Mike Warren Mike Warren - 1 year ago 60
C++ Question

error: passing ‘const Integer’ as ‘this’ argument of ‘Integer Integer::pow(int)’ discards qualifiers

I have

class that is supposed to simulate an integer mod n. Thus, it has constructors like:

Integer::Integer(int x)
: m(x), n(0)


Integer::Integer(int x, int y)
: n(y), m(x)
// if this->n greater than 1
if (this->n > 1)
// mod this->m by this->n
this->m %= this->n;
// if this->m is negative
if (this->m < 0)
// add this->n to it
this->m += this->n;

There is also
, which are needed for completeness of this question:

Integer Integer::inverse()
// Extended Euclidean Algorithm
int t = 0,
r = this->n,
newT = 1,
newR = this->m;
while (newR != 0)
int quotient = r / newR,
tCopy = t,
rCopy = r,
newTCopy = newT,
newRCopy = newR;
t = newT;
newT = tCopy - quotient * newTCopy;
r = newR;
newR = rCopy - quotient * newRCopy;
if (r > 1)
throw Integer(-1);
if (t < 0) t = t + this->n;
return Integer(t, this->n);

Integer Integer::squared()
return Integer(this->m * this->m, this->n);

Integer Integer::pow(int x)
// if x less than 0, return this->inverse().pow(-x)
if (x < 0) return this->inverse().pow(-x);
// if x is 0, return Integer(1)
if (x == 0) return Integer(1, this->n);
// if x is 1, return *this
if (x == 1) return *this;
// if x is 2, return this->squared()
if (x == 2) return this->squared();
// if x greater than 2
if (x > 2)
// if x is even
if (x % 2 == 0)
// return this->pow(x/2).squared()
return this->pow(x/2).squared();
// return this->pow(x/2).squared() * (*this)
return this->pow(x/2).squared() * (*this);

Problem I'm having is when I go to implement
Integer::isQuadraticResidue() const

bool Integer::isQuadraticResidue() const
// if this->n is zero
if (this->n == 0)
// this doesn't belong to Integers mod anything. check for perfect square instead
double baseSquareRoot = std::sqrt((double)this->m);
return (baseSquareRoot == (double)((int)baseSquareRoot));
// this is quadratic residue iff this->pow((this->n + 1) / 2) == Integer(1, this->n)
return (this->pow((n + 1) / 2).m == 1);

I get following error:
error: passing ‘const Integer’ as ‘this’ argument of ‘Integer Integer::pow(int)’ discards qualifiers
. I think it has everything to do with the
on the end. What do?

EDIT: Class header file looks something like:

#ifndef INTEGER_H
#define INTEGER_H

#include <iostream>

class Integer
Integer(int, int);
// functions
Integer inverse();
Integer squared();
Integer pow(int);
bool isQuadraticResidue() const;
Integer sqrt();
int m, n;



Answer Source

This is an issue with const correctness, where a const function is trying to call a non-const function.

// ...

bool Integer::isQuadraticResidue() const;
Integer Integer::pow(int x);

// ....

In this situation, this is an Integer* in pow(), and a const Integer* in isQuadraticResidue(); this means that pow() can call isQuadraticResidue(), because it's legal to add CV-qualifiers, but not the other way around (because pow() would have to accept this as a non-qualified Integer*, losing the const qualifier).

This is unallowed, because allowing it would mean that isQuadraticResidue() breaks its guarantee that it won't modify the instance, whether directly or indirectly. While it itself doesn't change state, it assumes that pow() does change state, because pow() isn't also const (and thus doesn't promise not to change state). Due to this, isQuadraticResidue() is unable to call pow(), because doing so would risk breaking its guarantee.

Considering that, there are two solutions for this issue.

  1. Remove const from isQuadraticResidue(). This would naively solve the problem, but isn't recommended, because then you wouldn't be able to reap the benefits of const correctness.
  2. Make all member functions that don't modify Integer's logical state const, as well. This will require a bit more effort, but is overall safer. As they can then be called on a const Integer just as well as they can on an Integer, you will then be able to pass your instances around as const whenever they don't need to be modified, giving you a greater degree of safety.

    Integer Integer::inverse() const;
    Integer Integer::squared() const;
    Integer Integer::pow()     const;

    This will need to be changed both in the functions' prototypes, and in their definitions.

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