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I'm currently deep-diving into the way pointers work.

Something for me unexplainable happened when executing the following lines of code:

        std::vector<OptimizerPlanOperatorPtr> sources;
        for (const auto &source : sourceOperators){
            OptimizerPlanOperator planOperator = OptimizerPlanOperator(source);
            sources.push_back(static_cast<std::shared_ptr<OptimizerPlanOperator>(&planOperator));
        }

all sourceOperators differ, however when checking the elements of sources, they all point to the same OptimizerPlanOperator.

When I ran the debugger, I realized that in every loop step, all values of sources change to the recent value.

My assumption is, that I poorly initialized the pointer here which somehow results in the value the pointer refers to being overridden.

Can somebody show a solution or explain, what I did wrong here?

JVS
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    What is `OptimizerPlanOperatorPtr`? What are you trying to do with that `static_cast` over there? – Yksisarvinen Mar 31 '22 at 11:58
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    Side note: "overridden" != "overwritten", even though they sound alike in American. – molbdnilo Mar 31 '22 at 12:08
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    `OptimizerPlanOperator(source)` probably should return a `std::shared_ptr` so then you would have `auto planOperator = OptimizerPlanOperator(source);` and `sources.push_back(planOperator);` and no dangling pointer or trying to free a stack variable – drescherjm Mar 31 '22 at 12:11

2 Answers2

4

You are storing the location of an object whose lifetime ends with the current iteration and handing ownership of it to a shared_ptr. Both are problems that lead to undefined behaviour.

Casting a pointer to std::shared_ptr does not automagically make the pointed-to object into a shared object and extend its lifetime, and it is equivalent to std::shared_ptr<OptimizerPlanOperator>(&planOperator).

The simplest solution is to not do this stepwise but all at once:

for (const auto &source : sourceOperators){
    sources.push_back(std::make_shared<OptimizerPlanOperator>(source));
}
molbdnilo
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3

Your planOperator is a local variable on the stack, and when you are passing it to the cast static_cast<std::shared_ptr<OptimizerPlanOperator>(&planOperator), you are passing in the address of a local variable, so as soon as the iteration is over, that pointer &planOperator becomes garbage.

Albert Slepak
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