c++ - 如何解决这个前向类声明问题?
问题描述
我有以下情况:
#include <iostream>
#include <vector>
#include <string>
using namespace std;
// Forward declarations
class Owner;
class Item;
class Owner {
public:
string name;
vector<Item> item;
Owner(string _name, initializer_list<Item> _item) : name(_name), item(_item) {
for (size_t i = 0; i < item.size(); i++)
item[i].owner = this;
}
// change the owner when copying...
Owner(const Owner& o) {
name = o.name;
item = o.item;
for (size_t i = 0; i < item.size(); i++)
item[i].owner = this; // is this OK?
}
Owner& operator = (const Owner& o) {
name = o.name;
item = o.item;
for (size_t i = 0; i < item.size(); i++)
item[i].owner = this;
return *this;
}
};
class Item {
public:
string name;
Owner* owner;
Item(string _name) : name(_name) {}
string getInfo() {
return name + " owned by " + owner->name;
}
};
int main(){
Item Cup("Cup"), Plate("Plate");
Owner John("John", { Cup, Plate });
cout << Cup.getInfo() << endl; // expecting "Cup owned by John"
return 0;
}
一个所有者可以拥有许多项目,但每个项目还必须包含有关其所有者的信息。当我运行这个时,我得到
错误 C2027 使用未定义类型“项目”
我认为这可以通过使用前向类声明来解决,但没有这样的运气。另外,我猜我的复制构造函数和赋值运算符还可以吗?
解决方案
前向声明类型只是解决方案的一部分。当依赖项是循环的时,您还需要移动至少一种类型的函数的定义 - 通常放在 .cpp 文件中(但如示例中所示,不是必需的)。
工作示例:
#include <iostream>
#include <vector>
#include <string>
using namespace std;
// Forward declarations
class Owner;
class Item {
public:
string name;
Owner* owner;
Item(string _name) : name(_name) {}
string getInfo();//<-- only declaration now
};
class Owner {
public:
string name;
vector<Item> item;
Owner(string _name, initializer_list<Item> _item) : name(_name), item(_item) {
for (size_t i = 0; i < item.size(); i++)
item[i].owner = this;
}
// change the owner when copying...
Owner(const Owner& o) {
name = o.name;
item = o.item;
for (size_t i = 0; i < item.size(); i++)
item[i].owner = this; // is this OK?
}
Owner& operator = (const Owner& o) {
name = o.name;
item = o.item;
for (size_t i = 0; i < item.size(); i++)
item[i].owner = this;
return *this;
}
};
//or put in .cpp file
string Item::getInfo() {
return name + " owned by " + owner->name;
}
int main(){
Item Cup("Cup"), Plate("Plate");
Owner John("John", { Cup, Plate });
cout << Cup.getInfo() << endl; // expecting "Cup owned by John"
return 0;
}
此外,您将项目复制到所有者中,因此当您打印原件时,它只打印“杯子拥有者”并且所有者“约翰”丢失。目前尚不清楚这应该以哪种方式修复 - 但可以例如。如果项目应该共享,则std::shared_ptr
通过存储来修复。
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