c++ - Using auto keyword to fill doubly-indexed vector -

trying better learn auto keyword. i'd allocate doubly-indexed vector using auto keyword, when print vector (which should 0's), instead prints empty space, in correct "shape" of vector.

mwe:

#include <iostream> #include <vector>  void displayvector(const std::vector<std::vector<double> >& vec) {   std::cout << "vector: \n\n";   (auto : vec)     {       (auto j : i)         {           std::cout << j << " ";         }       std::cout << "\n";     }   std::cout << "\n"; }  int main() {   std::vector<std::vector<double> > vec1, vec2;    /* want work */   vec1.resize(5);   (auto : vec1)     {       i.resize(3);     }   displayvector(vec1);    /* "old" way of doing */   vec2.resize(5);   (int = 0; < 5; ++i)     {       vec2[i].resize(3);     }   displayvector(vec2);  } 

this results in

vector:         vector:   0 0 0  0 0 0  0 0 0  0 0 0  0 0 0  

what proper method of doing this? 1 preferred? accepting critiques on code implementation :)

in usage, on own, auto work values, resize copies of vectors within vec1.

change loop vec1 use reference

for (auto &i : vec1) {   i.resize(3); } 

also change both loops within displayvector() use const auto & (a const reference rather value). won't change output, avoid copying vectors around.

note: v154c1 , neil kirk have said same things in comments.