Active Template Library

ActiveTemplateLibrary (ATL) is a MicrosoftCorporation CeePlusPlus-based framework.

It was originally designed for developers of ComComponents (see ActivexTechnology). ActiveX controls had to be small enough to download quickly over a dial-up connection, so that they could be easily embedded in a web page. The alternative, MicrosoftFoundationClasses (MFC), produced much larger executables when linked statically, and required a several-megabyte set of runtime libraries when linked dynamically. In contrast, ATL-based ActiveX controls were typically much smaller--ATL classes use UpsideDownInheritance to allow derivation and polymorphism without vtables, and can often be developed without even linking against the C++ runtime libraries. However, ATL gives the developer a very restricted subset of the features provided by MFC.

Later, the WindowsTemplateLibrary was based on ATL. WTL attempts to extend ATL to more MFC-like features, without giving up the small footprint and inheritance model of ATL.

See http://www.microsoft.com/mind/0598/extreme0598.asp