c++ - How to transfer collection of small vector in a unique vector without copy -

i have map<t,vector<double> >, t=char values vector<double> of length<n n=5. transfer each vector<double> map big vector<double> have length n*mapsize, each vector inserted @ index 5*k. if possible without copying.

#include <vector> #include <map> using namespace std; int main() {     typedef vector<double> vec;     typedef map<char, vector<double> >::iterator itmap;      //create map<char,vector<double>> 2 keys holding resp 2-3 elem vectors     double v_value[] = {1.1,2.4};     vec v(v_value, v_value + sizeof(v_value)/sizeof(double));     map<char,vector<double> > mymap;     mymap['a'] = v;     v.push_back(10);     mymap['b'] = v;      //create vector should small vectors     vec receipt;     receipt.reserve(10);      for(itmap = mymap.begin(); != mymap.end(); ++it) {         it->second.resize(5);         receipt.insert(receipt.end(),it->second.begin(),it->second.end());     } } 

edit: edited solution, copy though.

as explained in comment, it's not possible without copying; each of vectors in map separately managed dynamic array , new vector new big array. values have copied. wouldn't worry this, unless have profiled , seen significant issue.

in spirit, std::copy rescue... de de, de de, dum de dum...

#include <vector> #include <map> #include <algorithm>  template<typename t, size_t n> size_t length(t (&)[n]) {    return n; }  int main() {     typedef std::vector<double> vec;     typedef std::map<char, vector<double> >::iterator itmap;      //create map<char,vector<double>> 2 keys holding resp 2-3 elem vectors     double v_value[] = {1.1,2.4};     vec v(v_value, v_value + length(v_value));     std::map<char,vector<double> > mymap;     mymap['a'] = v;     v.push_back(10);     mymap['b'] = v;      //create vector should small vectors     vec receipt(10);      for(itmap = mymap.begin(); != mymap.end(); ++it) {         std::copy(it->second.begin(), it->second.end(), std::back_inserter(receipt));     } } 

for user defined and/or more complex types double, may more efficient use std::move if target compilers support c++11.

i have demo'd small trick calculating length of array... confused sizeof.

ps hate using namespace std.