Using lambdas in local scope in c++

Question

Is it OK to declare lambdas in local scope like this:

// reference to method is just a call back function. 
void(*referenceToSomeMethod)();

// store call back function
void foo(void(*pointerToMethod)())
{
    referenceToSomeMethod = pointerToMethod; // save reference to method    
}

int main()
{

    referenceToSomeMethod = nullptr;

    if (1 == 1)
    {
        // use of lambda on local scope
        foo([]() {
            printf("Executing callback function\n");
        });


    } // leaving scope lambda may not longer be valid?

    // simulate some work
    Sleep(10000);

    if (referenceToSomeMethod != nullptr)
        referenceToSomeMethod(); // execute lambda
}

In real life I wait until an event occurs to fire the callback method. Can I run the risk of the callback pointer pointing to a function that no longer exists?

I know this works in c# and other languages that have a garbage collector. But will this work on c++?

Why not use std::function<void()> and pass by r-value reference. Thus allowing you to capture the lambda and move it into place? Thus allowing you presrve any captured arguments. — Loki Astari
– Loki Astari, Commented Jul 27, 2020 at 18:11
I cannot use std::function I am creating this a class library for a micro-controller (arduino). But yes I have to store the parameter as a reference somewhere else it is the same idea. — Tono Nam
– Tono Nam, Commented Jul 28, 2020 at 3:38

user2039981 · Accepted Answer · 2020-07-27 17:23:27Z

2

Lambdas with empty brackets are equivalent with function pointers and will be copied by value in the same way. Answer yes.

answered Jul 27, 2020 at 17:23

user2039981

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6 Comments

Tono Nam Over a year ago

And if my lambda takes in parameters by value I guess that will also be a yes correct? Thanks for the help @super

QuentinUK Over a year ago

No. A local object is created with the captured value in it, which then goes out of scope.

Tono Nam Over a year ago

So to be safe it will be best to declare the callback functions in global scope correct?

Loki Astari Over a year ago

@super. I was unaware that lambdas with empty brackets were equivalent to function pointers. Do you have a reference for this (the above code works so it seems to be true (which actually surprised me)).

user2039981 Over a year ago

@MartinYork This: stackoverflow.com/a/28746827/2378102 . Furthermore you can put a + infront of a capturing lambda for some magic, check it out!

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