访问派生 class 对象列表的元素抛出 "read access violation" 异常，而列表由函数填充

Question

我有 3 种类型的派生 class 对象的列表，每种都包含字符串和两个整数。

#include "pch.h"
#include <iostream>
#include <list>
#include <string>
#include <algorithm>
#include <iterator>

class Shape
{
private:
    int x, y;
public:
    Shape() {}
    Shape(int &x_, int &y_) : x(x_), y(y_) {}
    virtual void Draw() {
        std::cout << this->x << ", " << this->y << "}\n";
    }
};

class Circle : public Shape
{
private:
    std::string type;
public:
    Circle() {}
    Circle(int &x_, int &y_) : Shape(x_, y_) { this->type = "Circle"; }
    void Draw() {
        std::cout << this->type << ":   {";
        Shape::Draw();
    }
};

class Triangle : public Shape
{
private:
    std::string type;
public:
    Triangle() {}
    Triangle(int &x_, int &y_) : Shape(x_, y_) { this->type = "Triangle"; }
    void Draw() {
        std::cout << this->type << ":   {";
        Shape::Draw();
    }
};

class Square : public Shape
{
private:
    std::string type;
public:
    Square() {}
    Square(int &x_, int &y_) : Shape(x_, y_) { this->type = "Square"; }
    void Draw() {
        std::cout << this->type << ":   {";
        Shape::Draw();
    }
};

void FillWithShapes(int n, std::list<Shape*> &ls) {
    int x, y, type;
    Circle cir;
    Triangle tri;
    Square sq;
    for (int i = 0; i < 10; i++) {
        type = rand() % 3;
        x = rand() % 100;
        y = rand() % 100;
        if (type == 0) {
            cir = Circle(x, y);
            ls.push_back(&cir);
        }
        else if (type == 1) {
            tri = Triangle(x, y);
            ls.push_back(&tri);
        }
        else if (type == 2) {
            sq = Square(x, y);
            ls.push_back(&sq);
        }
    }
}

int main()
{
    std::list<Shape*> shapes;
    FillWithShapes(10, shapes);
    std::for_each(shapes.begin(), shapes.end(), [](Shape *s) { s->Draw(); });
}

访问 lambda 中的列表元素时出现读取访问冲突异常：

Exception thrown: read access violation.
s->**** was 0xCCCCCCCC.

但是当我将函数 FillWithShapes 的代码直接放到 main() 时，它工作得很好：

Square:   {18, 95}
Triangle:   {82, 21}
Circle:   {2, 53}
Square:   {18, 95}
Triangle:   {82, 21}
Square:   {18, 95}
Circle:   {2, 53}
Circle:   {2, 53}
Triangle:   {82, 21}
Square:   {18, 95}

我不久前才开始学习 c++，所以我不知道在这种情况下是什么原因导致了这个异常，尽管我可能在这里遗漏了一些简单但重要的东西。

更新： 修复了在堆上创建指针的函数：

void FillWithShapes(int n, std::list<Shape*> &ls) {
    int x, y, type;
    Circle *cir = new Circle();
    Triangle *tri = new Triangle();
    Square *sq = new Square();
    for (int i = 0; i < 10; i++) {
        type = rand() % 3;
        x = rand() % 100;
        y = rand() % 100;
        if (type == 0) {
            *cir = Circle(x, y);
            ls.push_back(cir);
        }
        else if (type == 1) {
            *tri = Triangle(x, y);
            ls.push_back(tri);
        }
        else if (type == 2) {
            *sq = Square(x, y);
            ls.push_back(sq);
        }
    }
}

Answer 1

您正在用指向您的 FillWithShapes() 函数的局部变量并存储在堆栈中的指针填充列表。它们的生命周期在函数 returns 之后结束 - 因此您遇到访问冲突是合理的。

当您将代码提升到 main() 时，局部变量的生命周期现在是 main() 的生命周期，即贯穿您的程序并经过形状列表的最后一次访问 -所以没有违规。

您可能想阅读：What and where are the stack and heap?

Answer 2

在您的函数 FillWithShapes 中，您正在创建 Circle、Sqaure 等类型的对象并将这些指针推入向量。一旦该函数超出范围，这些指针就不再有效。

您可以在堆上创建对象，然后将对象推送到 vector 中，并在使用向量完成后承担解除分配的负担。

访问派生 class 对象列表的元素抛出 "read access violation" 异常，而列表由函数填充

Accessing elements of derived class object list throws "read access violation" exception, while list is filled by function

c++

pointers

local-variables

derived-class

access-violation