overloading operator+ constructor issue

Question

I am learning C++ from a book and I find references and operator overloading really difficult. While studying operator overloading, I can't understand why this program won't work without a constructor that takes an integer as a parameter.

#include <iostream> using namespace std; class counter { public: counter(); counter(int value); ~counter(); int getValue() const { return itsValue; } int setValue(int value) { itsValue = value; } counter operator+(const counter&); private: int itsValue; }; counter::counter() : itsValue(0) {} counter::counter(int value) : itsValue(value) {} counter counter::operator+(const counter& rhs) { return itsValue + rhs.getValue(); } int main() { std::cout << "Hello World!\n"; }

I can't understand why the program won't work in the absence of these lines:

counter(int value);

AND

counter::counter(int value) : itsValue(value) {}

Why do you think the program won't work without those lines? (You're right that it doesn't, but the question should show the full error message) — Mooing Duck
– Mooing Duck, Commented Oct 13, 2020 at 22:21

cigien · Accepted Answer · 2020-10-14 01:04:39Z

If you don't have this constructor:

counter(int value);

then you can't do things like:

int n = 3; counter c = counter{n}; counter c = counter{1}; counter c = 2; // etc ...

The issue with your operator+ is that you are doing exactly that when you convert the int value itsValue + rhs.getValue() to a counter which is the return type:

counter counter::operator+(const counter& rhs) { return itsValue + rhs.getValue(); }

Without the appropriate constructor, there is no way to construct a counter from an int, and you get an error.

You can choose not to provide an int constructor, by manually constructing a counter object, and returning that:

counter counter::operator+(const counter& rhs) const { counter c; c.itsValue = itsValue + rhs.getValue(); return c; }

Note that operator+ should be const-qualified.

Here's a demo.

A few other fixes: setValue should return void. You haven't defined the destructor, but you have provided a declaration. Just remove the declaration, since the implicitly generated destructor is sufficient.

"This of course, requires a constructor for counter that takes an int" I think that what OP actually does not understand, I would elaborate this "of course"
@Slava Hmm, not sure how much to edit, but I tried a bit. Is this better?
I mean OP probably does not understand/know that single argument ctor works for type conversion.

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