How to implement monkey patch in C++?

c++

Solution

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.

Problem

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