If I have some c++ code as a string quantity (data) in a c++ program, can I execute the contents of that string?
As using the CodeDOM in C# or the eval function present
Non of the mainstream C++ implementations have that feature, as C++ is not reflective.
However, take a look at Ch, it might be what you are looking for:
http://www.softintegration.com/
http://en.wikipedia.org/wiki/Ch_interpreter
You can embed Ch interpreter into your C++ application and run dynamic C++ code inside it.