Say you have a variable length array declared in a function, and you try to initialize it using memset in a lambda function, like this.

int n; cin>>n;
int cnt[n];
auto reset = [&](){
    memset(cnt, 0, sizeof(cnt));
};

It will not work correctly because sizeof(cnt) will return sizeof(int*) instead of n*sizeof(int).
That is because variable length array seems to decay to pointer when using labmda functions.

But surprisingly, this happens only in C++17, not in C++20.
In C++ 20, sizeof(cnt) will return n*sizeof(int).

Here is code demonstating this. You can try to run it in custom invocation.

#include <bits/stdc++.h>
using namespace std;

int main()
{
    int arr1[100];
    cout << "sizeof arr1(outside lambda) = " << sizeof(arr1) << endl;
    auto lambda1 = [&](){
        cout << "sizeof arr1(in lambda) = " << sizeof(arr1) << endl;
    };
    lambda1();
    int n = 100;
    int arr2[n];
    cout << "sizeof arr2(outside lambda) = " << sizeof(arr2) << endl;
    auto lambda2 = [&](){
        cout << "sizeof arr2(in lambda) = " << sizeof(arr2) << endl;
    };
    lambda2();
}

Just be cautious while using arrays in lambda functions. I got wrong answer in a problem due to this, and it took me a lot of time to figure out that this was causing the issue.

Anyone is welcome to post comment of any related facts/insight, or maybe some explanations about why this happens, and why this doesn't happens in C++20.