CODEWITHSUNDEEP
c++gcclambda
ejkoy
ejkoy

Reputation: 207

GCC capturing of `this` in lambda

I have a code like:

#include <cstdio>
#include <functional> 

class test {
public:
    void Do() {
        this->test_fun();
    }

    void Init() {
        this->number = 1008;
        this->test_fun = [this] {
            this->test_fun = nullptr;
            printf("number %d\n", this->number); //gcc crash here, this == nullptr
        };

    }

public:
    std::function<void()> test_fun;
    int number = 0;
};


int main() {
    test t;
    t.Init();
    t.Do();

    return 0;
}

It can run on MSVC2013, but crash on gcc on the line: printf("number %d\n", this->number);

What am I doing wrong?

GCC Version: gcc version 4.8.2 20140120 (Red Hat 4.8.2-15) (GCC)

Upvotes: 3

Views: 136

Answers (1)

user2249683
user2249683

Reputation:

Your code introduces undefined behavior, due to the the self destruction of the lambda with 

this->test_fun = nullptr;

This example should be equivalent:

#include <cstdio>
#include <cassert>
#include <memory>

class test {
public:
    struct Lambda {
        test* this_ptr;
        Lambda(test* this_ptr) : this_ptr(this_ptr) {}
        ~Lambda() {
            this_ptr = nullptr;
        }
        void operator () () {
            printf("this_ptr %p\n", (void*)this_ptr);
            // self destruction
            this_ptr->test_fun = nullptr;
            // undefined behavior starts here ...
            printf("this_ptr %p\n", (void*)this_ptr);
            printf("number %d\n", this_ptr->number);
        }
    };

    void Do() {
        assert(this->test_fun.get());
        (*this->test_fun)();
    }

    void Init() {
        this->number = 1008;
        this->test_fun = std::make_unique<Lambda>(this);
    }

public:
    std::unique_ptr<Lambda> test_fun;
    int number = 0;
};


int main() {
    test t;
    t.Init();
    t.Do();

    return 0;
}

Upvotes: 3

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