Is there any way to see which constructor will be called for this statement "Student s3=func(s1, s4)"?

Question

I want a compiler (online/ offline) which will not do any copy elison. I want to see the actual output which we've learnt in theory of "constructor and destructors of c++".

Which constructor will be called for the statement "Student s3=func(s1, s4)". None of my copy constructor, parameterized constructor and overloaded assignment operator is called for this statement. Then, how is this object constructing?

I have tested using this compiler too: https://rextester.com/l/cpp_online_compiler_visual

#include<cstring>
#include<iostream>
using namespace std;
class Student
{
    char* name;
    int id;
public:
    Student(char* n, int i)
    {
        name= new char[strlen(n)+1];
        strcpy(name, n);
        id=i;
        cout<<"Constructor "<<name<<endl;
    }
    Student(const Student& s)
    {
        name= new char[strlen(s.name)+1];
        strcpy(name, s.name);
        id=s.id;
        cout<<"Copy constructor "<<name<<endl;
    }
    void operator = (const Student &s )
    {
        name= new char[strlen(s.name)+1];
        strcpy(name, s.name);
        id=s.id;
        cout<<"Assignment Operator "<<name<<endl;
    }
    ~Student()
    {
        cout<<"Destructing "<<name<<endl;
        delete[] name;
    }
};
Student func(Student s, Student t)
{
    return s;
}
int main()
{
    Student s1("abcd", 16);
    Student s4("wxyz", 17);
    Student s3=func(s1, s4);
}

Copy elision is **part of the language**. 'which we've learnt in theory of "constructor and destructors of c++"'. Your course sounds like it teaches an inaccurate, but probably still useful, model. — Caleth, Apr 17 '19 at 10:04
@Caleth I am really sorry. I didn't want to mean that. I tried to mean, I want to see the output without copy elision. Thanks. — Sanjida, Apr 17 '19 at 13:42

Ignatius · Accepted Answer · 2019-04-17T09:27:00.717

I think you are mistaken. A copy constructor is clearly called.

To make this clearer, re-write your main function to:

int main()
{
    Student s1("abcd", 16);
    Student s4("wxyz", 17);
    cout << "initializing s3\n";
    Student s3=func(s1, s4);
    cout << "done\n";
}

And the output will be (the "comments" are not part of actual output, but comments by myself.)

Constructor abcd
Constructor wxyz
initializing s3
Copy constructor wxyz // for the second parameter of the function
Copy constructor abcd // for the first parameter of the function
Copy constructor abcd // for s3
Destructing abcd // for the first parameter of the function
Destructing wxyz // for the second parameter of the function
done
Destructing abcd
Destructing wxyz
Destructing abcd

To make this even clearer, you can re-write also your function to take reference parameters so that new instances are not created.

Student func(Student &s, Student &t)
{
    return s;
}
int main()
{
    Student s1("abcd", 16);
    Student s4("wxyz", 17);
    cout << "initializing s3\n";
    Student s3=func(s1, s4);
    cout << "done\n";
}

Then the output will be:

Constructor abcd
Constructor wxyz
initializing s3
Copy constructor abcd
done
Destructing abcd
Destructing wxyz
Destructing abcd

where you can see a copy constructor is called once, obviously to initialize s3.

Now, if you compile with with the gcc option -fno-elide-constructors (as Quentin suggested), the output will be: (comments not part of actual output)

Constructor abcd
Constructor wxyz
initializing s3
Copy constructor abcd // for the return value of the function
Copy constructor abcd // for s3
Destructing abcd // for the return value of the function
done
Destructing abcd
Destructing wxyz
Destructing abcd

Thank you so much. This is really kind. How to add the option "-fno-elide-constructors" in code blocks? — Sanjida, Apr 17 '19 at 13:45
I don't know about Code Blocks, but try: https://stackoverflow.com/questions/33208733/how-to-add-compiler-flags-on-codeblocks — Ignatius, Apr 17 '19 at 15:04

score 2 · Answer 2 · edited Jun 20 '20 at 09:12

GCC (whether it is hosted online or not) has -fno-elide-constructors:

-fno-elide-constructors

The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline.

In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.

Is there any way to see which constructor will be called for this statement "Student s3=func(s1, s4)"?

2 Answers2