dependent scope; need typename in front;

Question

I want to create a template as follows. I want to delete a list of items from vector vec1. And the indexes of the items I want to delete are stored in index_list.

#include <vector>

using namespace std;

template <typename a_type>
bool vector_remove(vector< a_type > & vec1, vector< int > index_list)
{
    //index_list is sorted in order from small to large.

    if(index_list.size() > vec1.size())
    {
        cout << "ERROR in 'vector_remove()': index_list is longer than vec1."<<endl;
        return false;
    }
    if(index_list.size() == vec1.size())
    {
        vec1.clear();
        return true;
    }
    vector< int >::iterator ind_pt = index_list.begin();
    vector< a_type >::iterator vec1_pre = vec1.begin();
    vector< a_type >::iterator vec1_pos = vec1.begin();
    int vec1_ind = 0;
    while(ind_pt != index_list.end() && vec1_pos != vec1.end())
    {
        if(*ind_pt == vec1_ind)
        {
            ind_pt ++;
            vec1_pos ++;
            vec1_ind ++;
        }
        else if( *ind_pt > vec1_ind )
        {
            *(vec1_pre) = *(vec1_pos);
            vec1_pos ++;
            vec1_pre ++;
            vec1_ind ++;
        }
        else
        {
            cout << "ERROR in 'vector_remove'." <<endl;
            return false;
        }
    }
    while(vec1_pos != vec1.end())
    {
        *(vec1_pre) = *(vec1_pos);
        vec1_pos ++;
        vec1_pre ++;
    }
    // the above codes are to put all the rejected elements to the end of the vec1.

    // pop back all the rejected elements.
    while(vec1_pre != vec1.end() )
    {
        vec1.pop_back();
    }

    return true;
}

But it returns a lot of errors:

In file included from demo.cpp:3:0:
my_vector.h: In function ‘bool vector_remove(std::vector<a_type>&, std::vector<int>)’:
my_vector.h:21:2: error: need ‘typename’ before ‘std::vector<a_type>::iterator’ because ‘std::vector<a_type>’ is a dependent scope
  vector< a_type >::iterator vec1_pre = vec1.begin();
  ^
my_vector.h:21:29: error: expected ‘;’ before ‘vec1_pre’
  vector< a_type >::iterator vec1_pre = vec1.begin();
                             ^
my_vector.h:22:2: error: need ‘typename’ before ‘std::vector<a_type>::iterator’ because ‘std::vector<a_type>’ is a dependent scope
  vector< a_type >::iterator vec1_pos = vec1.begin();
  ^
my_vector.h:22:29: error: expected ‘;’ before ‘vec1_pos’
  vector< a_type >::iterator vec1_pos = vec1.begin();
                             ^
my_vector.h:24:38: error: ‘vec1_pos’ was not declared in this scope
  while(ind_pt != index_list.end() && vec1_pos != vec1.end())
                                      ^
my_vector.h:34:6: error: ‘vec1_pre’ was not declared in this scope
    *(vec1_pre) = *(vec1_pos);
      ^
my_vector.h:45:8: error: ‘vec1_pos’ was not declared in this scope
  while(vec1_pos != vec1.end())
        ^
my_vector.h:47:5: error: ‘vec1_pre’ was not declared in this scope
   *(vec1_pre) = *(vec1_pos);
     ^
my_vector.h:54:8: error: ‘vec1_pre’ was not declared in this scope
  while(vec1_pre != vec1.end() )

could anyone help me solve this?

Solution

The compiler says

my_vector.h:21:2: error: need ‘typename’ before ‘std::vector::iterator’ because ‘std::vector’ is a dependent scope

So you need to write

typename vector< a_type >::iterator vec1_pre = vec1.begin();
typename vector< a_type >::iterator vec1_pos = vec1.begin();

See Where and why do I have to put the "template" and "typename" keywords? for the reasons behind it.

One last remark: In C++11 you can use auto and don't have to think anymore:

auto vec1_pre = vec1.begin();
auto vec1_pos = vec1.begin();

Comments

[Neil Kirk]

The error message tells you what you need. Put the typename keyword on those lines

[ooga]

Try putting typename in front of those lines.

[tqjustc]

@ooga okay. Great, it works. it is amazing. But could you tell me when I should put 'typename' in front of the lines?

[ooga]

@tqjustc, When it's a declaration that depends on a template parameter.

[WhozCraig]

Sure we could tell you, wait, someone already did.