Sorry to reply to my own post, but I found a solution, though it requires a code change to boost: struct all_futures_lock { unsigned count; boost::scoped_array<boost::unique_lock<boost::mutex> > locks; all_futures_lock(std::vector<registered_waiter>& futures): count(static_cast<unsigned>(futures.size())),locks(new boost::unique_lock<boost::mutex>[count]) { for(unsigned i=0;i<count;++i) { locks[static_cast<std::ptrdiff_t>(i)]=boost::unique_lock<boost::mutex>(futures[i].future->mutex); } } void lock() { boost::lock(locks.get(),locks.get()+count); } void unlock() { for(unsigned i=0;i<count;++i) { locks[static_cast<std::ptrdiff_t>(i)].unlock(); } } }; Note the 3 static casts, the first is to avoid a warning, and the second two are to explicitly convert the unsigned variable to std::ptrdiff_t. VS 8.0 file "Microsoft Visual Studio 8\VC\include\crtdefs.h" line 516 defines ptrdiff_t as being "_w64 int" type. Thus, the T& scoped_array<T>::operator[](std::ptrdiff_t i) const function has an argument mismatch with unsigned. I realize that unsigned should convert to int just fine, so I'm not sure if this is a compiler issue or not. First Question: Is there anyone with a bit more experience on implicit conversions who can shed some light on whether or not the unsigned should be implictly converted to int, or whether a default function should be matched? Other question, is where is this *other* function coming from? Looking through the preprocessed file, I cannot find any other places where there is a operator[] being used on a scoped_array of any type. Second Question: Is is safe for me to change this header file, and build against it, while linking against the libraries which were built without this change? If not, can anyone suggest a better way to achieve this without having to modify the boost header? Thanks! -RD