CODEWITHSUNDEEP
c++templates
Fred Finkle
Fred Finkle

Reputation: 2017

Using friend to access private variable in templated classes

I get the following error from the program below:

use of undeclared identifier 'value'

Why? And, how do I fix it using friend classes?

Thanks in advance.

template <typename T>
class F2 : public F1<T>
{
public:
  F2(T o) : F1<T>(o) {}

  void fun() {
    std::cout << value << std::endl;
  }
};

template <typename T>
class F1
{
public:
    template <typename U>
    friend class F2;

    F1(T o) : value(o) {}

protected:
  T value;
};

int main()
{
    F2<int> f(3);
}

Upvotes: 0

Views: 139

Answers (2)

David
David

Reputation: 28178

There are multiple problems here:

1) F2 isn't templated, so everywhere you're using T in F2 is incorrect

2) F1 (the base class) is defined after F2, move F2 to below F1

3) A base class is friending it's derived class. It shouldn't know about its derived class.

Is this your actual code? If so is the error you wrote your actual error? I would expect numerous errors for an array of reasons.

Upvotes: 0

Neil
Neil

Reputation: 55382

The C++ standard prescribes that all names that are not dependent on template parameters are bound when parsing a template function, rather than when it is instantiated, which is otherwise when the reference to the inherited value could be detected.

The quick workaround is to use this->value; because this is dependent on the template parameter it is bound when the function is instantiated and the reference to the inherited value is therefore permitted.

Upvotes: 2

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