C++ 依赖范围；前面需要typename；
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dependent scope; need typename in front;
提问by tqjustc
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.
我想创建一个模板如下。我想从 vector 中删除项目列表vec1。我要删除的项目的索引存储在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:
但它返回了很多错误：
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?
谁能帮我解决这个问题？
回答by Danvil
The compiler says
编译器说
my_vector.h:21:2: error: need ‘typename' before ‘std::vector::iterator' because ‘std::vector' is a dependent scope
my_vector.h:21:2: 错误：在“std::vector::iterator”之前需要“typename”，因为“std::vector”是一个依赖范围
So you need to write
所以你需要写
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.
请参阅在哪里以及为什么必须放置“模板”和“类型名称”关键字？其背后的原因。
One last remark: In C++11 you can use autoand don't have to think anymore:
最后一句话：在 C++11 中，你可以使用auto，不用再想了：
auto vec1_pre = vec1.begin();
auto vec1_pos = vec1.begin();
