CODEWITHSUNDEEP
c++templatesstliteratortypename
Yuri G.
Yuri G.

Reputation: 47

C++ Templates and STL vector problem

I need help in the simple matter

Im trying to create class 

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

template<class T> class merge_sort
{
protected:

    vector<T> merge(const vector<T> &a, const vector<T> &b)
    {
        vector<T> v;

        typename vector<T>::iterator A;
        A= a.begin();
        typename vector<T>::iterator B;
        B= b.begin();
...

but compiler gives me next error:

no match for ‘operator=’ in ‘A = ((const std::vector<int, std::allocator<int> >*)a)->std::vector<_Tp, _Alloc>::begin [with _Tp = int, _Alloc = std::allocator<int>]()’  merge.cpp   /merge_sort line 23 C/C++ Problem

Upvotes: 1

Views: 2775

Answers (3)

Alexandre C.
Alexandre C.

Reputation: 56956

Use 

typename vector<T>::const_iterator A = a.begin();
typename vector<T>::const_iterator B = b.begin();

because a and b are const references, the const version of begin is called, and it returns a const_iterator, not an iterator. You cannot assign const_iterators to iterators, like you cannot assign pointers-to-const to pointers.

Upvotes: 7

ThomasMcLeod
ThomasMcLeod

Reputation: 7769

You are confusing a typedef with a decaration.

If you want to declare a typedef with a dependent type, then you do need to use the typename keyword:

typedef typename vector<T>::const_iterator iter
iter A = a.begin( );
iter B = b.begin( );

BTW, typename is required even without the typedef.

Upvotes: 0

Binary Worrier
Binary Worrier

Reputation: 51711

typename vector<T>::iterator A;

Should be

typename vector<T>::const_iterator A;

Same for B

Update

My C++ skills are rusty, but

Because the two vectors passed to merge are const references, you cannot use a standard iterator to move over them, because the standard iterator allows you to modify the contents of the vector. Therefore, you must use const_iterator's which will not allow you to modify the vector contents.

Apologies if my C++ Fu isn't up to scratch, I remember enough C++ to fix the problem, but haven't used C++ in anger in . . . wow 7 years (is it really that long? Bugger me but I'm getting old).

As I said, feel free to edit this answer if you can provide better explanations.

Upvotes: 3

Related Questions