How to return a vector from a thread in C++?

struct Data_1{

    
    string stm;
    ...
};

void get_child_tree_thread(void *arguments){    

 vector<Data_1> v_result;

  Data_1 a,a1;

  a.stm = "1234";

  a1.stm = "1235";
  
  v_result.push_back(a);

  v_result.push_back(a1);
  
  std::vector<Data_1>* vec = new std::vector<Data_1>(v_result.size());
    
  vec->assign(v_result.begin(),v_result.end()); 
  
  **return vec;**
 
}
int main(void)
{
    
    if (pthread_create(&thread,NULL, get_child_tree_thread,NULL)){
            cerr << "create thread  error" << endl;         
            cout << "create thread fail->"  << endl;
            exit(0);
        }
    sleep(3);
    void *threadResult = NULL;
    cout << "main 3" << endl;
    pthread_join(thread, &threadResult);
        
    std::vector<Data_1> *v_child_out = (std::vector<Data_1> *)((std::vector<Data_1>* )threadResult);
    
    cout << "p join v_child_out size" << v_child_out->size()<<  endl;       
        
  
}

how to cast vector properly? ->

std::vector<Data_1> *v_child_out = (std::vector<Data_1> *)((std::vector<Data_1>* )threadResult);

cout << "p join v_child_out size" << v_child_out->size()<<  endl;       

Your get_child_tree_thread() function is declared wrong. It needs to return void* not void (your compiler should have caught that mistake):

void* get_child_tree_thread(void *arguments){
  ...
  auto* vec = new std::vector<Data_1>(...);
  ...
  return vec; 
}

Then, your typecast of threadResult can be simplified by removing the redundant cast (you are casting twice when once will suffice):

int main()
{
    ...

    void *threadResult = nullptr;
    pthread_join(thread, &threadResult);

    auto *v_child_out = static_cast<std::vector<Data_1>*>(threadResult);
    ...
    delete v_child_out;
}

You can then simplify the code further by passing a pointer to a vector object to the thread via its input argument, and get rid of new entirely, eg:

void* get_child_tree_thread(void *argument){

  auto *v_result = static_cast<std::vector<Data_1>*>(argument);

  ...
  v_result->push_back(...);
  ...

  return nullptr;
}

int main()
{
    std::vector<Data_1> v_child_out;

    if (pthread_create(&thread, nullptr, get_child_tree_thread, &v_child_out)){
        std::cerr << "create thread error" << std::endl;
        std::cout << "create thread fail->" << std::endl;
        return 0;
    }
    sleep(3);

    std::cout << "main 3" << std::endl;
    pthread_join(thread, nullptr);

    std::cout << "p join v_child_out size" << v_child_out.size() << std::endl;
}

Now, that said, you should not be using pthreads at all. Use C++'s native std::thread instead:

#include <thread>

void get_child_tree(std::vector<Data_1> &v_result){
  ...  
  v_result.push_back(...);
  ...
}

int main()
{
    std::vector<Data_1> v_child_out;

    std::thread thd(get_child_tree, std::ref(v_child_out));
    sleep(3);

    std::cout << "main 3" << std::endl;
    thd.join();
    
    std::cout << "p join v_child_out size" << v_child_out.size() << std::endl;         
}

Or even std::async() and std::future:

#include <future>

std::vector<Data_1> get_child_tree(){

  std::vector<Data_1> v_result;

  ...  
  v_result.push_back(...);
  ...

  return v_result;
}

int main()
{
    auto f = std::async(std::launch::async, get_child_tree);

    sleep(3);

    std::cout << "main 3" << std::endl;

    auto v_child_out = f.get();

    std::cout << "p join v_child_out size" << v_child_out.size() << std::endl;         
}