push_back versus operator[] assignment in c++ vectors -


can please explain me in detail why following code vectory assignment size of vecy zero? also, begin , end iterators stuck @ first node. seems reserve works push , need construct vector size if want iterators vectors , size work expected. assuming push_back doing type of allocation straight assignment not in case? looking details explaining can make sure understand happening reserve , push_back versus constructing size element , doing assignment in vecx example.

#include <iostream> #include <vector>      int main ( int argc, char *argv[])     {       std::vector<int> vecx(2);       vecx[0] = 1;       vecx[1] = 2;       std::cout << " vecx0 item: " << vecx[0] << std::endl;       std::cout << " vecx1 item: " << vecx[1] << std::endl;       std::cout << " vectorx size: " << vecx.size() << std::endl;        std::vector<int> vecy;       vecy.reserve(2);       vecy[0] = 1;       vecy[1] = 2;       std::cout << " vecy0 item: " << vecy[0] << std::endl;       std::cout << " vecy1 item: " << vecy[1] << std::endl;       std::cout << " vectory size: " << vecy.size() << std::endl;     }  output   vecx0 item: 1  vecx1 item: 2  vectorx size: 2  vecy0 item: 1  vecy1 item: 2  vectory size: 0 

std::vector<int> vecy;       vecy.reserve(2);       vecy[0] = 1;       vecy[1] = 2;

this code wrong , evokes undefined behavior1. when reserve vector, set capacity, not size.

you need either push_back, or construct vector did in example 1.

"undefined behavior" : invokes undefined behavior because of out-of-range call operator[] if call vector::operator[n] n > vec.size(), behavior undefined.


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