what function were called when initiate a string with both char array and = operator

Question

According to the explanation in C++ primer 5th.

Initiate a string with char array and using = operator. It will actually do below two things:

1: Call constructor which accept a const char * to create a temporary string object.

2: Call copy constructor to initiate the true variable;

chapter 13.1 page 618
string null_book = "9-999-99999-9"; // copy initialization

I made a test. and it seems that when I initiate A object with a cahr array. copy construtor have never been called.

#include <iostream>
int b =5;
using namespace std;
class A
{
public:
    A(const char * ch) :chr(*ch) {cout << "contruct ";};
    A(const A & a) : chr(0) {cout << "copy_construc ";::b = 2;}  ;
    A &operator=(const A & a)  {cout << "assignment"; return *this;};
    char chr;
};

int main() {
    A a = "qweqeasd";
    cout << b;
    cout << a.chr;
    A c = A("wrwsx");
    cout << b;
    cout << c.chr;
}

output:

contruct 5qcontruct 5w

It's not because there is an `=` that it's going to be a copy constructor. — Matthieu Brucher, Mar 04 '19 at 12:43
Yes, exactly. It will call the constructor with your value. Normal construction. Yes, the duplicate is not the best one here. — Matthieu Brucher, Mar 04 '19 at 12:49
@MatthieuBrucher Yeah you're right I misread, maybe you can provide a better one? — Hatted Rooster, Mar 04 '19 at 12:52
Maybe https://stackoverflow.com/questions/1758142/why-copy-constructor-is-not-called-in-this-case even if there is no explicit `A` in this case, but explains what happens as if OP wrote `A a = A("xxxx");`? — Matthieu Brucher, Mar 04 '19 at 12:58
@MatthieuBrucher Thanks, I think that question is the same with my question. — BAKE ZQ, Mar 04 '19 at 13:24

score 0 · Accepted Answer · answered Mar 04 '19 at 13:23

0

What you have here is called copy-initialization.

A a = "qweqeasd";

If T is a class type and the cv-unqualified version of the type of other is T or a class derived from T, the non-explicit constructors of T are examined and the best match is selected by overload resolution. The constructor is then called to initialize the object.

Here the best match is the constructor A(const char * ch) and that is why the output starts with contruct .

answered Mar 04 '19 at 13:23

P.W

26,289
6
39
76

please stop answering duplicates, especially when they are already mentioned. – Matthieu Brucher Mar 04 '19 at 14:30
I answered **after** the post was reopened after being closed as a duplicate. – P.W Mar 04 '19 at 14:34
I gave a possible duplicate, and we were discussing duplicates. For an almost gold badge, this is a shame. – Matthieu Brucher Mar 04 '19 at 14:48
Well, the goldbadge holder was mistaken, your comment suggested that you were not sure (maybe...). It's still not closed as a dupe after nearly two hours when such things happen within minutes. There is only one close vote on it. Let's not "shame" others. – P.W Mar 04 '19 at 14:55

what function were called when initiate a string with both char array and = operator

1 Answers1