Question

must you convert from strings to double? if so. how?

Are there functions for trig that will accept textbox string data as is?

Is there a way to pull the data from the textbox as a numeric value, not as a string?

Answer 1

must you convert from strings to double?

Yes.

if so. how?

The C++ way is to use the string streams; in particular, you’ll probably want to use istringstream. A viable alternative is to follow the C way, i.e. use sscanf with the appropriate format specifier (“%f”).

Another C++ option is to use the boost lexical_cast, but starting to use boost just for lexical_cast is a bit overkill in my opinion.

Example with istringstream:

#include <sstream>
// ...
std::istringstream strParser(yourString);
double yourDouble;
strParser>>yourDouble;
if(strParser.fail())
{
    // the string couldn't be converted to a double
}
else if(!strParser.eof())
{
    // the string hasn't been fully consumed; this may or may not be a problem, depending on your needs
}

Are there functions for trig that will accept textbox string data as is?

AFAIK no, there’s no need of them (although you can write them quite easily).

Is there a way to pull the data from the textbox as a numeric value, not as a string?

The WinAPIs provide a handful of such “convenience functions” for use in dialogs, but the only one I can recall that provides such help is GetDlgItemInt, which, as the name may suggest, works only for integers.

Addendum

I see now that you are using C++/CLI (you mentioned System::String): well, you should have said that, this changes the options quite a bit.

In the managed world, your best option is to use the Double::Parse method; to catch bad-formatted strings, you should either catch the exceptions thrown by Double::Parse or call Double::TryParse before calling Double::Parse.

Addendum bis

Uh, I forgot, since you’re using the .NET Framework probably it should be better to use the .NET trigonometric functions (class System.Math).