iterator issues

I have a function that returns the sum of elements in an std::vector, but I'm having issues with iterators. (line numbers added for clarity)

Code:

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4 : template<class T> T sum(std::vector<T>& v)
5 : {
6 :  T tmp = 0;
7 :  for (std::vector<T>::iterator it = v.begin(); it != v.end(); it++)
8 :  {
9 :    tmp += *it;
10:  }
11:  return tmp;
12: }

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I'm compiling this on OpenSuse linux 10.3, kernel version 2.6.22.5-31-default, using GCC 4.2.1, and I get the following errors:

xxx.cpp:7: error: expected ';' before 'it'
xxx.cpp:7: error: 'it' was not declared in this scope

Code:

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template<typename T> T sum(std::vector<T>& v)

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Try it with typename instead of class.

vector<T>::iterator is a dependent type. you need the typename keyword to tell the compiler that it's a type and not a value.

Code:

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4 : template<class T> T sum(std::vector<T>& v)
5 : {
6 :  T tmp = 0;
7 :  for (typename std::vector<T>::iterator it = v.begin(); it != v.end(); it++)
8 :  {
9 :    tmp += *it;
10:  }
11:  return tmp;
12: }

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Quote:

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Originally Posted by hk_mp5kpdw

Code:

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template<typename T> T sum(std::vector<T>& v)

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Try it with typename instead of class.

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typename and class are the same thing when used there.

Quote:

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Originally Posted by Meldreth

vector<T>::iterator is a dependent type. you need the typename keyword to tell the compiler that it's a type and not a value.

Code:

---

4 : template<class T> T sum(std::vector<T>& v)
5 : {
6 :  T tmp = 0;
7 :  for (typename std::vector<T>::iterator it = v.begin(); it != v.end(); it++)
8 :  {
9 :    tmp += *it;
10:  }
11:  return tmp;
12: }

---

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that fixed it. thank you very much.

oh yeah, and you can use the accumulate function in the numeric header to do this too.

Code:

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#include <iostream>
#include <numeric>
#include <vector>

using namespace std;

int main()
{
    vector<int> v;
    v.push_back(1);
    v.push_back(2);
    v.push_back(3);
    v.push_back(4);
    v.push_back(5);
    cout << accumulate(v.begin(), v.end(), 0) << '\n';
}

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Quote:

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Originally Posted by Meldreth

typename and class are the same thing when used there.

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Not always. This is valid:

Code:

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template <template <typename> class X>
class foo {};

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Whereas this is not:

Code:

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template <template <typename> typename X>
class foo {};

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Quote:

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Originally Posted by brewbuck

Not always.

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yes, always. without fail. even if you come up with a similar but not quite the same example that suggests otherwise. it's explained on this page if you still can't understand why your example doesn't prove me wrong.

Quote:

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Originally Posted by Meldreth

yes, always. without fail. even if you come up with a similar but not quite the same example that suggests otherwise. it's explained on this page if you still can't understand why your example doesn't prove me wrong.

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What's your point?

Quote:

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Originally Posted by brewbuck

What's your point?

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Meldreth's point is that typename is 'an alternative to "class" when declaring template arguments', so hk_mp5kpdw's suggestion was on the wrong track. I really don't see what is the fuss when "there" was not clearly specified, but the context is otherwise obvious.

EDIT:
Well, actually, Stroustrup's definition is inaccurate: "template arguments" should be changed to "template parameters".

Quote:

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Well, actually, Stroustrup's definition is inaccurate: "template arguments" should be changed to "template parameters".

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I would say "template type parameters". It seems easier to infer the intended meaning when choosing between "template type parameters", "template template parameters", and "template value parameters".

Soma