c++ 向量指针的大小不正确 vector<Object>* ptr, ptr->size 是错误的

如何解决c++ 向量指针的大小不正确 vector<Object>* ptr, ptr->size 是错误的

这是我的代码

#include <iostream>
#include <vector>
#include <string>

using namespace std;

struct Student // EdgeContainer
{
    string name = "";
    vector<string> *addresses = nullptr;
    Student()
    {
        cout << __FILE__ << " " << __LINE__ << " Student()" << endl;
    }

    Student(const Student &stu)
    {
        cout << __FILE__ << " " << __LINE__ << " Student(const Student &stu)" << endl;
        name = stu.name;
        addresses = stu.addresses;
    }

    void print()
    {
        cout << name << " " << addresses << " size=" << addresses->size() << endl;
        if (addresses != nullptr)
        {
            for (int i = 0; i < addresses->size(); i++)
            {
                cout << i << " " << (*addresses)[i] << endl;
            }
        }
        cout << "----------------\n";
    }
};

struct Printer //ToyView
{
    vector<Student *> students;
    vector<vector<string>> stu_addresses;
    void add_student()
    {
        for (int i = 0; i < 10; i++)
        {
            vector<string> address;
            address.push_back("addr1");
            address.push_back("addr2");
            stu_addresses.push_back(address);
            Student *stu = new Student;
            students.push_back(stu);
            students[i]->name = "stu" + to_string(i);
            students[i]->addresses = &stu_addresses[i];
            cout << i << " stu->address=" << stu->addresses << " size=" << stu->addresses->size() << endl;
        }
    }

    void print()
    {
        cout << " students size=" << students.size() << endl;
        for (int i = 0; i < students.size(); i++)
        {
            cout << i << " stu->address=" << students[i]->addresses << endl;
            // students[i]->addresses = &stu_addresses[i];
            students[i]->print();
        }
    }
};

int main()
{
    Printer printer;
    printer.add_student();
    printer.print();
    return 0;
}

这是一些结果：

../vector_cp.cpp 13 Student()
0 stu->address=0x563888ac6f30 size=2
../vector_cp.cpp 13 Student()
1 stu->address=0x563888ac7418 size=2
../vector_cp.cpp 13 Student()
2 stu->address=0x563888ac74f0 size=2
../vector_cp.cpp 13 Student()
3 stu->address=0x563888ac7508 size=2
../vector_cp.cpp 13 Student()
4 stu->address=0x563888ac76c0 size=2
../vector_cp.cpp 13 Student()
5 stu->address=0x563888ac76d8 size=2
../vector_cp.cpp 13 Student()
6 stu->address=0x563888ac76f0 size=2
../vector_cp.cpp 13 Student()
7 stu->address=0x563888ac7708 size=2
../vector_cp.cpp 13 Student()
8 stu->address=0x563888ac7a10 size=2
../vector_cp.cpp 13 Student()
9 stu->address=0x563888ac7a28 size=2
 students size=10
0 stu->address=0x563888ac6f30
stu0 0x563888ac6f30 size=18446744073709549283

我希望students[0]->addresses.size() 应该是2，但它显示了垃圾数。 取消注释此行后 // student[i]->addresses = &stu_addresses[i]; 代码实际有效，即student[0]->addresses.size()。 我仍然无法弄清楚为什么会这样。有人可以帮忙吗？

解决方法

stu_addresses.push_back(address);
// ...
students[i]->addresses = &stu_addresses[i];

推回 stu_addresses 中的元素可能会重新分配您的向量，并且 stu_addresses 元素上先前有效的指针/迭代器将失效。

参见https://en.cppreference.com/w/cpp/container#Iterator_invalidation