When developing the 64-bit version of an application, you should also be very attentive to the issue of program distribution - you might encounter some peculiar problems when installing the program on a 64-bit operating system, and if you forget about them, you will get a non-working installation package.
First of all you should understand that the program installer itself (the exe-file that launches the installation process) can technically be either a 32-bit application or a 64-bit one. If you make this installer 64-bit, it will not work on a 32-bit system. Note that you will not see a message like: "You are trying to install a distribution kit of a 64-bit program on a 32-bit system". It will simply generate a message about a damaged file. Thus, it is most often reasonable to make the installer a 32-bit application even if it will be installed only on a 64-bit system.
An important issue of operation of 32-bit programs in the 64-bit environment is the redirection mechanism implemented in Windows. This mechanism arranges the work of obsolete 32-bit applications so that when trying to access, for instance, the folder "c:\program files", they are automatically redirected to "c:\program files (x86)". The access to some register sections is also automatically redirected.
When developing installers, the redirection mechanism may often lead to a situation when the files appear in some other place than the developer has expected, or the register entries relate to some other sections than they should. You may read about the redirection system in MSDN Help system: "File System Redirector".
All these specifics can be set in any contemporary installer but you should not forget about it while creating a distribution kit compatible with 64-bit operating systems.
When developing installers for 64-bit versions, developers often make one common installer that contains both the 32-bit and 64-bit versions of the applications and their components. In this case, do not forget to add full sets of components and dependent libraries. For example, developers often add Visual C++ Redistributable Package into the installation package for applications developed with Visual C++. This package must be included both for the x86 and x64 versions.
If your installer uses some third-party units to implement complex functionality, you should remember about the restriction - units built for the 64-bit mode cannot load 32-bit dynamic libraries and vice versa.
The rightholder of the course "Lessons on development of 64-bit C/C++ applications" is OOO "Program Verification Systems". The company develops software in the sphere of source program code analysis. The company's site: http://www.viva64.com.