Converting a string to a number via stringstream takes two steps:

 Constructing the stream from the string
 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:

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:

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

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