c++ - 共享指针 (shared_ptr) 的 Boost Fusion 容器导致段错误 (sigsegv) 或垃圾结果

Question

编辑:事实证明这是临时变量的问题。基本上，我无知地使用 C++，好像它像 Java 或 C# 一样工作，但事实并非如此。希望这将是一个很好的警示故事。

编辑:此问题似乎仅在结合使用 joint_view 和 shared_ptr 时才会发生。原始指针似乎在相同的场景中工作得很好，就像在一个简单的 fusion 容器中一次性构建其所有项目的共享指针一样，无需添加任何其他内容。详情如下:

我正在使用 mingw gcc 4.5.1 使用 boost fusion 容器并取出内容时遇到一个特殊问题。我有一个自定义类，它包装在 std::shared_ptr 中，然后将其传递给 fusion make_list() (或 make_vector() ，似乎并不重要)。如果我能一次性将所有对象放入容器中，那就一切顺利。当我向容器添加另一个共享指针时，问题似乎出现了，这会产生一个joint_view。我使用 fusion::for_each() 进行迭代，并传入一个函数对象来打印出该值。如果我迭代共享指针的普通 fusion 容器而不是 joint_view 或其中没有共享指针的 joint_view ，它可以正常工作，但否则会出现段错误或垃圾值。

下面是我为尝试隔离问题而编写的测试程序。关于问题可能是什么的任何想法吗？完全有可能我只是错过了一些我应该/不应该做的事情:(

#include <iostream>
#include <memory>

//BOOST SMART POINTERS
//I only use boost's shared_ptr for ONE test, results are the same.
#include <boost/shared_ptr.hpp>
#include <boost/make_shared.hpp>

//BOOST FUSION
#include <boost/fusion/algorithm/iteration/for_each.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <boost/fusion/container/generation/make_list.hpp>
#include <boost/fusion/include/make_list.hpp>
#include <boost/fusion/container/generation/make_vector.hpp>
#include <boost/fusion/include/make_vector.hpp>
#include <boost/fusion/algorithm/transformation/push_back.hpp>
#include <boost/fusion/include/push_back.hpp>

using namespace std;
using namespace boost::fusion;

struct TestStructInt {
    int test_int;

    TestStructInt(int init_num) : test_int(init_num) {};
};

struct TestStructString {
    string test_string;

    TestStructString(string init_str) : test_string(init_str) {};
};

struct do_junk{
    void operator()(string t) const {
        cout << t << endl;
    }

    void operator()(string* t) const {
        cout << *t << endl;
    }

    void operator()(std::shared_ptr<int> t) const {
        cout << *t << endl;
    }

    void operator()(std::shared_ptr<string> t) const {
        cout << *t << endl;
    }

    void operator() (boost::shared_ptr<string> t) const {
        cout << *t << endl;
    }

    void operator() (TestStructInt t) const {
        cout << t.test_int << endl;
    }

    void operator() (std::shared_ptr<TestStructInt> t) const {
        cout << t->test_int << endl;
    }

    void operator() (TestStructString t) const {
        cout << t.test_string << endl;
    }

    void operator() (std::shared_ptr<TestStructString> t) const {
        cout << t->test_string << endl;
    }

    void operator() (std::shared_ptr<TestStructString*> t) const {
        cout << (*t)->test_string << endl;
    }
};

int main()
{
    string foo1 = "foo1";
    string foo2 = "foo2";
    string foo3 = "foo3";
    int bar1 = 1;
    int bar2 = 2;
    int bar3 = 3;
    string* foo1_ptr = &foo1;
    string* foo2_ptr = &foo2;
    string* foo3_ptr = &foo3;
    auto foo1_obj = make_shared<TestStructString>(TestStructString("foo1"));
    auto foo2_obj = make_shared<TestStructString>(TestStructString("foo2"));
    auto foo3_obj = make_shared<TestStructString>(TestStructString("foo3"));

    //works fine
    auto list_test1 = make_list(foo1, foo2);

    //works fine
    auto list_test2 = make_list(foo1_ptr, foo2_ptr);

    //seems to work, but is undefined behavior
    auto list_test3
        = make_list(
            std::make_shared<int>(bar1), std::make_shared<int>(bar2)
        )
    ;

    //seems to work, but is undefined behavior
    auto list_test4
        = make_list(
            std::make_shared<string>(foo1), std::make_shared<string>(foo2)
        )
    ;

    //seems to work, but is undefined behavior
    auto list_test5
        = make_list(
            std::make_shared<TestStructInt>(TestStructInt(1))
            , std::make_shared<TestStructInt>(TestStructInt(2))
        )
    ;

    //seems to work, but is undefined behavior
    auto list_test6
        = make_list(
            std::make_shared<TestStructString>(TestStructString("foo1"))
            , std::make_shared<TestStructString>(TestStructString("foo2"))
        )
    ;

    //seems to work, but is undefined behavior
    auto list_test7
        = make_list(TestStructString("foo1"), TestStructString("foo2"))
    ;

    //seems to work, but is undefined behavior
    auto joint_view_test1 = push_back(make_list(foo1, foo2), foo3);

    //seems to work, but is undefined behavior
    auto joint_view_test2 = push_back(make_list(foo1_ptr, foo2_ptr), foo3_ptr);

    //seems to work, but is undefined behavior
    auto joint_view_test3
        = push_back(
            make_list(
                TestStructString(foo1), TestStructString(foo2)
            )
            , TestStructString(foo3)
        )
    ;

    //integer values I pass in are coming out different
    auto joint_view_test4
        = push_back(
            make_list(
                std::make_shared<int>(bar1), std::make_shared<int>(bar2)
            )
            , make_shared<int>(bar3)
        )
    ;

    //pass in foo1, foo2, and foo3, but only get foo3's value back out for each
    auto joint_view_test5
        = push_back(
            make_list(
                std::make_shared<string>(foo1), std::make_shared<string>(foo2)
            )
            , make_shared<string>(foo3)
        )
    ;

    //causes seg fault when running do_junk()
    auto joint_view_test6
        = push_back(
            make_vector(
                std::make_shared<string>(foo1), std::make_shared<string>(foo2)
            )
            , std::make_shared<string>(foo3)
        )
    ;

    //causes seg fault when running do_junk()
    auto joint_view_test7
        = push_back(
            make_list(
                boost::make_shared<string>(foo1)
                , boost::make_shared<string>(foo2)
            )
            , boost::make_shared<string>(foo3)
        )
    ;

    //integer values I pass in are coming out different
    auto joint_view_test8
        = push_back(
            make_list(
                std::make_shared<TestStructInt>(TestStructInt(1))
                , std::make_shared<TestStructInt>(TestStructInt(2))
            )
            , std::make_shared<TestStructInt>(TestStructInt(3))
        )
    ;

    //causes seg fault when running do_junk()
    auto joint_view_test9
        = push_back(
            make_list(
                std::make_shared<TestStructString>(TestStructString("foo1"))
                , std::make_shared<TestStructString>(TestStructString("foo2"))
            )
            , std::make_shared<TestStructString>(TestStructString("foo3"))
        )
    ;

    //causes seg fault when running do_junk()
    auto joint_view_test10
        = push_back(
            make_list(
                std::make_shared<TestStructString*>(new TestStructString("foo1"))
                , std::make_shared<TestStructString*>(new TestStructString("foo2"))
            )
            , std::make_shared<TestStructString*>(new TestStructString("foo3"))
        )
    ;

    //seems to work, but is undefined behavior
    auto joint_view_test11
        = push_back(
            make_list(
                foo1_obj
                , foo2_obj
            )
            , foo3_obj
        )
    ;

    cout << "@@ list1" << endl;
    boost::fusion::for_each(list_test1, do_junk());
    cout << "@@ list2" << endl;
    boost::fusion::for_each(list_test2, do_junk());
    cout << "@@ list3" << endl;
    boost::fusion::for_each(list_test3, do_junk());
    cout << "@@ list4" << endl;
    boost::fusion::for_each(list_test4, do_junk());
    cout << "@@ list5" << endl;
    boost::fusion::for_each(list_test5, do_junk());
    cout << "@@ list6" << endl;
    boost::fusion::for_each(list_test6, do_junk());
    cout << "@@ list7" << endl;
    boost::fusion::for_each(list_test7, do_junk());
    cout << "@@ joint_view1" << endl;
    boost::fusion::for_each(joint_view_test1, do_junk());
    cout << "@@ joint_view2" << endl;
    boost::fusion::for_each(joint_view_test2, do_junk());
    cout << "@@ joint_view3" << endl;
    boost::fusion::for_each(joint_view_test3, do_junk());
    cout << "@@ joint_view4" << endl;
    boost::fusion::for_each(joint_view_test4, do_junk());
    cout << "@@ joint_view5" << endl;
    //boost::fusion::for_each(joint_view_test5, do_junk());
    cout << "@@ joint_view6" << endl;
    //boost::fusion::for_each(joint_view_test6, do_junk());
    cout << "@@ joint_view7" << endl;
    //boost::fusion::for_each(joint_view_test7, do_junk());
    cout << "@@ joint_view8" << endl;
    //boost::fusion::for_each(joint_view_test8, do_junk());
    cout << "@@ joint_view9" << endl;
    //boost::fusion::for_each(joint_view_test9, do_junk());
    cout << "@@ joint_view10" << endl;
    //boost::fusion::for_each(joint_view_test10, do_junk());
    cout << "@@ joint_view11" << endl;
    boost::fusion::for_each(joint_view_test11, do_junk());
    cout << "@@" << endl;

    return 0;
}

Answer 1

fusion::joint_view 保留其元素作为引用。您需要非常小心，它不引用临时变量(它们将在您的设置中的迭代时间超出范围)。您的示例中似乎就是这种情况。