Code blocks & lambdas c++11

Question

please, can any one explain me, how conditionalVariable will be stored in this case, to be used while check_calls_on_current_floor calling outside the condition block?

std::function<bool()> check_calls_on_current_floor;
if (/*Some condition*/)
{
    const int conditionalVariable = /*some value*/;

    check_calls_on_current_floor = [&](){ 
        return conditionalVariable == 10; };
}
check_calls_on_current_floor();

It seems like in this case we, can access this variable outside the condition block, in case we got lambda from there.

possible duplicate of Using pointer-to-member to read value of cons object's field — BЈовић
– BЈовић, Commented Nov 13, 2013 at 6:38

Benjamin Lindley · Accepted Answer · 2013-11-13 05:09:51Z

3

It's a dangling reference. It's undefined behavior to make that call after the if block. It's very similar to returning a reference to a local variable from a function. It's even more similar to this:

struct ref_holder
{
    ref_holder(const int & r) :ref(r) {}
    const int & ref;
};

int main()
{
    std::unique_ptr<ref_holder> ptr;
    if (true)
    {
        const int conditionalVariable = 10;

        ptr.reset(new ref_holder(conditionalVariable));
    }
    ptr->ref == 10; // undefined behavior
}

edited Nov 13, 2013 at 5:09

answered Nov 13, 2013 at 5:03

Benjamin Lindley

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Comments

Timothy Shields · Accepted Answer · 2013-11-13 05:12:42Z

1

It's somewhat analogous to this:

int x = 0;
int* z = &x;
if (condition)
{
    int y = 1;
    z = &y;
}

If the condition holds, then z will be pointing to y which has gone out of scope.

edited Nov 13, 2013 at 5:12

answered Nov 13, 2013 at 5:07

Timothy Shields

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