let the user use a function in c++ [duplicate]

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Dynamic source code in C++

is it possible to let the user type in a function and then r开发者_如何学Goun that function without using a lot of if's or a huge switch?

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It is not possible to execute arbitrary c++ code in your program, since you than need a c++ compiler inside your program. But you could try to embed Python to your program. Boost python makes this relatively easy. The user can than write a python function that is executed and can interact with the classes and functions of your program. You need to make your functions explicitely visible to python.

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What ever a user types in will be text, or a string. The only way I know to have it get mapped to a function is to use if/else or switch statements. That or the cringe inducing option of mapping each of your functions to a UI widget.

The end of the story, is it's your code. You have to write, and live with it. Just be careful, your program may be wildly successful, and you may not write code anymore, and then someone else will have to maintain your code. So be nice to the maintenance programmer who may follow you, and write code that isn't too tricky to figure out.

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I assume you want something like eval from php.

You can try to play with command design pattern, but I doubt it will be an easy task. Basically you need to write simple C++ interpreter.

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What type of function do you mean? A C++ function? If so, then you will have to either (1)interpret it or (2)compile and execute it. Interpretation would be the more likely choice here. I'm not sure if there are libraries out there already to do this but I'd assume there are.

If you don't like mega-if's or huge switches, you may be SoL on any solution for anything ever, but then again there is seldom one perfect way to do things. Consider looking in to various logic structures and algorithms to see how to do something that would normally be the job of a 23-case switch could be done another way. Like I said initially, however, sometimes you really do just need a million nested if's to do what you want to.

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No, in C++ this is not possible. C++ is a compiled language. When the program runs, the compiler doesn't need to be accessible, or even installed on the machine that runs the program.

If you want to do this in C++, you need to write your own interpreter that parses whatever the user enters.

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Here is my best idea, but it is a tad memory intensive.

First, create a class, lets call it MyFuncPtr to store a union of several different types of pointers to functions and an integer to tell which type it is. Overload the () operator to call the function stored with a variable length argument list. Make sure to include some sort of run-time argument checking.

Finally create a map of strings to MyFuncPtrs. Store your functions in this map along with their names. Then all you need to do is feed the name into the [] command to get a function that can be easily called. Templates could probably be used to aid in the making of MyFuncPtr instances.

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This would be the easiest if it were plain C functions and no name mangling is performed on the symbols (use extern "C" { ... })

With some platform-specific code you can get the address of a function by its name. Then you cast the address as a function pointer which you can use to call the function.

On windows you must be using GetProcAddress and dlsym on Posix compliant platforms.