emplace_back a temporary vector in-place to a vector of vector

Question

suppose we have an array of int , and now we want to convert to vector of vector , in condition that all number less than or equal 5 must be the first element of the sub-vector of the new vector vv , and also the length of each sub-vector should no bigger than 3. //

int array[] = {9, 8, 6, 5, 4, 8, 9, 7, 6, 5};

and it should be converted to

vv = { { 9, 8, 6 }, { 5 }, { 4, 8, 9 }, { 7, 6 }, { 5 } };

In order to do that, I can loop through the array and found the elements of the next sub-vector to emplace_back. So that I am considering that after finding the numbers to to pushed back, create temporary vector in-place.

Say, I found 9, 8, 6 goes to the next sub-vector, and 5 goes to next after, we do:

vector< vector < int > > vv;
vector < int >  v
v.push_back(9);  
v.push_back(8);  
v.push_back(6);  
vv.push_back(v);
v.clear();
v.push_back(5);
vv.push_back(v)
v.clear();

Now vv is { { 9, 8, 6 }, { 5 } }. This is quite cumbersome. Can I have the temporary vector created in-place to emplace_back directly to achieve the same result.

vv.emplace_back( vector <int > ( 3, 9, 8, 6 ) ); // this is wrong
vv.emplace_back( vector< int > ( 1, 5 ) );       // this is good.

or equivalently,

vv.emplace_back( vector <int > { 9, 8, 6 } ); // this is good.
vv.emplace_back( vector< int > ( 1, 5 ) );    // this is good too.

Thanks!

Answer 1

If you know the values in advance you can just do list initialization:

std::vector<std::vector<int>> vv{
  { 9, 8, 6 }, { 5 }
};

Answer 2

You can use list initialization:

vector<vector<int>> vv{{9,8,6}, {5}};

live example on wandbox.org

Answer 3

You can do:

vector< vector < int > > vv;
...
vv = { { 9, 8, 6 }, { 5 } };

or:

vector< vector < int > > vv;
...
vv.emplace_back( { 9, 8, 6 } );
vv.emplace_back( { 5 } );

you should not create temporary explicitly when calling emplace_back method you basically make it equivalent to push_back , whole point of emplace is to forward parameters to element ctor directly (same for emplace methods in other containers).