Is there a good C++ tool for parsing a decimal?

I have a place in my code where I have an element iter of type std::string::const_iterator and if *iter is the beginning of a decimal (i.e. the beginning of a string of the form -00.01, .60, 12319.1, 99, etc.) then I want to advance iter until it has passed through the decimal string and I want to store the decimal string.

For a concrete setup of the scenario:

for (std::string::const_iterator iter(str.begin()), offend(str.end()); iter != offend; ++iter)
{
    if (iter == '(')
    {
       //... blah blah blah
    }
    else if (iter is the beginning of a decimal representation)
    {
       parse the decimal representation, store it in a string, advance iter to the character at the end of the decimal representation (or to offend -- whichever comes first)
    }
    else if (some other condition involving iter)
    {
       // ... blah blah blah
    } 
    //...
} 

Is there a C++ tool that I can exploit for this scenario?