temporary object in range-based for

c++, c++11, qt

Solution

Since you're using C++11, you could use initialization list instead. This will pass valgrind:

int main() {
  for (auto i : QList<int>{1, 2, 3})
    std::cout << i << std::endl;
  return 0;
}

The problem is not totally related to range-based for or even C++11. The following code demonstrates the same problem:

QList<int>& things = QList<int>() << 1;
things.end();

or:

#include <iostream>

struct S {
    int* x;

    S() { x = NULL; }
    ~S() { delete x; }

    S& foo(int y) {
        x = new int(y);
        return *this;
    }
};

int main() {
    S& things = S().foo(2);
    std::cout << *things.x << std::endl;
    return 0;
}

The invalid read is because the temporary object from the expression `S()` (or `QList<int>{}`) is destructed after the declaration (following C++03 and C++11 §12.2/5), because the compiler has no idea that the method `foo()` (or `operator<<`) will return that temporary object. So you are now refering to content of freed memory.

Problem

I know that in general the life time of a temporary in a range-based `for` loop is extended to the whole loop (I've read C++11: The range-based for statement: "range-init" lifetime?). Therefore doing stuff like this is generally OK: ``` for (auto &thingy : func_that_returns_eg_a_vector()) std::cout << thingy; ``` Now I'm stumbling about memory issues when I try to do something I thought to be similar with Qt's `QList` container: ``` #include <iostream> #include <QList> int main() { for (auto i : QList<int>{} << 1 << 2 << 3) std::cout << i << std::endl; return 0; } ``` The problem here is that valgrind shows invalid memory access somewhere inside the `QList` class. However, modifying the example so that the list is stored in variable provides a correct result: ``` #include <iostream> #include <QList> int main() { auto things = QList<int>{} << 1 << 2 << 3; for (auto i : things) std::cout << i << std::endl; return 0; } ``` Now my question is: am I doing something dumb in the first case resulting in e.g. undefined behaviour (I don't have enough experience reading the C++ standard in order to answer this for myself)? Or is this an issue with how I use `QList`, or how `QList` is implemented?

Original source

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