- Can I pass a shared_ptr to a thread as a temporary ? - 5 Updates
- std::copy can't copy raw bytes from a file - 3 Updates
| "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>: Feb 10 03:39PM -0800 On 2/10/2020 3:15 PM, Chris M. Thomasson wrote: > // this does not work wrt the > // last time I looked at shared_ptr > shared_ptr<bar> local = g_bar; // it would be nice to check for nullptr... if (! local) continue; |
| Paavo Helde <myfirstname@osa.pri.ee>: Feb 11 08:37AM +0200 On 11.02.2020 0:04, Chris M. Thomasson wrote: > I don't really use std::shared_ptr all that much, preferring "intrusive" > counts that embed the refcount within the object itself. No need to avoid std::shared_ptr for this reason, std::make_shared() and std::allocate_shared() effectively do exactly this. One problem with home-grown intrusive refcounting schemas is that it would be too easy/too tempting to create new smartpointers during the main object construction and destruction, leading to conceptual problems and race conditions. Using std::shared_ptr will avoid such problems, by design. |
| Bonita Montero <Bonita.Montero@gmail.com>: Feb 11 08:07AM +0100 > No need to avoid std::shared_ptr for this reason, std::make_shared() and > std::allocate_shared() effectively do exactly this. That's a matter of taste. |
| "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>: Feb 11 02:23PM -0800 On 2/10/2020 10:37 PM, Paavo Helde wrote: > would be too easy/too tempting to create new smartpointers during the > main object construction and destruction, leading to conceptual problems > and race conditions. I am not exactly sure what you mean here? Can you give a simple example? > Using std::shared_ptr will avoid such problems, by > design. Fwiw, shared_ptr does not provide strong thread safety, last time I looked. This can be dangerous in certain scenarios that require it. Here is the pattern, pseudo-code: static shared_ptr<bar> g_bar; // nullptr void multiple_threads() { for (;;) { // this does not work wrt the // last time I looked at shared_ptr shared_ptr<bar> local = g_bar; if (! local) continue; local->foobar(); } } void single_thread() { shared_ptr<bar> local(new bar()); g_bar = local; } Last time I checked, shared_ptr works only if a thread has a prior reference. The multiple_threads() function violates this. Now, there is a home-brew smart pointer than can easily handle such scenarios. Afaict, its basically crowbar proof: http://atomic-ptr-plus.sourceforge.net This has a patent: https://patents.google.com/patent/US5295262 expired but fee related. Humm... |
| "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>: Feb 11 02:29PM -0800 On 2/9/2020 7:28 AM, Paavo Helde wrote: > std::thread constructor where it will be copied or moved appropriately > before the thread starts. Any time later one will need more elaborate > synchronization. Afaict, this pretty much sums it all up. |
| Frederick Gotham <cauldwell.thomas@gmail.com>: Feb 11 07:36AM -0800 I wanted to copy the contents of a binary file into a vector of unsigned chars. Here's what I tried to do: ifstream mofile("some_binary_file.exe", std::ios::binary); if ( !mofile.is_open() ) return; vector<char unsigned> tmp_vec; tmp_vec.reserve(10000ull); std::copy( istream_iterator<char unsigned>(mofile), istream_iterator<char unsigned>(), back_inserter(tmp_vec) ); This code compiles and appears to run just fine, but the file contents are copied properly. I had to change it to: ifstream mofile("some_binary_file.exe", std::ios::binary); if ( !mofile.is_open() ) return; vector<char unsigned> tmp_vec; tmp_vec.reserve(10000ull); char ctmp; while ( mofile.get(ctmp) ) //std::copy doesn't work here { tmp_vec.push_back( *reinterpret_cast<char unsigned*>(&ctmp) ); } I've had trouble in the past with using "std::copy" to copy raw bytes from a file like this, and I've always had to replace it with a loop that does byte-by-byte reads. Anyone know why the first one doesn't work properly? |
| "Öö Tiib" <ootiib@hot.ee>: Feb 11 09:29AM -0800 On Tuesday, 11 February 2020 17:37:03 UTC+2, Frederick Gotham wrote: > return; > vector<char unsigned> tmp_vec; > tmp_vec.reserve(10000ull); I suspect the skipws format flag for the mofile stream is set. So it might be that you need ... mofile >> std::noskipws; ... here. > } > I've had trouble in the past with using "std::copy" to copy raw bytes from a file like this, and I've always had to replace it with a loop that does byte-by-byte reads. > Anyone know why the first one doesn't work properly? I suspect that it skips whitespace. However if that is not the case then check other such annoyance flags of your mofile and/or locale. I may be wrong since I have used same code of ifstream::reading as 4096 byte chunks almost everywhere as long I remember. |
| Frederick Gotham <cauldwell.thomas@gmail.com>: Feb 11 01:06PM -0800 > for the mofile stream is set. > So it might be that you need ... > mofile >> std::noskipws; I looked at the differences in "meld" and it has loads of "0x20" bytes highlighted, which is the ASCII value for a space. Looks like you're right. I might try those extra flags when I'm at a PC tomorrow. |
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