a memory leak example in practice

memleak.cpp

/**
 * Here is an example of mem leak I met today. I didn't write the codes, I just help finding the memleak.
 * Of course the real code is much more complicated, I simplify it here.
 *
 * To prove there is memleak, just compile and run:
 * $ gcc a.cpp --std=c++11 -lstdc++ && ./a.out
 *
 * And you will see below:
 * 4 bytes leaked.
 *
 * The interesting thing is why only 4 bytes is leaked.
 **/
#include <vector>
#include <queue>
#include <functional>
#include <iostream>

int size = 0;

struct A {
    int *p = 0;

    ~A() { if (p) { delete p; size -= 4;} }

    A &operator =(A &other) {
        if (p) { delete p; size -= 4; }
        p = other.p;
        other.p = 0;
        return *this;
    }

    void foo() { p = new int(3); size += 4;}
};

void bar(A *pa) {
    A a;
    a.foo();
    *pa = a;
}

int main(int, char **) {
	{
		std::queue<std::function<void (void)>> jobs;

		const int N = 200;

		// mem leak version
		std::vector<A> v;
		for (int i = 0; i < N; i++) {
			A a;
			jobs.emplace(std::bind(bar, &a));
			v.push_back(a);
		}

		// // ok version
		// std::vector<A> v(N);
		// for (int i = 0; i < N; i++) {
		// 	jobs.emplace(std::bind(bar, &v[i]));
		// }

		while (!jobs.empty()){
			auto job = std::move(jobs.front());
			jobs.pop();
			job();
		}
	}

	std::cout << size << " bytes leaked." << std::endl;

	return 0;
}