Parsing string as a line of code in C++

Question

Is it possible that a string variable can be parsed as an actual line of code in C++? For example, can this string, "x=0", be parsed as actual code and set the value of x (some random variable in the program) to zero? What I plan to do with this is that I want to make a simple plotter in C++. The user enters the function (The function will be in terms of x and y and will have the value zero) to plot as a string (like 2*y+x), which then will be converted to a code object and then evaluated accordingly using a loop.

Answer 1

The short answer is "No". You can't compile C/C++ "on the fly" like that, as it's a compiled language, not an interpreted one.

But here's an idea: you could embed a JavaScript interpreter, using the SpiderMonkey API, which can interpret all your example code snippets, as JavaScript syntax is very similar to C/C++ in this regard.

Answer 2

Because C++ is a compiled and linked language it is not suitable for on-the-fly evaluation.

But I've achieved something similar to your aims in the past with C++ by embedding a Python interpretter to evaluate Python code as strings on the fly and pass the results to the C++ code.

Some other popular scripting languages that can be embedded in a C++ program are Lua and Squirrel.

In Java I've done the same by embedding a Groovy interpretter.

You need to integrate the scripting language interpretter into your code by "embedding" it and then pass values from the scripting language code to your C++ code by a process of "marshaling"

If you really want C++ syntax that can be interpretted, it is theoretically possible to develop a dynamic parser and interpretter for a subset of the language, but C++ is a complex language and such a task would be an enormous undertaking fraught with difficulty and essentially a case of using the wrong tool for the job.

Answer 3

The short answer is "Yes". Compiling C++ on the fly works just fine using a C++ JIT. From llvm.org

A Just-In-Time (JIT) code generation system, which currently supports X86, X86-64, ARM, AArch64, Mips, SystemZ, PowerPC, and PowerPC-64.

The assumption is that you're willing to link most of a compiler into your program in order to achieve this. With concerted effort you should be able to write "eval" on top of the existing API.