std::vector< std::vector<int> > my2dArray;

std::vector<int> mySizes;
for (int i = 0; i < my2dArray.size(); i++ )
{
  mySizes.push_back( my2dArray[i].size() );
}

typedef std::vector<int> IntVec;
std::vector<size_t> sizes(my2dArray.size());
std::transform(my2dArray.begin(), my2dArray.end(), sizes.begin(),
               std::mem_fun_ref(&IntVec::size));

std::vector<int> mySizes(my2dArray.size());
auto getSize = [](const std::vector<int> & v) { return v.size(); };
std::transform(my2dArray.begin(), my2dArray.end(), mySizes.begin(), getSize);

#include <string>
#include <iostream>
#include <algorithm>
#include <vector>

class Sizes
{
public:
    Sizes( std::vector<int>& v ) : sizeVec( v ) {}
    void operator() ( std::vector<int>& v ) {
        sizeVec.push_back( v.size() );
    }
    std::vector<int>& sizeVec;
};

void outFunc (int i) {
    std::cout << " " << i;
}
int _tmain(int argc, _TCHAR* argv[])
{
    std::vector<std::vector<int>> twodVec;

    std::vector<int> vec;
    vec.push_back( 6 );
    twodVec.push_back( vec );
    vec.push_back( 3 );
    twodVec.push_back( vec );
    vec.push_back( 8 );
    twodVec.push_back( vec );
    vec.push_back( 3 );
    twodVec.push_back( vec );

    std::vector<int> sizeVec;
    Sizes sizes(sizeVec);
    std::for_each( twodVec.begin(), twodVec.end(), sizes );
    std::for_each( sizeVec.begin(), sizeVec.end(), outFunc );

    return 0;
}

class Sizes
{
public:
    void operator() ( std::vector<int>& v ) {
        sizeVec.push_back( v.size() );
    }
    std::vector<int> sizeVec;
};

Sizes sizes;
sizes = std::for_each( twodVec.begin(), twodVec.end(), sizes );
std::for_each( sizes.sizeVec.begin(), sizes.sizeVec.end(), outFunc );