Re: Redefintion of a function in template class

From:
Victor Bazarov <v.bazarov@comcast.invalid>
Newsgroups:
comp.lang.c++
Date:
Mon, 28 Feb 2011 12:27:22 -0500
Message-ID:
<ikgltq$ivi$1@news.eternal-september.org>
On 2/28/2011 11:56 AM, syang8 wrote:

I'd like to ask the reason the following code works. Original, I
expected that there is going to be a "function redefinition" error.
Could anyone tell me whether this is a compiler specific (I am using
GNU g++ 4.3.4) issue or this actually conforms to the C++ standard? I
appreciate.

A function f is defined in a template class A. Class B inherited from
A<int> redefines the function f in A<int>. The output shows that the
definition f in A<int> is actually changed, while the other instance
of the template, e.g. A<float>, remains untouched.


What you have here is called a "specialization" of the template member.
  Read about it.

//===================================
#include<iostream>

template<class T>
class A
{
public:

     typedef A<T> BaseT;
     int f();
};

template<class T>
int A<T>::f()
{
     return 1;
}

class B : public A<int>
{
};

template<>
int
B::BaseT::f()
{
     return 3;
}

int main()
{
     A<int> a1;
     A<float> a2;
     B b;
     std::cout<< a1.f()<< std::endl;
     std::cout<< a2.f()<< std::endl;
     std::cout<< b.f()<< std::endl;
}

// ======================================
// The output of the program is
// 3
// 1
// 3


V
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