# Caveats When Not Using C++ RTTI

- Introduction
- Bind() Issues
- wxAny Issues

See also:

- Runtime Type Information (RTTI)
- wxEvtHandler::Bind()
- wxAny

## Introduction

Note:
  C++ RTTI is usually enabled by default in most wxWidgets builds. If you do not know if your build has C++ RTTI enabled or not, then it probably is enabled, and you should not worry about anything mentioned in this section.

While in general wxWidgets standard Runtime Type Information (RTTI) is used throughout the library, there are some places where it won't work. One of those places is template classes.

When available, C++ RTTI is used to address this issue. If you have built the library with C++ RTTI disabled, an internal RTTI system is substituted. However, this system is not perfect and one proven scenario where it may break is a shared library or DLL build. More specifically, a template class instance created in one physical binary may not be recognized as its correct type when used in another one.

## Bind() Issues

wxWidgets 2.9.0 introduced a new Dynamic Event Handling system, using wxEvtHandler::Bind<>() and Unbind<>(). This functionality uses templates behind the scenes and therefore is vulnerable to breakage in shared library builds, as described above.

Currently only Unbind<>() needs the type information, so you should be immune to this problem simply if you only need to use Bind<>() and not Unbind<>().

Also, if you only bind and unbind same event handler inside same binary, you should be fine.

## wxAny Issues

wxAny is a dynamic type class which transparently uses templates to generate data type handlers, and therefore is vulnerable to breakage in shared library builds, as described above

You should be fine if you only create and use wxAny instances inside same physical binary. However, if you do need to be able to use wxAny freely across binary boundaries, (and for sake of code-safety, you probably do), then specializations for wxAnyValueTypeImpl<> templates need to be defined in one of your shared library (DLL) files. One specialization is required for every data type you use with wxAny. Easiest way to do this is using macros provided in wx/any.h. Note that you do not need to define specializations for C built-in types, nor for wxString or wxDateTime, because these are already provided in wxBase. However, you do need to define specializations for all pointer types except char* and wchar_t*.

Let's define a specialization for imaginary type 'MyClass'. In your shared library source code you will need to have this line:

        WX_IMPLEMENT_ANY_VALUE_TYPE(wxAnyValueTypeImpl<MyClass>)

In your header file you will need the following:

        wxDECLARE_ANY_TYPE(MyClass, WXIMPORT_OR_WXEXPORT)

Where WXIMPORT_OR_WXEXPORT is WXEXPORT when being included from the shared library that called the WX_IMPLEMENT_ANY_VALUE_TYPE() macro, and WXIMPORT otherwise.

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