C ++“警告：返回对临时的引用” - 但事实并非如此

Question

I have a very simple method, and const overload of it.

Sy_animatable::PropertyTimeLine&
Sy_animatable_imp::getPropertyTimeLine( const QString& property )
{
    if ( !properties_.contains( property ) ) {
        throw Sy_unknownAnimPropertyException( property );
    }

    return properties_[property];
}

const Sy_animatable::PropertyTimeLine&
Sy_animatable_imp::getPropertyTimeLine( const QString& property ) const
{
    if ( !properties_.contains( property ) ) {
        throw Sy_unknownAnimPropertyException( property );
    }

    return properties_[property];  // "warning: returning reference to temporary"
}

I don't understand the warning for two reasons:

1. properties_ is a member variable and it's subscript operator (it's a QMap ) returns a reference, so there shouldn't be any temporaries and it is persistant for the lifetime of the object.
2. Why is the warning appearing in the const overload and not the original?

I could #pragma the line to hide the warning, but I really want to know why it is giving me the warning - any suggestions?

Answer 1

Looks like the [] -operator for QMap has strange semantics that sometimes generate a const-reference to a temporary (if there's no element with the given key), and the lifetime of that one isn't extended far enough.

Try return properties_.find(property).value(); instead.

Answer 2

In QMap, operator[]() is kind of quirky; it can both insert (key, value) pairs in the map, and be used for looking up a value. The documentation states:

To look up a value, use operator or value():

 int num1 = map["thirteen"]; int num2 = map.value("thirteen"); 

If there is no item with the specified key in the map, these functions return a default-constructed value.

QMap::value() returns a default-constructed value if the specified key isn't in the map -- which means a value is created using the default constructor. This is the temporary the warning you are getting is referring to. While operator[]() will not silently insert a (key, value) pair if it already exists (but will if it doesn't), using .value() is the way to go although I'm not sure the warning will go away.