I'm learning C++ lambda function. Why does it have this output?

April 29, 2022

This is my code

#include<iostream>
int* p = nullptr;
auto fun()
{
    int a = 1;
    p = &a;
    std::cout << &a << std::endl;
    auto z = [&a]() {std::cout << &a << "   "; a++; std::cout << "lambda call " << a << std::endl; };
    return z;
}
int main()
{
    auto z = fun();
    std::cout << *p << "\n";
    z();
    z();
    
    fun()();
}

and the output is

0x7fffd10af15c
1
0x7fffd10af15c   lambda call 21880
0x7fffd10af15c   lambda call 21880
0x7fffd10af15c
0x7fffd10af15c   lambda call 21880

The version of my compiler is gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
Why do i have this output? Is it an undefined behavior?

>Solution :

Yes, the problem is that a is a local variable in the fun function and gets destroyed by the time fun finishes. That means, the returned lambda z is referencing an area on the stack where a used to be, but now when z is called this area is used for something else (this is why you see 21880).

In order to avoid this problem you need to prevent a being destroyed when leaving fun‘s scope. One way is to declare a as a global variable, just like p. The other one is to make it a static variable.