type of iterator c++

Question

I'm trying to write my own sort function that can sort vector container.

I'm hoping to call it in the following manner:

this:sort(arr.begin() , arr.end() - 1)

arr.end() - 1 - because arr.end() returns the next element, actually not element, returns iterator to end.

So i want to call my function with iterator on any type.I tried to run this code:

#include<iostream>
#include<vector> 
using namespace std;

template<typename T1> 
void func(vector<T1>::iterator it) {
    cout << "\n" << *it; 
}

int main() {
    vector<int> arr(10);

    for(int i = 0;i < 10;++i)
        arr[i] = i+1;

    func<int>(arr.begin()); 
}

But compiler says:

main.cpp:10:23: error: variable or field ‘func’ declared void  void func(vector<T1>::iterator it)
                              main.cpp:10:32: error: expected ‘)’ before ‘it’  void func(vector<T1>::iterator it)
                                 main.cpp: In function ‘int main()’: main.cpp:26:5: error: ‘func’ was not declared in this scope
     func<int>(arr.begin());
     func main.cpp:26:10: error: expected primary-expression before ‘int’
     func<int>(arr.begin());

Please explain me my mistake.Thanks

You may wish to read [here](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) and [here](https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h) — UKMonkey, Jan 09 '18 at 11:34
*Never* include anything from `bits` directory - do `#include ` instead! — Aconcagua, Jan 09 '18 at 11:35
Привіт , можеш пояснити як , будь ласка?? — Denis Timchuk, Jan 09 '18 at 11:38
You're also not returning from the main, given that the return type is int, so you should do something like: return 0; — , Jan 09 '18 at 11:39
This is an English only site, please write only in English here... — Aconcagua, Jan 09 '18 at 11:41
Note that `typename std::vector::iterator` is non deducible (you would have to call `foo(..)`). Simpler to use `template void foo(It it)`.(You might then use `std::iterator_traits` to check category). — Jarod42, Jan 09 '18 at 12:16
_arr.end() - 1 - because arr.end() returns the next element, actually not element, returns iterator to end_ what? — , Jan 09 '18 at 12:36
You are right, but i want to write mergesort using buffer array with specific type. — Denis Timchuk, Jan 09 '18 at 13:49

Yola · Answer 1 · 2018-01-09T11:43:55.560

1

Compiler doesn't know that iterator is a type inside vector<T1>

template<typename T1> void func(typename vector<T1>::iterator it) {
    cout << "\n" << *it; }

Inside a template, some constructs have semantics which may differ from one instantiation to another. Such a construct depends on the template parameters.

You should write typename for such dependent types.

edited Jan 09 '18 at 11:43

answered Jan 09 '18 at 11:38

Yola

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Oh , thank you a lot.And one question in which cases we should write typename ?? – Denis Timchuk Jan 09 '18 at 11:41
1

That's what this answer is lacking... In case of [dependent names](http://en.cppreference.com/w/cpp/language/dependent_name). – Aconcagua Jan 09 '18 at 11:42

type of iterator c++

1 Answers1