Vector<string> в string

vector <string> test;
string str;

for(const auto& it : test) str += it;

#include <iostream>
#include <string>
#include <vector>
 
int main()
{
    std::vector <std::string> test;
    test.push_back("A");
    test.push_back("B");
    test.push_back("C");
    std::string str;
    str = test[0];
    
    std::cout<<str;
}

#include <iostream>
#include <string>
#include <algorithm>
#include <iterator>
#include <vector>
#include <list>
#include <memory>
using namespace std;
 
template< typename T, template<typename, typename> class Container >class ContainerTemplate:public Container<T, allocator<T>>
{
public:
  size_t inlet_whole_length(){
      size_t result(0);
          for(auto it: *this)
          {
              result+=it.size();
          }
      return result;
    }
 
 
  T  whole_inlet_containers2container()
  {
      T result;
      result.reserve(inlet_whole_length());    
      for(auto a:*this)result.insert(result.end(), a.begin(), a.end());
      return result;
  }
 
};
 
int main()
{
    ContainerTemplate<string , vector> vs{{"mam","pap","bro"}};
    for(auto a:vs)cout<<a<<' ';
    cout<<endl;
    cout<<vs.inlet_whole_length()<<endl;
    auto str= vs.whole_inlet_containers2container();
    cout<<str<<endl;
 
    ContainerTemplate<string , list> ls{{"mam","pap","bro"}};
    for(auto a:ls)cout<<a<<' ';
    cout<<endl;
    cout<<ls.inlet_whole_length()<<endl;
    str= ls.whole_inlet_containers2container();
    cout<<str<<endl;
 
    ContainerTemplate<vector<int> , vector> vi{{{1,2,3},{4,5}}};
    for(auto a:vi)
       for(auto c:a)cout<<c<<' ';
    cout<<endl;
    cout<<vi.inlet_whole_length()<<endl;
    auto int_sum= vi.whole_inlet_containers2container();
   for(auto a:int_sum)
       cout<<a<<' ';
   return 0;
}

#include <iostream>
#include <vector>
#include <string>
#include <numeric>
 
using namespace std;
 
int main()
{
    const vector<string> v{"Hello ", "mam", "pap", "bro"};
    cout << accumulate(v.cbegin(), v.cend(), string{}) << endl;
}

 vector <string>v,v_TEST{"mam","pap","bro", "mam","pap","bro", "mam","pap","bro","mam","pap","bro"};
    const size_t cycles(10000);
    for(size_t i=0; i<cycles;++i)v.insert(v.end(),v_TEST.begin(),v_TEST.end());
 
    string str_TEST;
        auto start=clock();
        str_TEST=accumulate(v.cbegin(), v.cend(), string{});
        auto finish=clock();
        cout<<"whole result string length is "<<str_TEST.size()<<endl;
        cout<<"accumulate lasts "<<(finish-start)<<endl;
 
    str_TEST.clear();
        start=clock();
        ContainerTemplate<string , vector> vs_TEST{v};
        str_TEST= vs_TEST.whole_inlet_containers2container();
        finish=clock();
        cout<<"whole result string length is "<<str_TEST.size()<<endl;
        cout<<"strategy template lasts "<<(finish-start)<<endl;

#include<iostream>
#include <vector>
 
using namespace std;
 
int main()
{
    vector<string> vec = { "Hello", ", ", "world", "!" };
    string str;
    
    for(int i = 0; i < vec.size(); i++)
    str += vec[i];
    
    cout << str;
    
    return 0;
}

    std::string R;
    R.reserve(std::accumulate(v.begin(),v.end(),std::size_t{0},
    [](std::size_t l,const std::string &s) {return l+s.size();}));