Editing Active Template Library

{{Infobox software
| name                       = Active Template Library
| logo                       = 
| screenshot                 = 
| caption                    = 
| collapsible                = 
| author                     = 
| developer                  = [[Microsoft]]
| released                   = 
| latest release version     = 
| latest release date        = 
| latest preview version     = 
| latest preview date        = 
| programming language       = [[C++]]
| operating system           = [[Microsoft Windows]]
| platform                   = 
| size                       = 
| language                   = 
| genre                      = [[Library (computing)|Library]] or [[Software framework|framework]]
| license                    = [[Proprietary software|Proprietary]]
| website                    = 
}}{{Short description|Windows development library}}
The '''Active Template Library''' (ATL) is a set of template-based [[C++]] classes developed by [[Microsoft]], intended to simplify the programming of [[Component Object Model]] (COM) objects. The COM support in [[Microsoft Visual C++]] allows developers to create a variety of COM objects, [[OLE Automation]] servers, and [[ActiveX]] controls.<ref>{{cite book | title = ATL COM | edition = 1st | chapter = ATL and COM | page = [https://archive.org/details/atlcomprogrammer00grim/page/6 6] | last = Grimes | first = Richard | year = 1998 | publisher = Wrox Press | isbn = 1-86100-249-1 | chapter-url-access = registration | chapter-url = https://archive.org/details/atlcomprogrammer00grim/page/6 }}</ref><ref>{{cite book | title = Beginning ATL 3 COM Programming | edition = 1st | chapter = Building and calling a COM object using ATL | page = [https://archive.org/details/beginningatl3com00grim/page/63 63] | last = Grimes | first = Richard | year = 1999 | publisher = Wrox Press | isbn = 978-1-86100-120-7 | chapter-url-access = registration | chapter-url = https://archive.org/details/beginningatl3com00grim/page/63 }}</ref> 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 pointer]]s 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]] (MFC), but this leads to larger binaries that require support [[dynamic-link library|DLL]]s.<ref>{{cite book | title = Visual C++ 6 Unleashed | edition = 1st | chapter = Active Documents | page = 733 | last = Williams | first = Mickey | year = 2000 | publisher = Sams | isbn = 978-0-672-31241-0 }}</ref> ATL, on the other hand, is a more lightweight alternative in situations where the graphical user interface parts of MFC are not required.<ref>Microsoft MSDN: [https://msdn.microsoft.com/en-us/library/bk8ytxz5.aspx Recommendations for Choosing Between ATL and MFC]</ref>

Older versions of ATL (ATL prior to version 7.1, [[Visual_Studio#History|pre-Visual Studio 2005]]) are incompatible with [[Data Execution Prevention#Windows|DEP]] because it places executable [[thunk]]s in data heap.<ref>http://web.archive.org/web/20080130115007/http://blogs.msdn.com/michael_howard/archive/2008/01/29/new-nx-apis-added-to-windows-vista-sp1-windows-xp-sp3-and-windows-server-2008.aspx</ref><ref>https://learn.microsoft.com/en-us/archive/blogs/michael_howard/new-nx-apis-added-to-windows-vista-sp1-windows-xp-sp3-and-windows-server-2008</ref><ref>https://msrc.microsoft.com/blog/2009/06/understanding-dep-as-a-mitigation-technology-part-1/</ref> 
This problem can be mitigated with DEP-ATL thunk emulation with performance penalty.<ref>http://web.archive.org/web/20090123222148/http://support.microsoft.com/kb/948468</ref><ref>https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-updateprocthreadattribute</ref>

In ATL version 7 (Visual Studio 2003), which directly succeeded version 3 (Visual Studio 6.0), 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 [[Metadata (CLI)#Attributes|CLI attributes]], however these have not been particularly successful, and have been deemphasized in ATL version 8 (Visual Studio 2005); 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.<ref>{{cite web|url=http://msdn.microsoft.com/en-us/visualc/ee309358.aspx |title=ATL Security Update |publisher= Microsoft|work = MSDN| date=2009-07-28 |access-date=2011-09-19}}</ref>

Since Visual Studio 2013 (ATL version 12), all ATL code is static, eliminating the DLL.<ref>http://web.archive.org/web/20130827013146/http://blogs.msdn.com/b/vcblog/archive/2013/08/20/atl-and-mfc-changes-and-fixes-in-visual-studio-2013.aspx</ref><ref>https://devblogs.microsoft.com/cppblog/atl-and-mfc-changes-and-fixes-in-visual-studio-2013/</ref>

ATL version is defined by ATL_VER macros and can be queried via AtlGetVersion() function.<ref>https://learn.microsoft.com/en-us/cpp/atl/reference/atl-text-encoding-functions?view=msvc-170#atlgetversion</ref>

== Support classes ==
ATL includes many [[Resource Acquisition Is Initialization|RAII]] classes to simplify management of COM types. The most commonly used classes include:

* <code>CComPtr<T></code><ref>Microsoft MSDN: [https://msdn.microsoft.com/en-us/library/ezzw7k98.aspx CComPtr Class]</ref> general-purpose smart-pointer,
* <code>CComBSTR</code><ref>Microsoft MSDN: [https://msdn.microsoft.com/en-us/library/zh7x9w3f.aspx CComBSTR Class]</ref> BSTR wrapper,
* <code>CComVariant</code><ref>Microsoft MSDN: [https://msdn.microsoft.com/en-us/library/ac97df2h.aspx CComVariant Class]</ref> VARIANT wrapper, and
* <code>CComSafeArray<T></code><ref>Microsoft MSDN: [https://msdn.microsoft.com/en-us/library/3xzbsee8.aspx CComSafeArray Class]</ref> SAFEARRAY wrapper.

== Compiler COM support ==
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:

* <code>_com_ptr_t</code><ref>Microsoft MSDN: [https://msdn.microsoft.com/en-us/library/417w8b3b.aspx _com_ptr_t Class]</ref> smart-pointer that decorates the COM interface name with a "Ptr" suffix,
* <code>_bstr_t</code><ref>Microsoft MSDN: [https://msdn.microsoft.com/en-us/library/zthfhkd6.aspx _bstr_t Class]</ref> BSTR wrapper,
* <code>_variant_t</code><ref>Microsoft MSDN: [https://msdn.microsoft.com/en-us/library/x295h94e.aspx _variant_t Class]</ref> VARIANT wrapper, and
* <code>_com_error</code><ref>Microsoft MSDN: [https://msdn.microsoft.com/en-us/library/0ye3k36s.aspx _com_error Class]</ref> HRESULT wrapper.
Note that as of Visual Studio 2012, the compiler COM support classes does not include a SAFEARRAY wrapper.

== See also ==
* [[ATL Server]]
*[[C++/CX]]
* [[OLE Automation]]
* [[Standard Template Library]]
* [[Windows Template Library]]
* [[Microsoft Windows SDK]]
* [[List of C++ template libraries]]

== References ==
{{Reflist}}

== External links ==
* [https://msdn.microsoft.com/en-us/library/t9adwcde.aspx Official MSDN reference documentation]

{{Authority control}}

[[Category:C++ libraries]]
[[Category:Microsoft application programming interfaces|Template Library]]