c++ - Can an iterator be turned into a pointer? -


in "13.7.3 async()" of "a tour of c++" (first print), following provided example:

double comp4(vector<double>& v) {   if (v.size() < 10000)     return accum(v.begin(), v.end(), 0.0);    auto v0 = &v[0];   auto sz = v.size();    auto f0 = async(accum, v0, v0 + sz/4, 0.0);   auto f1 = async(accum, v0 + sz/4, v0 + sz/2, 0.0);   auto f2 = async(accum, v0 + sz/2, v0 + sz*3/4, 0.0);   auto f3 = async(accum, v0 + sz*3/4, v0 + sz, 0.0);    return f0.get() + f1.get() + f2.get() + f3.get(); }

where accum defined follows:

double accum(double* beg, double* end, double init) {   return accumulate(beg, end, init); }

vector, async , accumulate standard library versions.

running produces following error:

error: cannot convert ‘std::vector<double>::iterator {aka __gnu_cxx::__normal_iterator<double*, std::vector<double> >}’ ‘double*’ argument ‘1’ ‘double accum(double*, double*, double)’      return accum(v.begin(), v.end(), 0.0);

i can work if use return accum(&v[0], &v[0] + v.size(), 0.0); instead. have done wrong in original formulation? should vector<double>::iterator able convert double* or mistake book? issue doesn't seem mentioned in errata both first , second printings of book.

the way std::vector specified, it's entirely possible implement having vector<t>::iterator typedef of t*.

odds whoever wrote code happened have version of vector implemented way , got lucky.

your workaround correct way this, assuming definition of accum() doesn't change.

in opinion, accum should have been declared so:

template<typename t, typename ite> t accum(ite beg, ite end, t init);

but that's std::accumulate, it'd redundant.


Comments

Post a Comment

Popular posts from this blog

Is there a better way to structure post methods in Class Based Views -

i'm using class based views in django , never specify url in ajax calls , omit action parameter in form because know these requests go through post method of class based view. notice view - profile page make many requests depending on user updating on page - post method become convoluted , if/else mess example: def post(self, request, *args, **kwargs): # handle user changing available date if request.post.get('availabledate') != none: self.updateavailabledate(request) return jsonresponse({'result':'success'}) if request.post.get('newprojectname') != none: creative_user = creativeuserprofile.objects.get(id = request.user.id) project = project.create_project(creative_user, request.post['newprojectname']) project.save() return jsonresponse({'projectid': project.id }) if request.post.get('projectimage') != none , request.files['file']: proj
Read more

performance - Why is XCHG reg, reg a 3 micro-op instruction on modern Intel architectures? -

i'm doing micro-optimization on performance critical part of code , came across sequence of instructions (in at&t syntax): add %rax, %rbx mov %rdx, %rax mov %rbx, %rdx i thought had use case xchg allow me shave instruction , write: add %rbx, %rax xchg %rax, %rdx however, dimay found agner fog's instruction tables , xchg 3 micro-op instruction 2 cycle latency on sandy bridge, ivy bridge, broadwell, haswell , skylake. 3 whole micro-ops , 2 cycles of latency! 3 micro-ops throws off 4-1-1-1 cadence , 2 cycle latency makes worse original in best case since last 2 instructions in original might execute in parallel. now... cpu might breaking instruction micro-ops equivalent to: mov %rax, %tmp mov %rdx, %rax mov %tmp, %rdx where tmp anonymous internal register , suppose last 2 micro-ops run in parallel latency 2 cycles. given register renaming occurs on these micro-architectures, though, doesn't make sense me done way. why wouldn't register renamer
Read more

jquery - Responsive Navbar with Sub Navbar -

good day, forgive me nav vocabulary not best. if question confusing apologize. i'm working on navigation bar needs centered on page @ 768px , above. items in nav open (or perhaps toggle) subnav lives directly below. these nav should appear centered on page. i have been able work through responsive portion of nav bar, having main items align vertically , sub nav menu of each item roll out below, , gave them indent separation. working well. however, issue not responsive side, behavior @ width greater 768px. happens toggling menu's sub navigation pushes other main items down page below sub nav. here's code: $(document).ready(function () { //stick in fixed 100% height behind navbar don't wrap $('#slide-nav.navbar-inverse').after($('<div class="inverse" id="navbar-height-col"></div>')); $('#slide-nav.navbar-default').after($('<div id="navbar-height-col"></d
Read more