No, you cannot write code of the form

      #include <bitset>

      void foo (size_t n)
      {
	  std::bitset<n>   bits;
	  ....
      }
   

because n must be known at compile time. Your compiler is correct; it is not a bug. That's the way templates work. (Yes, it is a feature.)

There are a couple of ways to handle this kind of thing. Please consider all of them before passing judgement. They include, in no particular order:

A very large N in bitset<N>.   It has been pointed out a few times in newsgroups that N bits only takes up (N/8) bytes on most systems, and division by a factor of eight is pretty impressive when speaking of memory. Half a megabyte given over to a bitset (recall that there is zero space overhead for housekeeping info; it is known at compile time exactly how large the set is) will hold over four million bits. If you're using those bits as status flags (e.g., “changed”/“unchanged” flags), that's a lot of state.

You can then keep track of the “maximum bit used” during some testing runs on representative data, make note of how many of those bits really need to be there, and then reduce N to a smaller number. Leave some extra space, of course. (If you plan to write code like the incorrect example above, where the bitset is a local variable, then you may have to talk your compiler into allowing that much stack space; there may be zero space overhead, but it's all allocated inside the object.)

A container<bool>.   The Committee made provision for the space savings possible with that (N/8) usage previously mentioned, so that you don't have to do wasteful things like Container<char> or Container<short int>. Specifically, vector<bool> is required to be specialized for that space savings.

The problem is that vector<bool> doesn't behave like a normal vector anymore. There have been journal articles which discuss the problems (the ones by Herb Sutter in the May and July/August 1999 issues of C++ Report cover it well). Future revisions of the ISO C++ Standard will change the requirement for vector<bool> specialization. In the meantime, deque<bool> is recommended (although its behavior is sane, you probably will not get the space savings, but the allocation scheme is different than that of vector).

Extremely weird solutions.   If you have access to the compiler and linker at runtime, you can do something insane, like figuring out just how many bits you need, then writing a temporary source code file. That file contains an instantiation of bitset for the required number of bits, inside some wrapper functions with unchanging signatures. Have your program then call the compiler on that file using Position Independent Code, then open the newly-created object file and load those wrapper functions. You'll have an instantiation of bitset<N> for the exact N that you need at the time. Don't forget to delete the temporary files. (Yes, this can be, and has been, done.)

This would be the approach of either a visionary genius or a raving lunatic, depending on your programming and management style. Probably the latter.

Which of the above techniques you use, if any, are up to you and your intended application. Some time/space profiling is indicated if it really matters (don't just guess). And, if you manage to do anything along the lines of the third category, the author would love to hear from you...

Also note that the implementation of bitset used in libstdc++ has some extensions.

Bitmasks do not take char* nor const char* arguments in their constructors. This is something of an accident, but you can read about the problem: follow the library's “Links” from the homepage, and from the C++ information “defect reflector” link, select the library issues list. Issue number 116 describes the problem.

For now you can simply make a temporary string object using the constructor expression:

      std::bitset<5> b ( std::string("10110") );
   

instead of

      std::bitset<5> b ( "10110" );    // invalid