标准::向量不更改类内的数据

Std::vector not changing data inside class

本文关键字：数据 向量 标准      更新时间：2023-10-16

我正在用向量做一些事情，我正在尝试编写一个可以循环访问并向量内的对象执行操作的程序。我已经阅读了很多关于最小代码的信息，所以我编写了这个小应用程序来说明这个问题。

该程序的输出是：

Element 1 is equal to 2
Element 2 is equal to 4
Element 1 is equal to 2
Element 2 is equal to 4
Press any key to continue...

输出应为：

Element 1 is equal to 2
Element 2 is equal to 4
Element 1 is equal to 7
Element 2 is equal to 9
Press any key to continue...

为什么我没有达到预期的输出？

#include <vector>
#include <iostream>
// Some object
class A {
public:
    A(int data)
        : data(data)
    {}
    int data;
};
class B {
public:
    // A function that adds a new 'A' object to the vector in class b
    void push_back_to_vector(A &element);
    // A function that changes the data of the objects stored in vector_of_a
    void add_to_vector();
    // A vector to hold objects of A
    std::vector<A> vector_of_a;
};
void B::push_back_to_vector(A &element) {
    vector_of_a.push_back(element);
}
void B::add_to_vector() {
    for (size_t i = 0; i < vector_of_a.size(); i++) {
        // Add five to all the elements in the vector
        vector_of_a[i].data += 5;
    }
}
int main() {
    A element_1(2);
    A element_2(4);
    B b;
    b.push_back_to_vector(element_1);
    b.push_back_to_vector(element_2);
    std::cout << "Element 1 is equal to " << element_1.data << std::endl;
    std::cout << "Element 2 is equal to " << element_2.data << std::endl;
    // This should change element_1.data to 7 and element_2.data to 9
    b.add_to_vector();
    std::cout << "Element 1 is equal to " << element_1.data << std::endl;
    std::cout << "Element 2 is equal to " << element_2.data << std::endl;
    system("pause");
    return 0;
}

push_back复制新元素。如果你想做你似乎想做的事情，你需要存储指向A的指针，而不仅仅是A。

基本上，你的代码就是这样的：

A element_1(2);
A element_2(4);
std::cout << "Element 1 is equal to " << element_1.data << std::endl;
std::cout << "Element 2 is equal to " << element_2.data << std::endl;
std::cout << "Element 1 is equal to " << element_1.data << std::endl;
std::cout << "Element 2 is equal to " << element_2.data << std::endl;
system("pause");
return 0;

与B相关的一切都不起作用。试试这个：

class B {
public:
    // A function that adds a new 'A' object to the vector in class b
    void push_back_to_vector(A *element);
    // A function that changes the data of the objects stored in vector_of_a
    void add_to_vector();
    // A vector to hold objects of A
    std::vector<*A> vector_of_a;
};
void B::push_back_to_vector(A* element) {
    vector_of_a.push_back(element);
}
void B::add_to_vector() {
    for (size_t i = 0; i < vector_of_a.size(); i++) {
        // Add five to all the elements in the vector
        vector_of_a[i]->data += 5;
    }
}

并传递指针：

b.push_back_to_vector(&element_1);
b.push_back_to_vector(&element_2);

当然，请注意您在 B 中指向的堆栈上对象的生存期。

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