Question

What does the line:

template<typename _Tp1, typename _Seq1>
friend bool
operator==(const stack<_Tp1, _Seq1>&, const stack<_Tp1, _Seq1>&);

in http://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.4/a01367.html

do?

Why is _Tp1 repeated twice in arguements list?
Thanks,

Answer 1

It declares the equality operator between two stacks a friend function of this class, which is necessary for it to access private members.

The const stack<_Tp1, _Seq1> appear twice because there are 2 arguments.

Of course it can be written as

bool operator==(const stack<_Tp1, _Seq1>& y) const { return c == y.c; }

but the C++ standard (§[stack.ops] (23.3.5.3.4)) seems to require this operator to be a free function.