Active Template Library


The Active Template Library is a set of template-based C++ classes developed by Microsoft, intended to simplify the programming of Component Object Model objects. The COM support in Microsoft Visual C++ allows developers to create a variety of COM objects, OLE Automation servers, and ActiveX controls. ATL includes an object wizard that sets up primary structure of the objects quickly with a minimum of hand coding. On the COM client side ATL provides smart pointers that deal with COM reference counting. The library makes heavy use of the curiously recurring template pattern.

History

COM objects can also be created with Microsoft Foundation Classes, but this leads to larger binaries that require support DLLs. ATL, on the other hand, is a more lightweight alternative in situations where the graphical user interface parts of MFC are not required.
In ATL version 7, which directly succeeded version 3, a number of MFC classes like CString were made available in ATL, or more precisely moved to an ATLMFC common layer which is shared by both libraries. ATL version 7 also introduced attributes in C++ in an attempt to provide something similar to CLI attributes, however these have not been particularly successful, and have been deemphasized in ATL version 8 ; the various wizards no longer generate them by default. Version 7 also introduced new string conversion classes.
On July 28, 2009, Microsoft released a patch to ATL to fix a bug that could allow ActiveX controls created using ATL to be vulnerable to a remote code execution security flaw.
Since Visual Studio 2013, ATL code in Visual C++ 2013 is static, eliminating the DLL.

Support classes

ATL includes many RAII classes to simplify management of COM types. The most commonly used classes include:
Although not formally part of ATL, Microsoft Visual C++ also includes additional C++ RAII classes to simplify management of COM types. These compiler COM support classes can be used as replacement for or in combination with ATL, and includes:
Note that as of Visual Studio 2012, the compiler COM support classes does not include a SAFEARRAY wrapper.