C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Shrink vector capacity?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Joseph Turian Guest Posted: Wed Jan 19, 2005 8:47 pm    Post subject: Shrink vector capacity? Fellow hackers, Is it possible to shrink the capacity of a vector? i.e. Without new'ing and delete'ing a vector, can one return its memory to the heap? Here's what I get under the g++ implementation: vector<double> v = vector<double>(1024*1024); cerr << v.capacity() << "n"; // outputs 1048576 v.clear(); cerr << v.capacity() << "n"; // outputs 1048576 v = vector cerr << v.capacity() << "n"; // outputs 1048576 Joseph Back to top Victor Bazarov Guest Posted: Wed Jan 19, 2005 8:56 pm    Post subject: Re: Shrink vector capacity? Joseph Turian wrote: Quote: Is it possible to shrink the capacity of a vector? No guaranteed way. Quote: i.e. Without new'ing and delete'ing a vector, can one return its memory to the heap? Nothing in the language says that even delete'ing anything returns that memory to the heap. There is no re-use guarantee for free store. Quote: Here's what I get under the g++ implementation: vector<double> v = vector<double>(1024*1024); You're potentially wasting more memory than you need. Just do vector<double> v(1024*1024); Quote: cerr << v.capacity() << "n"; // outputs 1048576 v.clear(); 'clear' does not change capacity. It changes 'size'. Quote: cerr << v.capacity() << "n"; // outputs 1048576 v = vector Assignment does not necessarily change capacity. It only copies elements and as a side effect, changes 'size'. Reallocation would happen only if the existing 'v's capacity is less than needed. Quote: cerr << v.capacity() << "n"; // outputs 1048576 Try vector Again, no guarantees. The reason shrinking a vector is not an easy thing is simple: shrinking would require reallocation, and reallocation is the most expensive operation. V Back to top Joseph Turian Guest Posted: Wed Jan 19, 2005 9:05 pm    Post subject: Re: Shrink vector capacity? Victor, Quote: vector<double> v = vector<double>(1024*1024); You're potentially wasting more memory than you need. Just do vector<double> v(1024*1024); I don't understand. How is this potentially wasting more memory than the original expression? Quote: Nothing in the language says that even delete'ing anything returns that memory to the heap. There is no re-use guarantee for free store. What guarantee does it give? If I new a 1M element vector, delete it, and new another 1M element vector, am I at least guaranteed that the program needs only 1MB instead of 2? Quote: Try vector<double>().swap(v); FYI---at least on g++---this does in fact resize the capacity to 0. Joseph Back to top Victor Bazarov Guest Posted: Wed Jan 19, 2005 9:48 pm    Post subject: Re: Shrink vector capacity? Joseph Turian wrote: Quote: Victor, vector<double> v = vector<double>(1024*1024); You're potentially wasting more memory than you need. Just do vector<double> v(1024*1024); I don't understand. How is this potentially wasting more memory than the original expression? The original expression creates a temporary vector and then copy- initialises the 'v' vector with it. The compiler is allowed to forgo creation of the temporary (and most probably do), yet, according to the rules of the language, a temporary _may_ be created. Quote: Nothing in the language says that even delete'ing anything returns that memory to the heap. There is no re-use guarantee for free store. What guarantee does it give? None whatsoever, except that when there is no more memory left, it will let you know. Quote: If I new a 1M element vector, delete it, and new another 1M element vector, am I at least guaranteed that the program needs only 1MB instead of 2? Nope. That's an issue between your implementation and your OS. Quote: Try vector<double>().swap(v); FYI---at least on g++---this does in fact resize the capacity to 0. Good. V Back to top Efrat Regev Guest Posted: Thu Jan 20, 2005 12:41 am    Post subject: Re: Shrink vector capacity? Hello, Scott Meyer's explains in "Effective STL" how to use the std::vector's swap() method to shrink vectors. Perhaps this is relevant for you. It works as follows: // Creating a vector std::vector<int> my_vec(arbitrary_small_number); // Using it for huge settings my_vec.reserve(huge_number); for(int i = 0; i < huge_number; ++i) my_vec.push_back(i); // (Above code can be done more efficiently using std::generate, but that's irrelevant) // Do something with the vector my_do_something_function(my_vec); // Now we need the vector object, but not its elements { // We create a tiny vector std::vector // We swap the vectors my_vec.swap(my_tmp_vec); // Implicitly, my_tmp_vec's destructor called. Presumably, the allocator gains all of its memory. } // At this point my_vec is truly small (which is waht we wanted). I hope this helps, Ami P.S. Scott Meyer writes excellent books; even if this example doesn't help you, I highly recommend his writings. "Joseph Turian" <turian (AT) gmail (DOT) com> wrote Quote: Fellow hackers, Is it possible to shrink the capacity of a vector? i.e. Without new'ing and delete'ing a vector, can one return its memory to the heap? Here's what I get under the g++ implementation: vector<double> v = vector<double>(1024*1024); cerr << v.capacity() << "n"; // outputs 1048576 v.clear(); cerr << v.capacity() << "n"; // outputs 1048576 v = vector cerr << v.capacity() << "n"; // outputs 1048576 Joseph Back to top Joseph Turian Guest Posted: Thu Jan 20, 2005 8:31 pm    Post subject: Re: Shrink vector capacity? Ami, Thank you for the reply. I will definitely check out his books at my library. Any non-STL books of his that you particularly recommend? Joseph Back to top Howard Guest Posted: Thu Jan 20, 2005 10:06 pm    Post subject: Re: Shrink vector capacity? "Joseph Turian" <turian (AT) gmail (DOT) com> wrote Quote: Ami, Thank you for the reply. I will definitely check out his books at my library. Any non-STL books of his that you particularly recommend? Joseph "Effective C++, Second Edition." and "More Effective C++" are both indispensible, as far as I'm concerned! -Howard Back to top Attila Feher Guest Posted: Thu Jan 27, 2005 1:18 pm    Post subject: Re: Shrink vector capacity? Howard wrote: Quote: "Joseph Turian" <turian (AT) gmail (DOT) com> wrote in message news:1106253093.897922.78870 (AT) f14g2000cwb (DOT) googlegroups.com... Ami, Thank you for the reply. I will definitely check out his books at my library. Any non-STL books of his that you particularly recommend? "Effective C++, Second Edition." and "More Effective C++" are both indispensible, as far as I'm concerned! They are also available on a tree-saving, cost effective CD-ROM edition. Both books (and some more goodies) on one CD-ROM. -- Attila aka WW Back to top Victor Bazarov Guest Posted: Thu Jan 27, 2005 3:22 pm    Post subject: Re: Shrink vector capacity? Attila Feher wrote: Quote: Howard wrote: "Joseph Turian" <turian (AT) gmail (DOT) com> wrote in message news:1106253093.897922.78870 (AT) f14g2000cwb (DOT) googlegroups.com... Ami, Thank you for the reply. I will definitely check out his books at my library. Any non-STL books of his that you particularly recommend? "Effective C++, Second Edition." and "More Effective C++" are both indispensible, as far as I'm concerned! They are also available on a tree-saving, cost effective CD-ROM edition. I take it you're being sarcastic here. The tree-saving properties of compact discs hasn't been researched, AFAIK. As to cost effectiveness, how do you measure it? Yes, it costs about $1 to produce, so does a book nowadays. Besides, the sheer convenience of the CD... You just need a computer and a power source to read it. You can always take it on the road. It fits conveniently in a notebook case and weighs only 7 pounds (what's the weight of an average notebook PC?). 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