Change the logic of application at run-time - c++

I was wondering if it is possible to change the logic of an application at runtime? Meybe we could replace the implementation of an abstract class with another implementation? Or maybe we could replace a shared library at runtime...
update: Suppose that I've got two implementations of function foo(x, y) and can use any of them based on strategy pattern. Now I want to know if it's possible to add a third implementation of foo(x, y) without restarting the application.

You can use a plugin (a library that you will load at runtime) that expose a new foo function.
I remember we implemented something similar at school, a calculator in which we could add new operations at runtime, without having to restart the program. See dlsym and dlopen.
Addenda
Be very careful when dlclose-ing a plugin that it is not still used in some active call stack frame. On Linux you can call many thousands of times dlopen (so you could accept not dlclose-ing plugins, with some address space leak).

Exactly, as you said "replace the implementation of an abstract class with another implementation" if by it you mean, you can use runtime polymorphism and change the instances of concrete classes with instances of another set of concrete classes.
More specifically, there is a well-known pattern called Strategy pattern exactly for this purpose. Have a look at the wiki page, as it explains this very nicely, even with a code example along with diagram.

C++ mechanism of virtual functions does not allow you to change the implementation at run-time.
However, you can implement whatever implementation change at runtime with function pointers.

Here is an article on self-modifying code that I read recently: http://mainisusuallyafunction.blogspot.com/2011/11/self-modifying-code-for-debug-tracing.html

Related

How can one execute C++ code at runtime from a string representing a function?

I'm working on a project wherein I need to be able to save a function string to disk, so I am having the user pass a string of characters that is the actual code of the function and saving it to disk. The opposite is necessary as well; loading a string (from file) and executing as a function at runtime within C++. I need to load this function and return a function pointer to be used in my program. I'm looking at Clang right now, but some of it is a little over my head. So basically I have two questions;
Can Clang run code extracted from a string (loaded from disk)?
Can a compiled Clang function be represented with a function pointer pointing to it?
Any ideas?
The simple answer to your question is "yes", the slightly more complex answer is "not at all easily".
Doing it with C++ would require that you compile and link your function into a DLL/shared object, load it, then acquire the exported function. In addition, accepting such code from the user would be a terrible security risk
C++ is a very poor choice for such run-time execution, you would be far better off going with a language meant for that use, JavaScript or Python come to mind.
You can't easily do this in a compiled language.
For a compiled program to execute a C++ function that has been dynamically provided at runtime, that function would need to be compiled itself. You could make your program call the compiler at runtime to generate a callable library (e.g. one that implements an interface or abstract class and is callable via Dependency Injection), but this is complex and is a project in and of itself. This also means that your application must be packaged with the compiler or must only be installed on systems that contain a compatible compiler - somewhat realistic on Linux, not at all so on Windows.
A better solution would be to use an interpreter. JavaScript and Lisp both come with an eval() function that does exactly what you want - it takes a string (in the case of JavaScript) or a list (in the case of Lisp) and executes it as code.
A third possibility is to find a C++ interpreter that has an eval() function. I'm not sure if any exist. You could try to write one yourself.

Reflection + function pointers vs Observer pattern

I have got a program which gets commands via network and allocates these to a specific function. And now I want to implement a plugin feature where I can add a .dll file in a folder. The next step is to invoke the methods in the dll based on the command.
I have two ideas how to solve this problem but I do not know which of these is better/more performant:
Initializing all methods + commands from the dll with reflection and store them in a std::map<std::string, void(*func)(args...)>. When the program receives a command it looks up the associated function in the map and invokes it.
Load the dll into runtime and create an interface which hands over the std::string with the arguments to all dll's which have implemented it. The method in the dll uses if statements to check the command can be processed in there. (Observer pattern)
If there are better options which I have not mentioned let me know.
Although you are using incorrect terminology to describe what you want, I would say option 2 is "cleaner"
Forcing a singular interface that is implemented by plug-in DLLs is the best-practice way of establishing the sort of dependency injection you're seeking, in my opinion.

Is it possible to change the code in the program itself in c++?

About the last year I did Java(Android)-programming, and did C# the Year before that. About a month now I'm learning C++, and since I got over friends, inheritance and stuff, I got a few questions, since I haven't been working with it up until now:
Is there a way for a class to define friends later on, because they need to exchange information or something. e.g. is there a way to define a 'random' friend later on? what do you need for that? The function's name or the address of the class?
Or is there generally a way to change the code from the program itself, so that it won't be necessary to recompile? e.g. creating new functions, classes or so?
I'd be very happy about any answer about that.
What you want to do is not possible with C++. If you need that sort of dynamically changing the program, you are better advised using a more dynamic higher-level language like Lisp.
friends can only be added to a class by modifying its source code. This is a feature, not a bug.
There are two ways to extend functionality like that.
Use dynamically loaded modules with the extended functionality. These modules supply a specific interface, and can be compiled separately from the main program.
Add support for scripting - allow users to add write scripts, and run them from inside your program.
The first solution is easier, depending on how much control you want to give those scripts.

Can anyone explain the DYNAMIC_CLASSes in a few terms?

I've been developing for some time. And these beasts appear from time to time in MFC, wxWidgets code, and yet I can't find any info on what they do exactly.
As I understand, they appeared before dynamic_cast was integrated into core C++. And the purpose, is to allow for object creation on the fly, and runtime dynamic casts.
But this is where all the information I found, ends.
I've run into some sample code that uses DECLARE_DYNAMIC_CLASS and IMPLEMENT_DYNAMIC_CLASS within a DLL, and that is used for exported classes. And this structure confuses me.
Why is it done this way? Is that a plugin based approach, where you call LoadLibrary and then call the CreateDynamicClass to get a pointer which can be casted to the needed type?
Does the DECLARE/IMPLEMENT_DYNAMIC work over DLL boundaries? Since even class is not so safe to DLLEXPORT, and here we have a custom RTTI table in addition to existing problems.
Is it possible to derive my class from a DYNAMIC_CLASS from another DLL, how would it work?
Can anyone please explain me what these things are for, or where I can find more than a two sentences on a topic?
This stuff appends addional type information to your class, which allows to RTTI in runtime-independent manner, possibility of having factories to create your classes and many other things. You can find similar approach at COM, QMetaObject, etc
Have you looked at the definitions of DECLARE/IMPLEMENT_DYNAMIC?
In the MS world, all uppercase usually denotes a macro, so you can just look up the definition and try to work out what it's doing from there. If you're in Visual Studio, there's a key you can hit to jump to the definition - see what it says, and look that up and try to work from there.

How to implement monkey patch in C++?

Is it possible to implement monkey patching in C++?
Or any other similar approach to that?
Thanks.
Not portably so, and due to the dangers for larger projects you better have good reason.
The Preprocessor is probably the best candidate, due to it's ignorance of the language itself. It can be used to rename attributes, methods and other symbol names - but the replacement is global at least for a single #include or sequence of code.
I've used that before to beat "library diamonds" into submission - Library A and B both importing an OS library S, but in different ways so that some symbols of S would be identically named but different. (namespaces were out of the question, for they'd have much more far-reaching consequences).
Similary, you can replace symbol names with compatible-but-superior classes.
e.g. in VC, #import generates an import library that uses _bstr_t as type adapter. In one project I've successfully replaced these _bstr_t uses with a compatible-enough class that interoperated better with other code, just be #define'ing _bstr_t as my replacement class for the #import.
Patching the Virtual Method Table - either replacing the entire VMT or individual methods - is somethign else I've come across. It requires good understanding of how your compiler implements VMTs. I wouldn't do that in a real life project, because it depends on compiler internals, and you don't get any warning when thigns have changed. It's a fun exercise to learn about the implementation details of C++, though. One application would be switching at runtime from an initializer/loader stub to a full - or even data-dependent - implementation.
Generating code on the fly is common in certain scenarios, such as forwarding/filtering COM Interface calls or mapping OS Window Handles to library objects. I'm not sure if this is still "monkey-patching", as it isn't really toying with the language itself.
To add to other answers, consider that any function exposed through a shared object or DLL (depending on platform) can be overridden at run-time. Linux provides the LD_PRELOAD environment variable, which can specify a shared object to load after all others, which can be used to override arbitrary function definitions. It's actually about the best way to provide a "mock object" for unit-testing purposes, since it is not really invasive. However, unlike other forms of monkey-patching, be aware that a change like this is global. You can't specify one particular call to be different, without impacting other calls.
Considering the "guerilla third-party library use" aspect of monkey-patching, C++ offers a number of facilities:
const_cast lets you work around zealous const declarations.
#define private public prior to header inclusion lets you access private members.
subclassing and use Parent::protected_field lets you access protected members.
you can redefine a number of things at link time.
If the third party content you're working around is provided already compiled, though, most of the things feasible in dynamic languages isn't as easy, and often isn't possible at all.
I suppose it depends what you want to do. If you've already linked your program, you're gonna have a hard time replacing anything (short of actually changing the instructions in memory, which might be a stretch as well). However, before this happens, there are options. If you have a dynamically linked program, you can alter the way the linker operates (e.g. LD_LIBRARY_PATH environment variable) and have it link something else than the intended library.
Have a look at valgrind for example, which replaces (among alot of other magic stuff it's dealing with) the standard memory allocation mechanisms.
As monkey patching refers to dynamically changing code, I can't imagine how this could be implemented in C++...