c++如何从函数中保存Vector中的元素

Question

考虑以下代码。在list1中，我将保存在堆上的元素添加到矢量和打印。它像预期的那样工作。在list2中，我从一个函数中添加元素到向量上，该函数也在堆上分配元素。

我知道我必须在addNode的堆上分配Node，否则当函数返回时它将被释放。然而，通过最后的print语句，我可以看到堆上的节点仍然被分配，但它们没有显示在我的向量中。

#include <iostream>
#include <vector>
using namespace std;

/*
Simple node class for demo
*/
class Node
{
public:
string val;
Node(string value) { this->val = value; }
};
Node *addNode(vector<Node *> list)
{
// allocate space for node on the heap so it isn't destroyed after function returns
auto node = new Node("foo");
// add pointer to node onto vector
list.push_back(node);
return node;
}
/*
Simple function for printing vector contents
*/
template <typename T>
void printVector(T d)
{
cout << "Vector has size " << d.size() << " and elements: ";
for (auto p = d.begin(); p < d.end(); p++)
{
cout << (*p)->val << ",";
}
cout << "n";
}
int main()
{
// make a new vector
vector<Node *> list1;
// add elements allocated on the heap
list1.push_back(new Node("foo"));
list1.push_back(new Node("foo"));
list1.push_back(new Node("foo"));
printVector(list1); // prints: "Vector has size 3 and elements: foo,foo,foo,"
// make a new vector
vector<Node *> list2;
// add elements allocated on the heap from a function
addNode(list2);
addNode(list2);
// save one of the nodes to a variable for demonstration
auto node = addNode(list2);
printVector(list2); // prints: "Vector has size 0 and elements:"
cout << node->val << "n"; // prints: "foo"
return 0;
}

有人可以解释如何从函数添加元素到向量吗?

Answer 1

在addNode()中，您通过值传递vector，因此对调用方的vector进行复制，然后将Node*添加到副本中，原始vector不受影响。

您需要通过引用传递vector而不是:

Node* addNode(vector<Node*> &list)

与printVector()相同。您通过值传递vector，您只是不修改副本，但您仍然应该通过(const)引用传递vector，以避免完全复制:

template <typename T>
void printVector(const T &d)

---

附带说明，您正在泄漏您创建的Nodes。当你完成使用它们时，你需要delete它们:

int main()
{
vector<Node*> list1;
list1.push_back(new Node("foo"));
list1.push_back(new Node("foo"));
list1.push_back(new Node("foo"));
printVector(list1);
for(auto *p : list1)
delete p;
vector<Node*> list2;
addNode(list2);
addNode(list2);
auto node = addNode(list2);
printVector(list2);
cout << node->val << "n";
for(auto *p : list2)
delete p;
return 0;
}

最好使用std::unique_ptr为您管理:

#include <iostream>
#include <vector>
#include <memory>
using namespace std;
...
Node* addNode(vector<unique_ptr<Node>> &list)
{
list.push_back(make_unique<Node>("foo"));
return list.back().get();
}
template <typename T>
void printVector(const T &d)
{
cout << "Vector has size " << d.size() << " and elements: ";
for (const auto &p : d)
{
cout << p->val << ",";
}
cout << "n";
}
int main()
{
vector<unique_ptr<Node>> list1;
list1.push_back(make_unique<Node>("foo"));
list1.push_back(make_unique<Node>("foo"));
list1.push_back(make_unique<Node>("foo"));
printVector(list1);
vector<unique_ptr<Node>> list2;
addNode(list2);
addNode(list2);
auto node = addNode(list2);
printVector(list2);
cout << node->val << "n";
return 0;
}