Re: Using operator() as return a reference to class self

Carl Barron <>
22 Jun 2006 07:01:31 -0400
In article <e792fd$7s0$>, Mockey Chen
<> wrote:

Overload operator() is magic in C++, since you can implement
a functor using operator(), the behave like a normal function.
I found we can use a trick that operator() return a reference to
class self. According return value, we can do a recursive call
operator(), this idiom can be used to initialize a list like class,
such as list, deque, vector...
There is an example:

#include <list>
#include <iostream>
#include <iterator>
#include <algorithm>

template<typename T>
class cmlist : public std::list<T>
     // Note: here *MUST* return a reference to class self
     cmlist& operator()(const T& e)

         return *this;

int main()
     cmlist<int> il;

     // Note: using recursive call operator to
     // initialize the list.
     // avoid using 3 line to call 3 push_back()

         std::ostream_iterator<int>(std::cout, " "));

     return 0;

I don't recommend inheritance from classes without a
virtual destructor. I use an overloaded operator << to
do this. If I have a custom container class I see no major

    I prefer an overload of operator << to do this in general.
The only problem with a simple template solution like
template <class T,class A,class X>
std::list<T,A> & operator << (std::list<T,A> &list,const X &x)
    return *this;

occurs if X is in namespace std, but this can be handle with a wrapper
struct containing a const X &.

template <class X>
struct class_wrapper
    const X &x;
    class_wrapper(const X & a):x(a){}
    operator const X &() {return x;}

template <class X>
inline class_wrapper<X> wrap_class(const X &x)
{ return class_wrapper<X>(x);}

and using wrap_class(x) as the args of the << chain.

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