Why is this a compiler error? (g++)

Question

So I'm attempting to utilize a generic compare functor in my utility class.

I attempt to define it and call it like so

template <class T>
bool AVL_tree<T>::avl_insert(AVL_Node<T> *& top, const AVL_Node<T> * insertNode, bool & taller) {
    std::binary_function<T,T,bool>::first_argument_type insertNodeValue;
    insertNodeValue  = insertNode->data;
    std::binary_function<T,T,bool>::second_argument_type topValue;
    topValue = insertNode->data;
    std::binary_function<T,T,bool>::result_type cmp_result;
    cmp_result = comparer(insertNodeValue,topValue);
    std::binary_function<T,T,bool>::result_type cmp_result2;
    cmp_result2 = comparer(topValue,insertNodeValue);
    //Function continues from here
}

The specific compiler error is expected ; before insertNodeValue

This error is repeated for topValue and cmp_result;

I don't really understand why this is a syntax error, I'm working off this reference: http://www.cplusplus.com/reference/std/functional/binary_function/

Why all this: `std::binary_function::first_argument_type`? Instead of just `T`? — Benjamin Lindley, Apr 27 '11 at 23:46

score 2 · Accepted Answer · edited May 23 '17 at 12:03

2

It's a dependent name, so it requires the typename keyword:

typename std::binary_function<T,T,bool>::first_argument_type insertNodeValue;

Similarly for others. See the SO FAQ entry on dependent names.

edited May 23 '17 at 12:03

Community

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answered Apr 27 '11 at 23:43

Cat Plus Plus

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score 0 · Answer 2 · answered Apr 27 '11 at 23:44

0

Given that these are dependent types, your first step should probably be to add typename:

typename std::binary_function<T,T,bool>::first_argument_type insertNodeValue;

answered Apr 27 '11 at 23:44

Jerry Coffin

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Why is this a compiler error? (g++)

2 Answers2