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C++/dynamic bit-fields


I was wondering if there is such thing as a dynamic bit-field or any other way to implement it?
To illuminate the question how can i create a bit-field where the size of the members is provided at run-time and added to the developing struct or any other data type? Any method possible? THANKS IN ADVANCE!!

Short answer: no

You cannot modify types at runtime - all types and their structure have to be defined at compile time - C++ is a statically typed language. Bit fields are a low level feature best used when doing low-level interfacing with hardware registers (for example) that may have specific bit-layouts and obviously do not tend to change dynamically.

However, depending on what you are actually trying to achieve other approaches might be of use.

For example, the Boost libraries ( include a dynamic bitset which may or may not be of use.

Or you could perform the dynamic field allocation manually - e.g. using a std::vector<unsigned long> as the underlying container and wrapping it in a class that manages the bit fields within the vector.

However, such dynamic runtime management of fields is going to require space and probably time, overheads, to keep track of - for example that field n is m bits wide, and possibly which vector element and bit number within that element the field starts at.

For more on the basics of bit fields, including hints on how to perform such operations manually and how the sort of code compilers generate for non-bit field and bit field access differ, please see the my answers to the first two questions in this sequence:

Hope this gives you some hints.


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Ralph McArdell


I am a software developer with more than 15 years C++ experience and over 25 years experience developing a wide variety of applications for Windows NT/2000/XP, UNIX, Linux and other platforms. I can help with basic to advanced C++, C (although I do not write just-C much if at all these days so maybe ask in the C section about purely C matters), software development and many platform specific and system development problems.


My career started in the mid 1980s working as a batch process operator for the now defunct Inner London Education Authority, working on Prime mini computers. I then moved into the role of Programmer / Analyst, also on the Primes, then into technical support and finally into the micro computing section, using a variety of 16 and 8 bit machines. Following the demise of the ILEA I worked for a small company, now gone, called Hodos. I worked on a part task train simulator using C and the Intel DVI (Digital Video Interactive) - the hardware based predecessor to Indeo. Other projects included a CGI based train simulator (different goals to the first), and various other projects in C and Visual Basic (er, version 1 that is). When Hodos went into receivership I went freelance and finally managed to start working in C++. I initially had contracts working on train simulators (surprise) and multimedia - I worked on many of the Dorling Kindersley CD-ROM titles and wrote the screensaver games for the Wallace and Gromit Cracking Animator CD. My more recent contracts have been more traditionally IT based, working predominately in C++ on MS Windows NT, 2000. XP, Linux and UN*X. These projects have had wide ranging additional skill sets including system analysis and design, databases and SQL in various guises, C#, client server and remoting, cross porting applications between platforms and various client development processes. I have an interest in the development of the C++ core language and libraries and try to keep up with at least some of the papers on the ISO C++ Standard Committee site at


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