push-back

vector push_back over std::copy

由 梦想的初衷 提交于 2019-12-11 12:39:18
问题 I have a function which has an unordered set as a parameter . Since I am using openmp I am converting this unordered set to vector . I use a std::copy for this conversion . //pseudo code func( std::unorderedset s1) begin vector v1; std::copy(s1.begin,s2.end,std::back_inserter(v1.end()); #openmp scope for( i = 0 ; i < v1.size(); i++ ) { //accessing v1(i) } end However I feel std::copy is a costly operation . So what I think is, if I create a class variable vector and I keep populating this

Crash from a push_back() in a std vector

由 微笑、不失礼 提交于 2019-12-08 05:25:08
问题 This program works as expected: #include <iostream> #include <string> #include <vector> using namespace std; struct Thumbnail { string tag; string fileName; }; int main() { { Thumbnail newThumbnail; newThumbnail.tag = "Test_tag"; newThumbnail.fileName = "Test_filename.jpg"; std::vector<Thumbnail> thumbnails; for(int i = 0; i < 10; ++i) { thumbnails.push_back(newThumbnail); } } return 0; } If I copy and paste the main block of code in another project (still single threaded), inside any

Vector push_back Access Violation

由 核能气质少年 提交于 2019-12-07 22:59:58
问题 This is probably a silly error, but it's driving me nuts trying to fix it. I have a struct: struct MarkerData { int pattId; unsigned short boneId; Ogre::Matrix4 transToBone; Ogre::Vector3 translation; Ogre::Quaternion orientation; MarkerData(int p_id, unsigned short b_id, Ogre::Matrix4 trans) { pattId = p_id; boneId = b_id; transToBone = trans; } }; And a class: class TrackingSystem { public: void addMarker(int pattId, unsigned short boneId, Ogre::Matrix4 transToBone); private: std::vector

Why is it unsafe to call push_back in a for loop even when there is no memory reallocation?

由 浪子不回头ぞ 提交于 2019-12-07 18:46:43
问题 When I read this post: https://stackoverflow.com/a/42448319/3336423 I understand, that calling push_back in a for loop is unsafe because: If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated. Then, I assume that if I guarantee the capacity won't be exceeded, it would be safe.... But apparently not, all implementations below will crash after first push_back

Vector push_back only if enough memory is available

由 落花浮王杯 提交于 2019-12-07 10:16:59
问题 I am currently building a code that is dealing with massive amounts of memory using the vector class, dynamically. The code is building up the vector with push_back , where it is important to notice that the vector is 2 dimensional, representing the data matrix. Depending on circumstances, this matrix can be small, or become exceptionally large. For instance, data matrix can have few rows, with 1000 columns in every row, or it can get 1000 rows with the same amount of columns, full of double

In C++, will the vector function push_back increase the size of an empty array?

由 瘦欲@ 提交于 2019-12-07 08:04:24
问题 Quick question. Let's say I declare a vector of size 20. And then I want to add a few integers to it using push_back. vector<int> myVector(20); myVector.push_back(5); myVector.push_back(14); Is the capacity of my vector now 22, or is it still 20? Were 5 and 14 added to indices [19] and [20], respectively? Or are they at [0] and [1]? 回答1: After those statements its capacity is implementation-defined. (Please note that is different from its size.) vector<int> myVector(20); This creates a vector

Vector push_back Access Violation

由 纵饮孤独 提交于 2019-12-06 11:25:17
This is probably a silly error, but it's driving me nuts trying to fix it. I have a struct: struct MarkerData { int pattId; unsigned short boneId; Ogre::Matrix4 transToBone; Ogre::Vector3 translation; Ogre::Quaternion orientation; MarkerData(int p_id, unsigned short b_id, Ogre::Matrix4 trans) { pattId = p_id; boneId = b_id; transToBone = trans; } }; And a class: class TrackingSystem { public: void addMarker(int pattId, unsigned short boneId, Ogre::Matrix4 transToBone); private: std::vector <MarkerData> mMarkers; }; Now, in the addMarker method: void TrackingSystem::addMarker(int pattId,

Being Smart About Vector Memory Allocation

由 与世无争的帅哥 提交于 2019-12-06 04:44:46
问题 Let's say I have to iterate over a potentially very large vector of numbers and copy the even and odd elements into new, separate vectors. (The source vector may have any proportion of evens to odds; it could be all evens, all odds, or somewhere in-between.) For simplicity, push_back is often used for this sort of thing: for (std::size_t Index; Index < Source.size(); Index++) { if (Source[Index] % 2) Odds.push_back(Source[Index]); else Evens.push_back(Source[Index]); } However, I'm worried

Is there a back_inserter variant that takes advantage of move?

由 南楼画角 提交于 2019-12-05 16:34:24
In generic code I was trying to tell an output iterator (in practice a std::back_inserter_iterator to move a range of elements. To my surprise it looked as if elements were moved in a move-to-back_inserter operation. #include<algorithm> // move #include<iterator> // back_inserter #include<vector> int main(){ std::vector<std::vector<double> > v(10, std::vector<double>(100)); std::vector<std::vector<double> > w; assert( not v[0].empty() and w.size() == 0 ); std::copy(v.begin(), v.end(), std::back_inserter(w)); assert( not v[0].empty() and w.size() == 10 ); std::move(v.begin(), v.end(), std::back

Vector push_back only if enough memory is available

由 大城市里の小女人 提交于 2019-12-05 12:40:38
I am currently building a code that is dealing with massive amounts of memory using the vector class, dynamically. The code is building up the vector with push_back , where it is important to notice that the vector is 2 dimensional, representing the data matrix. Depending on circumstances, this matrix can be small, or become exceptionally large. For instance, data matrix can have few rows, with 1000 columns in every row, or it can get 1000 rows with the same amount of columns, full of double data types. Obviously, this can very easily become a problem, because 1000x1000x8 = 8 000 000 bytes,
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