C++ python.h - how to evaluate a string?

There is nothing like that in C/C++ because it is a compiled language. Those don't have functions like eval() because that would require the compiler (which translates strings into machine code) being part of the created application.

Python is basically an interpreted language (even when using a Python implementation precompiling to intermediate code). These require an executable on the target machine that finally executes the program by translating the source code or the intermediate code.

If you need such functionality in C/C++, you need an interpreter as part of the application or be called as external program.

In C++ you have two options, first and easiest is that you evaluate not that string, but fetch the needed operations from the UI and calculate it. (Get the numbers and operators from the button input) or you must parse the string data for numbers and operators yourself and than work it out. I found theDescCalc project on Github. But be aware: it may be too complex for your needs.

I would write a parser which is evaluate the input string to numbers and operators. Respect the rules of math like for:

a * b + c
(a + b) * c