conversion failures
When boost::lexical_cast fails, it throws boost::bad_lexical_cast ( derived from std::bad_cast )
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try
{
    Number = boost::lexical_cast<Type>(Text);
}
catch ( const boost::bad_lexical_cast &exc ) // conversion failed, exception thrown by lexical_cast and caught
{
    Number = 0; // give 'Number' an arbitrary value ( in this case zero )
                // if you don't give it any value, it would maintain the value it had before the conversion

    // A string containing a description of the exception can be found in exc.what()
}


C - stdio

Number to String


In C there is no stream library, but the function sprintf can be used for conversion
It works in a similar way to printf but it will put the characters in a C string ( a character array ) instead of stdout Using this is not as easy as with streams as the format string changes depending on the type of the number which needs to be converted

Example:
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int Number = 123; // number to convert

char Result[16]; // string which will contain the number

sprintf ( Result, "%d", Number ); // %d makes the result be a decimal integer

String to Number


As printf, also scanf has a related function which can read from a character array, sscanf 
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char Text[] = "456"; // string to be converted

int Result; // number which will contain the result

sscanf ( Text, "%d", &Result );

If sscanf fails ( ie: the string is not a number ), the value of the variable passed remains unchanged, in that case the function should return zero as no argument were read successfully, if the string passed is so bad that nothing can be read from it, it would return EOF:
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char Text[] = "456"; // string to be converted

int Result; // number which will contain the result

int Succeeded = sscanf ( Text, "%d", &Result );

if ( !Succeeded || Succeeded == EOF ) // check if something went wrong during the conversion
    Result = 0;


C - stdlib

The stdlib header contains some functions to convert text and numbers
Notice that some of these functions are not standard! These functions are:

- For examples refer to the individual reference pages -


Writing your own function

Using already existing libraries is easier and better but, just to show how some of the above solutions work, here are some examples of how to write functions to convert text to numbers and numbers to text using only the core language, the following examples are from the book "The C Programming Language"

Here is itoa ( Integer TO Alphabet )

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/* itoa:  convert n to characters in s */
void itoa(int n, char s[])
{
    int i, sign;

    if ((sign = n) < 0)  /* record sign */
        n = -n;          /* make n positive */
    i = 0;
    do {       /* generate digits in reverse order */
        s[i++] = n % 10 + '0';   /* get next digit */
    } while ((n /= 10) > 0);     /* delete it */
    if (sign < 0)
        s[i++] = '-';
    s[i] = '\0';
    reverse(s);
}

Here is the function reverse used in itoa:
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/* reverse:  reverse string s in place */
void reverse(char s[])
{
    int i, j;
    char c;

    for (i = 0, j = strlen(s)-1; i<j; i++, j--) {
        c = s[i];
        s[i] = s[j];
        s[j] = c;
    }
}

reverse uses the function strlen from the header cstring ( string.h in C )
This is easy to implement, here is an example:
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/* strlen: return length of s */
int strlen(char s[])
{
    int i = 0;
    while (s[i] != '\0')
        ++i;
    return i;
}

As you can see, is possible to create a ( bad ) conversion function with just some basic C
The same applies to the opposite conversion:
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/* atoi: convert s to integer */
int atoi(char s[])
{
    int i, n;
    n = 0;
    for (i = 0; s[i] >= '0' && s[i] <= '9'; ++i)
        n = 10 * n + (s[i] - '0');
    return n;
}

Of course these functions are bad for many reasons and should not be used in actual code
They just show the idea behind the conversion between an integer value and a character sequence