vector 与 智能指针使用注意事项

        看以下代码，执行时会有什么问题？

#include <vector>
#include <stdio.h>
#include <stdlib.h>
#include <memory>

class SimpleTest 
{
public:
    SimpleTest(int test_id): mId(test_id) 
    {}
    ~SimpleTest() {}
    void print()
    {
        printf("mId = %d\n", mId);
    }
private:
    int mId;
};

void Test()
{
    std::vector< std::shared_ptr<SimpleTest> > vc;
    vc.push_back(std::shared_ptr<SimpleTest>(new SimpleTest(5)));
    vc.push_back(std::shared_ptr<SimpleTest>(new SimpleTest(6)));
    vc.push_back(std::shared_ptr<SimpleTest>(new SimpleTest(7)));
    printf("vc size = %lu\n", vc.size());

    for (std::vector< std::shared_ptr<SimpleTest> >::iterator iter = vc.begin(); iter != vc.end(); iter++)
    {

        (*iter)->print();  
    }

    printf("after resize\n");
    vc.resize(5);
    for (std::vector< std::shared_ptr<SimpleTest> >::iterator iter = vc.begin(); iter != vc.end(); iter++)
    {

        (*iter)->print();

    }
}

int main()
{
    Test();
    return 0;
}

执行结果出现段错误：

 那为什么会出现段错误呢？我们可以GDB跟踪一下：

 很明显在调用了 vc.resize(5); 后，vc容器元素个数为 5 个，但其中有两个是空的。因为如果n大于当前容器的大小，则在末尾插入尽可能多的元素，使容器的大小达到 n。而插入的新元素，会调用元素的默认构造函数，而智能指针默认是构造一个空的智能指针，所以后两个是 0x0。然后再调用的话就会出现问题。所以我们在改变容器大小后，指针的调用时刻需要判断是否为空指针：

#include <vector>
#include <stdio.h>
#include <stdlib.h>
#include <memory>

class SimpleTest 
{
public:
    SimpleTest(int test_id): mId(test_id) 
    {}
    ~SimpleTest() {}
    void print()
    {
        printf("mId = %d\n", mId);
    }
private:
    int mId;
};

void Test()
{
    std::vector< std::shared_ptr<SimpleTest> > vc;
    vc.push_back(std::shared_ptr<SimpleTest>(new SimpleTest(5)));
    vc.push_back(std::shared_ptr<SimpleTest>(new SimpleTest(6)));
    vc.push_back(std::shared_ptr<SimpleTest>(new SimpleTest(7)));
    printf("vc size = %lu\n", vc.size());

    for (std::vector< std::shared_ptr<SimpleTest> >::iterator iter = vc.begin(); iter != vc.end(); iter++)
    {
        if(*iter != nullptr)
        {
            (*iter)->print();  
        }
    }

    printf("after resize\n");
    vc.resize(5);
    for (std::vector< std::shared_ptr<SimpleTest> >::iterator iter = vc.begin(); iter != vc.end(); iter++)
    {
        if(*iter != nullptr)
        {
            (*iter)->print();
        }
    }
}

int main()
{
    Test();
    return 0;
}