c++ classes, const function that calls non constant function

Question

in C++ classes, if I have a constant function (In my understanding constant functions are classes that cannot change anyting in the class, hope this is true) for example a function like this

int output(int value) const

Am I allowed to call a function within this function remove that actually changes anything in the class?

For example lets say I have a variable called number in private.

my output function adds one to the variable number, but not directly, it calls a non constant function called addOne which adds one to the variable number. I notice that I am getting an error when a output function is calling addOne(I know that output function is const but it modifies a value, but it modifies it indirectly). I wonder why I am getting an error, what error compiler actually detects here?

Thank You, and sorry if my question is hard to understand.

Thank you all for answering.

Are you wondering why your function that promises not to change anything isn't allowed to change anything? — molbdnilo
– molbdnilo, Commented Apr 12, 2015 at 0:59
It is possible to indirectly modify an object through a const method, if you can find a non-const pointer to it from somewhere else. — Neil Kirk
– Neil Kirk, Commented Apr 12, 2015 at 1:06
@NeilKirk again a good point :) And this comes back to the old discussion of "bit-wise const" via "logical const". — vsoftco
– vsoftco, Commented Apr 12, 2015 at 1:11
@molbdnilo that is exactly what I was wondering about. I understand it sounds stupid, but I was curious because change was happening indirectly. — user1335175
– user1335175, Commented Apr 12, 2015 at 1:24
@user1335175 Along the lines of "I'm not allowed to have a cookie, but my brother is, so it should be OK if he takes one and gives it to me"? No, that's still not OK. — molbdnilo
– molbdnilo, Commented Apr 12, 2015 at 1:33

antonpp · Accepted Answer · 2015-04-12 01:03:48Z

2

The answer is simple - no, you cannot. Every class method is a function that implicitly passes this as a parameter. When you declare a method as const, that means that it will actually pass const this to this function. It is both correct logically and syntactically not to call not const methods from const methods.

answered Apr 12, 2015 at 1:03

antonpp

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3 Comments

antonpp Over a year ago

@vsoftco, yeah sure. It is a good point to use mutable members when you want to cache some calculations that is done in const methods in order to get a better response time.

vsoftco Over a year ago

Actually I take it back, you cannot call non-const member functions from inside const methods, no matter what. I misread the question, upvoted your answer.

vsoftco Over a year ago

You can actually const_cast<Foo*>(this) and call a const method, but it only works ok on non-const objects. On const objects it leads to undefined behaviour.

erol yeniaras · Accepted Answer · 2015-04-12 01:16:51Z

2

const instances can only call const functions. Say:

class YourClass { ... int output(int value) const; int regularMethod(); // Non-const ... };

is your class with a const function. And the variables:

const YourClass obj1; YourClass obj2;

Here obj1 can call output method while it cant call regularMethod. obj2 can call both.

edited Apr 12, 2015 at 1:16

answered Apr 12, 2015 at 1:06

erol yeniaras

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3 Comments

Neil Kirk Over a year ago

This answer is too vague. What is a "regular" instance? Also, you can have a non-const instance accessed through a const reference.

Neil Kirk Over a year ago

obj2 can call the output function.

erol yeniaras Over a year ago

@NeilKirk: It was my bad sorry!

Steephen · Accepted Answer · 2015-04-12 01:19:34Z

Every const function() is getting a const this pointer as a default parameter, so you cannot change a member variable of a class even if you try to call a non_cost function from a const function and try to modify a member variable.

So if you try to call a non const function from a const function, you will get compile error.

vsoftco · Accepted Answer · 2015-04-12 02:09:15Z

The answer is NO. The long answer is perhaps YES, provided you use an abomination like this, via const_cast-ing this in the const calling function (but you have to make sure you invoke your function on non-const objects, otherwise it leads to undefined behaviour):

#include <iostream> struct Foo { int x = 0; void f() { ++x; } void modify_x() const { const_cast<Foo*>(this)->f(); // call non-const member function } }; int main() { Foo foo; std::cout << foo.x << std::endl; foo.modify_x(); std::cout << foo.x << std::endl; }

Thank You for the answer. Also thank you for additional clarification in regards to mutable, but at this point I never worked with mutable and have no idea what it is :).
@user1335175 actually I misread your question, and Anton's answer is the correct one, I clarified mine. mutable lets you modify non-const member variables via const methods. But it cannot allow calling non-const methods from inside const methods.

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