Copy constructor error when initilizing a vector of custormer typed objects

Question

Class Foo only has a default constructor and a copy constructor. A vector of size 10 initialized by object of type Foo is somehow incorrect.

#include <iostream>
#include <vector>

class Foo
{
    public:
        Foo() = default; // Error1
        Foo(Foo& foo) {  // Error2
            std::cout << "copied" << std::endl;
        }

};

int main( void )
{
    Foo f;
    std::vector<Foo> vec(10, f); // Error3
    return 0;
}

There are 3 errors for the sample code above:

Error1: candidate constructor not viable: requires 0 arguments, but 1 was provided

Error2: candidate constructor not viable: 1st argument ('const Foo') would lose const qualifier

Error3: in instantiation of member function 'std::__1::vector >::vector' requested here

When I remove the copy constructor or the vector in the main function, there are no errors.

Question:

Which part of the code is wrong and why?

p.s.

When the vector definition is replaced by Foo ff(f), errors are gone as well. Is the const qualifier is request by the vector?

A copy constructor that modifies the thing it copies from looks strange. — juanchopanza
– juanchopanza, Commented Nov 28, 2018 at 13:41
@SLN I guess std::vector adds const before copying to ensure the initial object is not changed in some weird copy constructors like the ones juanchopanza had in mind while posting his comment. — Detonar
– Detonar, Commented Nov 28, 2018 at 13:56
@SLN - Think of it like this. If I gave you my lecture to notes to copy after you missed a class, it'd be pretty poor form to doodle all over my notebook, won't it? — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Nov 28, 2018 at 14:05

Daniel Langr · Accepted Answer · 2018-11-28 14:13:47Z

4

Look at the signature of the invoked constructor of std::vector:

vector(size_type n, const T& value, const Allocator& = Allocator());

The argument f is passed by const reference (parameter value). That parameter therefore cannot be bound to non-const parameter foo of type Foo& of Foo's copy constructor.

Same (simpler) case:

Foo f;
const Foo& value = f;
Foo& foo = value;  // error

edited Nov 28, 2018 at 14:13

answered Nov 28, 2018 at 14:02

Daniel Langr

24.2k3 gold badges55 silver badges110 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

StoryTeller - Unslander Monica Over a year ago

@bolov - Sorry what? You can't bind a non-const reference to an object that is const or is referred to by a const reference. This answer is perfectly correct.

bolov · Accepted Answer · 2018-11-28 14:05:23Z

1

The constructor you use is std::vector::vector (2)

vector( size_type count, 
                 const T& value,
                 const Allocator& alloc = Allocator());

The parameter value which will be copied inside the constructor is const, and because your copy constructor cannot copy from const the implementation of the constructor cannot create copies to fill the vector.

answered Nov 28, 2018 at 14:05

bolov

76.9k17 gold badges156 silver badges248 bronze badges

Comments

Detonar · Accepted Answer · 2018-11-28 14:10:59Z

0

The constructor documentation of std::vector shows the following signature:

explicit vector (size_type n, const value_type& val = value_type(),
             const allocator_type& alloc = allocator_type());

This shows that it requires val to be a const reference. Because it most likely won't cut this const in it's internal processing it seems to pass val like this to the copy constructor. Because Foo's copy constructor expects an non-const reference to be passed val would lose it's const qualifier. Because this is not allowed the compiler raises the error

Error: candidate constructor not viable: 1st argument ('const Foo') would lose const qualifier

answered Nov 28, 2018 at 14:10

Detonar

1,4298 silver badges19 bronze badges

Collectives™ on Stack Overflow

Copy constructor error when initilizing a vector of custormer typed objects

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related