Question

I am looking for the way to initialize a vector with the minimal amount of copy.

struct T {
   std::vector<int> v;
   //some stuff here ; pod
   T(std::vector<int> vv):v(vv){}://a non default constructor
};

Now to initialize a vector, i do

std::vector<T> vec(n);
for (auto it = vec.begin() ;it != vec.end(),++it) 
{
  // do some stuff to *it;
}

the “do some stuff here” basically just redo what is done in the constructor, and so i have two initialization of T.v…
I also thought of doing

std::vector<T> vec;
for (int i = 0 ; i< n;++i) 
{
   std::vector<int> vv = // blabla
   T t(vv);
   vec.push_back(t);
}

which again result on vv being copied…

So how do i initialize a vector of struct, where each element is initialized using a non default constructor (call with different argument for each element) ?

Answer 1

Use reserve and construct elements in place:

std::vector<T> vec;
vec.reserve(n);     // now guaranteed O(1) insertions at the end

for (unsigned i = 0; i != n; ++i)
{
   vec.emplace_back(vv);  // or whatever
}

You can also use the old-style push_back(); with any luck, the compiler will optimize out several unneeded copies. You should probably also add an iterator-based and an initializer-list constructor to T, just to give you some flexibility.