Copy constructor not being called, trouble understanding how the assignment operator works

Question

I couldn't really think of a specific title, as I don't really know how to word it. I'm really confused on this, as my previous question did not really get answered, but only make me more confused. My other question asked

Setting std::string to 0 during definition vs setting std::string to 0

User mnciitbhu said that he was getting an error on GCC 5.2. I am using Microsoft Visual Studio so I tried it on Code::Blocks, and there were no errors. Besides that, I wanted to try this out for myself. I have a class - Test

class Test {

public:

    Test(const Test& T) {

        std::cout << "COPY CTOR\n";

    }

    Test(){

        std::cout << "DEFAULT CTOR\n";

    }

    Test(int NUMBER) {

        std::cout << "INT CTOR CALLED\n";

    }


};

Inside my main function, when I do

Test t1 = 5;
t1 = 10;

the output is

INT CTOR CALLED

Now where I am getting confused is when you do Test t1 = 5, a user on the other question said it will create a temporary Test, then call the copy constructor. This never calls the copy constructor. Also, in reference to my other question; when I did

 // text = std::string
 text = NULL;

one of the answers said that it didn't work because it did not define operator=. My Test class never defined it either, but doing

t1 = 10;

still works and calls the constructor again. One of the users on the other question said that in this context, it call the assignment operator. This did not, as you can tell from the output. So could someone please explain, in great detail, why

std::string text = NULL;
text = NULL;

gives me an error, and

Test t1 = 5;
t1 = 10;

works, and doesn't call the copy constructor, and calls the constructor twice.

What is the = actually doing? Why does it call the constructor when I do t1 = 10

The output of your example should contains `INT CTOR CALLED` instead of `DEFAULT CTOR`, typo? — fghj, Nov 14 '15 at 18:09
So, what is your question? if you want to compile time error on you code add `explicit` to your `Test` constructor, like this `explicit Test(int NUMBER)` — fghj, Nov 14 '15 at 18:18
Or you can force compilation of your code with `Test& operator=(const Test&) = delete;` in `Test` definition. `operator=` if not defined generated by compiler it self, in several cases. — fghj, Nov 14 '15 at 18:19
Since you didn't overload the assignment operator, you have no idea whether it's called or not. (And *copy elision* is a legitimate optimisation.) — molbdnilo, Nov 14 '15 at 18:20
Regarding the apparent lack of calls to the copy constructor: http://stackoverflow.com/questions/12953127/what-are-copy-elision-and-return-value-optimization — Benjamin Lindley, Nov 14 '15 at 18:22

score -3 · Answer 1 · answered Nov 14 '15 at 18:20

-3

Usually NULL is defined as ((void*)0) and std::string has no idea of how to construct strings from void* pointers, std::basic_string simply has no such constructors.

Try

std::string text = (const char*)0;

instead.

answered Nov 14 '15 at 18:20

c-smile

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NULL is defined as integer in c++, not a pointer. Your definition of NULL is from C. – Eugene Nov 14 '15 at 18:23
`std::string` can be constructed from `const char*`, therefore it can be constructed from `NULL`. – juanchopanza Nov 14 '15 at 18:38
@Eugene `NULL` is technically implementation defined in C and C++. – Jason Nov 14 '15 at 18:53
You should never write code that looks like that. – Neil Kirk Nov 14 '15 at 19:01
@NeilKirk Exactly, and the same apply to `std::string text = NULL` – c-smile Nov 14 '15 at 19:30

Copy constructor not being called, trouble understanding how the assignment operator works

1 Answers1