When Lambda capture by reference goes wrong

Here is a code that demonstrates lambda capture by value vs reference:

	#include <iostream>
	#include <functional>
	#include <vector>
	void add_func(std::vector<std::function<int(int)>>& fvec)
	{
	int x = 6;
	fvec.emplace_back([=](int value)
	//fvec.emplace_back([&](int value)
	{
	return value % x;
	});
	}
	int main()
	{
	std::vector<std::function<int(int)>> fvec;
	add_func(fvec);
	auto f = fvec.back();
	std::cout << f(10) << std::endl;
	return 0;
	}

view raw lambda_capture.cpp hosted with ❤ by GitHub

Running this code would yield 4.

When line 10 is used instead of line 9, running this code yields 10 instead of 4.

Ideally, 4 should be returned as the remainder of 10 divided by 6. If line 10 is used, however, the lambda function created encloses a reference to the variable x which goes out of scope as soon as the function add_func() completes execution.

After add_func() is called, the lambda function stored inside the vector uses a reference to x which is no longer valid (x only exists while add_func() is being executed). Therefore, the lambda function uses a reference to a variable with undefined value. Calling the lambda function therefore will yield to undefined results.