a question about QVariant

[calmspeaker]

Hi all
In the source code of Qt, we could see the below code

return QVariant();

return QVariant();

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VS9 will report a warning:

warning C4172: returning address of local variable or temporary

We all know returning address of local var is very dangerous!
I want to know is that(return QVariant() correct?

[yogeshgokul]

This is returning a name less empty object. So no probs !!

[nightghost]

Depends on the Signature.

// bad, return by reference a local variable
QVariant& myClass::myFunction() {
  return QVariant();
}
 
// ok, return by copy
QVariant myClass::myFunction() {
  return QVariant();
}
 
// Also ok, return by reference
QVariant& myClass::myFunction(QVariant& my_variant) {
  return my_variant;
}

// bad, return by reference a local variable
QVariant& myClass::myFunction() {
  return QVariant();
}

// ok, return by copy
QVariant myClass::myFunction() {
  return QVariant();
}

// Also ok, return by reference
QVariant& myClass::myFunction(QVariant& my_variant) {
  return my_variant;
}

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See (for example) here for a detailed expanation

[calmspeaker]

This is returning a name less empty object. So no probs !!

Thank you for replying!
What do you mean by empty object?
In Qt doc:
QVariant::QVariant ()

Constructs an invalid variant.
In Qt source code:

struct Private
    {
        inline Private(): type(Invalid), is_shared(false), is_null(true) { data.ptr = 0; }
        inline Private(const Private &other)
            : data(other.data), type(other.type),
              is_shared(other.is_shared), is_null(other.is_null)
        {}
        union Data
        {
            char c;
            int i;
            uint u;
            bool b;
            double d;
            qlonglong ll;
            qulonglong ull;
            void *ptr;
            PrivateShared *shared;
        } data;
        uint type : 30;
        uint is_shared : 1;
        uint is_null : 1;
    };

struct Private
    {
        inline Private(): type(Invalid), is_shared(false), is_null(true) { data.ptr = 0; }
        inline Private(const Private &other)
            : data(other.data), type(other.type),
              is_shared(other.is_shared), is_null(other.is_null)
        {}
        union Data
        {
            char c;
            int i;
            uint u;
            bool b;
            double d;
            qlonglong ll;
            qulonglong ull;
            void *ptr;
            PrivateShared *shared;
        } data;
        uint type : 30;
        uint is_shared : 1;
        uint is_null : 1;
    };

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I deduce that the QVariant() is not empty.It at least contains type of which value is Invalid!

[wysota]

Empty in the terms of semantics ("without any user relevant data").