nitin12384's blog

Caution while using variable length array in lambda in C++17.
By nitin12384, history, 21 month(s) ago, In English

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.

Demo Code
Output in GNU G++17 7.3.0
Output in GNU G++20 11.2.0 (64 bit, winlibs)

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.

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21 month(s) ago, # |
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I was learning forward declaration of lambda. This is giving me runtime error. Can you help?

code
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    21 month(s) ago, # ^ |
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    You must include the "functional" library to declare lambda as a function<> type and define the function's body before calling it.

    #include <iostream>
#include <functional>

using namespace std;

int main()
{
    function<int(int)> get2;
    
    get2 = [&](int a)
    {
        return a;
    }; 

    cout << get2(3);
    return 0;
}
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21 month(s) ago, # |
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It's not standard-related (variable length array not a part of C++ and never was). That behavior changed from GCC 10.1.

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