Copy constructor with non-const argument

[Joseph Hesse]

If a non-const argument is given for a copy constructor, with a
non-empty body, does the code in the body execute first or is the
initialization first? In the following example the code in the body
executes first. Is this always the case?

Thank you,
Joe

#include <iostream>

class X {
public:
X(int u = 0) : x(u) {}
X(X &rhs) : x(rhs.x) { x += 2; } // non-const argument for copy
constructor
int getx() const { return x; }
private:
int x;
};

int main() {
X x1(5);
X x2(x1);
std::cout << x2.getx() << '\n'; // g++ and clang++ output 7
}

[Markus Schaaf]

Am 16.02.23 um 16:34 schrieb Joseph Hesse:

> If a non-const argument is given for a copy constructor, with a
> non-empty body, does the code in the body execute first or is the
> initialization first?

First initialization, then body.

> In the following example the code in the body
> executes first.

No.

>
> Thank you,
> Joe
>
> #include <iostream>
>
> class X {
> public:
> X(int u = 0) : x(u) {}
> X(X &rhs) : x(rhs.x) { x += 2; } // non-const argument for copy
> constructor
> int getx() const { return x; }
> private:
> int x;
> };
>
> int main() {
> X x1(5);
> X x2(x1);
> std::cout << x2.getx() << '\n'; // g++ and clang++ output 7
> }

Of course. `X x1(5)` calls `X::X(int)`, which initializes `x1.x`
to 5. Then `X x2(x1);` calls `X::X(X&)`, which initializes `x2.x`
to 5, and then adds 2.

BR

[Andrey Tarasevich]

On 02/16/23 7:34 AM, Joseph Hesse wrote:
> If a non-const argument is given for a copy constructor, with a
> non-empty body, does the code in the body execute first or is the
> initialization first?

Member and base initialization is always carried out first.

> In the following example the code in the body
> executes first.

Where did you get this strange idea?

> #include <iostream>
>
> class X {
> public:
>   X(int u = 0) : x(u) {}
>   X(X &rhs) : x(rhs.x) { x += 2; }  // non-const argument for copy
> constructor
>   int getx() const { return x; }
> private:
>   int x;
> };
>
> int main() {
>   X x1(5);
>   X x2(x1);
>   std::cout << x2.getx() << '\n';  // g++ and clang++ output 7
> }

Aaand? '7' is the expected output. What made you conclude that "body
executes first" here?

--
Best regards,
Andrey