C++: why constructor "A(A a) {}" is illegal?

Question

I met a quiz saying that the code below is ill-formed because "It is illegal to have a constructor whose first and only non-default argument is a value parameter for the class type."

I couldn't understand that. Why things like A(A a) : val (a.val) {} is ruled as illegal? why such a line in the standard? Is it because this will lead to ambiguity with copy constructor?

#include <iostream>

struct A
{
  A() : val() {}
  A(int v) : val(v) {}
  A(A a) : val(a.val) {} 

  int val;
};

int main(int argc, char** argv)
{
  A a1(5);
  A a2(a1);

  std::cout << a1.val + a2.val << std::endl;

  return 0;
}

What happens if you use a header file? – Willem Van Onsem Jan 06 '15 at 05:25 — Willem Van Onsem, Jan 06 '15 at 05:25
@CommuSoft it's not related to header file. – athos Jan 06 '15 at 05:26 — athos, Jan 06 '15 at 05:26
Copy constructors cannot pass their argument by-value. – WhozCraig Jan 06 '15 at 05:28 — WhozCraig, Jan 06 '15 at 05:28

score 7 · Accepted Answer · answered Jan 06 '15 at 05:28

7

A(A a) : val(a.val) {} will cause an infinite recursion because constructor arguments are being copied by value (invoking copy constructor and again the copy constructor and ...)

answered Jan 06 '15 at 05:28

Mohit Jain

30,259
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so it's not possible to have both `A(A &a) : val(a.val) {}` and `A(A a) : val(a.val) {} `? is it because it's not able to differentiate them during function matching? – athos Jan 06 '15 at 05:37
Yes, you can not have both. – Mohit Jain Jan 06 '15 at 05:38

score 3 · Answer 2 · edited May 23 '17 at 12:20

A copy constructor is called when an object is passed by value. Copy constructor itself is a function. So if we pass argument by value in a copy constructor, a call to copy constructor would be made to call copy constructor which becomes a non-terminating chain of calls. Therefore compiler doesn’t allow parameters to be pass by value

Its already discussed on this SO post Why should the copy constructor accept its parameter by reference in C++?

score 1 · Answer 3 · answered Jan 06 '15 at 05:32

1

Everything changes with const:

#include <iostream>

struct A
{
    A () : _a(0) {}
    A (int a) : _a(a) {}
    A (const A& a) : _a(a._a) {}
    int _a;
};

int main()
{
    A a(5);
    A b(10);
    A c(a);

    std::cout << a._a + b._a + c._a << std::endl; // 20
    return 0;
}

answered Jan 06 '15 at 05:32

Joel

1,805
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2

i don't quite get what you mean..... – athos Jan 06 '15 at 05:36

C++: why constructor "A(A a) {}" is illegal?

3 Answers3

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