Why does const auto& lead to empty lists?

Asperamanca

Given that const references extend the lifetime of object instances (see e.g. Herb Sutter's explanation), why does the following code dump an empty list?

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    const auto& list = QStringList() << "abc"
                                     << "xyz";
    qDebug() << list;

    return a.exec();
}

Change to "const auto list =[...]", and the code works as expected.

SGaist

Hi,

What compiler are you using ?

This works with Apple LLVM version 9.0.0 (clang-900.0.39.2)

Asperamanca

Both MinGW 5.3 and VS 2015 show this behavior in my tests.

Konstantin Tokarev

This post is deleted!

SGaist

What about VS2017 ?

JKSH

@Asperamanca said in Why does const auto& lead to empty lists?:

const references extend the lifetime of object instances (see e.g. Herb Sutter's explanation)

Wow, TIL. I must say this feature feels unintuitive to me, however.

EDIT: Actually, this felt weird because I've never found a reason to define a const-ref local variable. However, this feature makes some sense when I think about a const-ref function parameter.

Asperamanca

@JKSH said in Why does const auto& lead to empty lists?:

@Asperamanca said in Why does const auto& lead to empty lists?:

const references extend the lifetime of object instances (see e.g. Herb Sutter's explanation)

Wow, TIL. I must say this feature feels unintuitive to me, however.

EDIT: Actually, this felt weird because I've never found a reason to define a const-ref local variable. However, this feature makes some sense when I think about a const-ref function parameter.

So you never do stuff like this?

for(const auto& item : container)
{
   // Do stuff
}

Because, theoretically you could now remove that item from the container within that loop (although this would rarely be a good idea), and the reference would not be invalid.

Asperamanca

@SGaist said in Why does const auto& lead to empty lists?:

What about VS2017 ?

Same as VS2015 and MinGW

JKSH

@Asperamanca said in Why does const auto& lead to empty lists?:

So you never do stuff like this?
for(const auto& item : container)
{
   // Do stuff
}

I do, I do.

I just I think of this like I do with function parameters, but I think of function parameters differently from local variables inside a function block (like const auto& list = ...;). Once I remember they're basically the same, it now makes sense.