Referencing deleted destructor

Question

I have the following code:

struct Foo
{
    int type;
    union
    {
        int intValue;
        double doubleValue;
        std::wstring stringValue;
    } value;
};

and then in the cpp file I have:

std::vector<Foo> row;
some_class_object->func( row );

and I got:

error C2280: 'void *Foo::__delDtor(unsigned int)': attempting to reference a deleted function

What is the problem here?

EDIT:

So I added this destructor:

~Foo()
{
    if( type ==3 )
        value.stringValue.~std::wstring();
}

and I got an error:

error C2061: syntax error: identifier 'wstring'.

Apparently std::string vs std::wstring matter in this case...

Didn't know about that.

EDIT2:

I am now getting:

 error C2280: 'Foo::<unnamed-type-value>::~<unnamed-type-value>(void)': attempting to reference a deleted function

Use `union` only to implement `variant`, then use it. Boost provides one for pre-C++17, std provides since C++17. — Jarod42, Mar 05 '21 at 09:34

bolov · Accepted Answer · 2021-03-05T07:22:18.850

5

The union contains a member (std::string) with a non-trivial destructor. This means that the union can't have a defaulted destructor (it wouldn't know which member's destructor to call). So you need to provide a custom destructor.

In your case define a union destructor that does nothing and then do the work in the struct destructor:

struct Foo {
    int type;

    union U {
        int intValue;
        double doubleValue;
        std::wstring stringValue;

        
        ~U() noexcept {}

    } value;

    ~Foo()
    {
        using std::wstring;

        if (type == 3)
            value.stringValue.~wstring();
    }
};

Please note that you need to do this for copy/move constructor/assignments as well.

In C++17 you have std::variant which is a safe union.

edited Mar 05 '21 at 07:22

answered Mar 05 '21 at 03:53

bolov

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so how do I write it? I am not using pointers, just a plain members.And I'm not there yet - I don't have C++17 compilers across platforms... – Igor Mar 05 '21 at 04:35
2

@Igor "*so how do I write it?*" - see [Is a Union Member's Destructor Called](https://stackoverflow.com/questions/40106941/) – Remy Lebeau Mar 05 '21 at 04:44
@RemyLebeau, I updated the OP with the code I tried and the error I got. – Igor Mar 05 '21 at 05:48
@Igor drop the `std::`, it is not part of the destructor's name: `value.stringValue.~wstring();` Just make sure you had used `new(&value.stringValue) wstring();` beforehand – Remy Lebeau Mar 05 '21 at 05:53
@Igor `using std::wstring; value.stringValue.~wstring();` – bolov Mar 05 '21 at 05:54
@bolov, I edited an OP with the new error. – Igor Mar 05 '21 at 06:48

score 0 · Answer 2 · answered Mar 05 '21 at 07:07

THE UNION has destructor deleted and you're attempting to use it as anonymous entity. You can't define destructor there without naming the type.

struct Foo
{
    int type;
    union ValueType
    {
        int intValue;
        double doubleValue;
        std::wstring stringValue;
        ~ValueType() {}
    } value;
    ~Foo();
};

Foo::~Foo()
{
    if( type ==3 )
        value.stringValue.std::wstring::~wstring();
}

It's far better to use standard tagged union template provided with compiler instead of rolling your own because of arcane code like this.

what is union template? Can you give a code sample or link documentation? — Igor, Mar 05 '21 at 12:59

Referencing deleted destructor

2 Answers2

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