c++ - 从函数返回一个包含另一个对象的对象

Question

为什么第一次调用 cout 后 drawManifestoGlobal 中的值会发生变化？看起来 canvas.panel.drawManifestoGlobal 被破坏了 - 为什么？

我该如何解决这个问题？

#include <iostream>

class DrawManifestoGlobal {
public:
    int value = 2;
};

class Panel {
public:
    void setDrawManifestoGlobal(DrawManifestoGlobal & _drawManifestoGlobal);
    DrawManifestoGlobal * drawManifestoGlobal;
};

class Canvas {
public:
    Canvas() {};
    Canvas(DrawManifestoGlobal _drawManifestoGlobal);
    DrawManifestoGlobal drawManifestoGlobal;
    Panel panel;
};


class SerDe {
public:
    Canvas doSerDe();
};

Canvas SerDe::doSerDe() {

    DrawManifestoGlobal drawManifestoGlobal;
    drawManifestoGlobal.value = 99; 
    Canvas canvas(drawManifestoGlobal);

    return canvas;
}

Canvas::Canvas(DrawManifestoGlobal _drawManifestoGlobal) {
    drawManifestoGlobal = _drawManifestoGlobal;
    panel.setDrawManifestoGlobal(drawManifestoGlobal);
}

void Panel::setDrawManifestoGlobal(DrawManifestoGlobal &_drawManifestoGlobal) {
    drawManifestoGlobal = &_drawManifestoGlobal;
}

int main () {
    SerDe serde;
    Canvas canvas;
    canvas = serde.doSerDe();

    std::cout << canvas.panel.drawManifestoGlobal->value << std::endl; // prints 99
    std::cout << canvas.panel.drawManifestoGlobal->value << std::endl; // prints 0 (!!!)
}

关于实现要求:Canvas 拥有 Panel 和 DrawManifestoGlobal，Panel 本身有指向 Canvas 的 DrawManifestoGlobal 的指针，因此 Canvas 对其发生的任何更改对 Panel 都是可见的。

Answer 1

Why does the value in drawManifestoGlobal change after the first time cout is called on it? It looks like canvas.panel.drawManifestoGlobal gets destructed - why?

doSerDe() 函数内的

drawManifesttoGlobal 是一个局部变量，在作用域结束时过期，但您正在设置指向它的指针(Panel::drawManifestoGlobal) 并将指针传递到函数外部(通过 canvas 的返回)。所以你有未定义的行为来访问一个已经被破坏的对象的值。编译器完全有权为同一对象打印两个不同的值。

如果您确实需要一个指针(您不需要)，您将需要动态分配 drawManifestoGlobal(最好使用 unique_ptr)。

class Panel {
public:
  void setDrawManifestoGlobal(std::unique_ptr<DrawManifestoGlobal> _drawManifestoGlobal);
  std::unique_ptr<DrawManifestoGlobal> drawManifestoGlobal;
};

Canvas SerDe::doSerDe() {
  std::unique_ptr<DrawManifestoGlobal> drawManifestoGlobal(make_unique<DrawManifesttoGlobal>());
  drawManifestoGlobal.value = 99; 
  return {std::move(drawManifestoGlobal)};
}

void Panel::setDrawManifestoGlobal(std::unique_ptr<DrawManifestoGlobal> drawManifestoGlobal) {
  drawManifestoGlobal = std::move(_drawManifestoGlobal);
}

事实上，您的代码中的任何地方都不需要指针!