C++ functional operator struct not doing anything?

Question

I have the following code in one of my problems for a certification program I'm in and can't figure out why the vector of A objects aren't being changed when the for-each statement is called with the doubler() struct. The code is below, if anyone could shed some light on why this is the case that would be great; I'm sure it's something minuscule I'm overlooking.

class A {
    int a;
public:
    A(int a) :a(a) {}
    int getA() const { return a; } void setA(int a) { this->a = a; }
    bool operator < (const A & b) const { return a<b.a; }
};
struct myprinter { void operator() (const A & a) { cout << a.getA() << ", "; } };
struct doubler
{
    void operator() (A a) { a.setA(a.getA() * 2); }
};

int main() {
    int mynumbers[] = { 3, 9, 0, 2, 1, 4, 5 };
    vector<A> v1(mynumbers, mynumbers + 7);
    set<A> s1(mynumbers, mynumbers + 7);
    for_each(v1.begin(), v1.end(), doubler()); for_each(v1.begin(), v1.end(), myprinter())
    for_each(s1.begin(), s1.end(), doubler()); for_each(s1.begin(), s1.end(), myprinter());
    return 0;
}

My output from the program is as follows: 3, 9, 0, 2, 1, 4, 5, 0, 1, 2, 3, 4, 5, 9,

I was expecting each of the values to be doubled, but for some reason the for-each statement with doubler() did not do anything.

Also note that you cannot directly modify the element of a `std::set`. — NathanOliver, Jan 27 '17 at 15:39
This is exactly [this question](http://stackoverflow.com/questions/41851517/using-for-each-to-modify-std-containers-even-though-you-shouldnt/41852333#41852333). I wonder why. — Hatted Rooster, Jan 27 '17 at 15:39
@GillBates really striking similarity. I smell some kind of plagiarism — 463035818_is_not_an_ai, Jan 27 '17 at 15:41
They're probably both following the same certification program, so they're getting the same question (with the same problem code). — Sander De Dycker, Jan 27 '17 at 15:44
@SanderDeDycker ah that makes sense. I misunderstood certification program as this code being part of some bigger code that can be used to certify something ;) — 463035818_is_not_an_ai, Jan 27 '17 at 15:46
@tobi303 : that said - it's still a bit dodgy to have the internet help you get certified. It makes these certifications largely meaningless. — Sander De Dycker, Jan 27 '17 at 15:48
@SanderDeDycker we should all get the certificate because we helped to spot the error — 463035818_is_not_an_ai, Jan 27 '17 at 15:49
I had tried changing it to pass by reference and then ran into the problem of trying to change the set values which I am clear on now. Thanks for the help guys; sorry for the duplicate post! — Ndesai94, Jan 27 '17 at 16:49
P.S. The cert is a complete waste of time.. but the boss wants me to get it for some reason. — Ndesai94, Jan 27 '17 at 16:50

score 0 · Answer 1 · edited May 23 '17 at 12:01

0

Your operator passes a by value, making a copy of it. Take a reference instead.

void operator() (A & a) { a.setA(a.getA() * 2); }
//        Add this ^

Edit: It's been indicated in the comments that you are applying your operation on an std::set. The elements of a set cannot be modified. See this answer for more information on why you cannot modify the elements in a set.

edited May 23 '17 at 12:01

Community

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answered Jan 27 '17 at 15:38

François Andrieux

28,148
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score 0 · Answer 2 · answered Jan 27 '17 at 15:39

0

In your doubler::operator () implementation, you're passing in the A object by value. It's doubling a copy of the A object, not the A object itself. You probably want:

struct doubler
{
    void operator() (A& a) { a.setA(a.getA() * 2); }
};

answered Jan 27 '17 at 15:39

Chris Vig

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C++ functional operator struct not doing anything?

2 Answers2