Numerical Check

I'm just wondering if anyone knows about any pre-made IsNumeric() checks for the string or String datatypes in C++? Or should I just build one myself? I seem to remember there being one in the #include <String> directory, but I'm not sure.

Also for some reason I'm getting an error when I try and declare a String saying "String" was not declared. e.g

#include <String>

int main()
{
   String szStuff;
}

error: 'String' was not declared in this scope

I'm sure <String> is a directory independent from <string> but maybe I'm just over-looking something simple.
I apologise in advance for what I expect to be a ridiculously simple school-boy error but I just can't figure this out at the moment.
Thanks for your help />