c++c++11lambdaclang

Capturing std::function objects in lambda


Below code fails with BAD_ACCESS when I call s_capture_void_int() in the last line and I do not understand why. I suppose that when I assign lambda expression to a global variable it supposed to copy itself together with captured values. So in my understanding dangling references should not appear. But it looks like I'm missing something.

std::function<void()> s_capture_void_int;
void capture_void_int (const std::function<void(int)>& param)
{
    s_capture_void_int = [param]() {
        param(1);
    };
}
void capture_local_lambda()
{
    auto local_lambda = [](int) {
    };
    s_capture_void_int = [local_lambda]() {
        local_lambda(1);
    };
}
BOOST_AUTO_TEST_CASE( test_lambda_captures )
{
    //Case 1: this works
    auto func2 = [](int){};
    {
        std::function<void(int)> func2_fn(func2);
        s_capture_void_int = [func2_fn]() { func2_fn(1); };
    }
    s_capture_void_int();

    //case 2: even this works.
    capture_local_lambda();
    s_capture_void_int();

    //case 3: but this fails.
    auto func3 = [](int){};
    {
        std::function<void(int)> func3_fn(func3);
        capture_void_int(func3_fn);
    }
    s_capture_void_int(); //<- it crashes here
}

I don't understand two things here:


Solution

  • I'd say it's a compiler bug. It works fine in GCC. Perhaps the param in capture_void_int is incorrectly captured by reference (since it's a reference) when it should instead be captured by value.