代码如下。我有一个名为book_b的数据成员,并且在函数OB :: x()中,此unordered_map获取插入的对象。在第一次插入时,键为10,并且在key = 10处插入的新对象工作正常。但是,当key = 10再次出现时,我预计将创建一个新对象并将其插入到key = 10处(将先前的对象替换为key = 10处)。但是,一旦OB :: x()返回,当我们返回OB :: y()时,就好像从未插入新对象一样。
我认为这应该起作用,因为我通过将book_b对象传递给了指向修改其状态的函数的指针?我担心我的基本理解有问题。
class OB{
public:
void y(O lo);
void x(std::unordered_map<int, PL>* book, int a, long b);
private:
std::unordered_map<int, PL> book_b;
std::unordered_map<int, PL> book_a;
};
void OB::y(O lo){
//Code which obtains parameters to use in x() from lo
int a = o.getA();
long b = o.getB();
//This is the data member the below function will insert an object in to
std::unordered_map<int,PL>* book = &book_b;
//This is the function which should be changing the state of book.
//It works on the first call (when a new object is inserted) but on repeated calls
//(where the object may be replaced with a new object with the same key) it acts
//as if the new key-value pair wasnt replacing the existing key-value pair.
x(book, a, b);
}
//Works when book is empty and we insert a new PL object, however, when I go to "overwrite"
//an existing PL object with the same key (a) it doesn't hold state once the function returns
void OB::x(std::unordered_map<int,PL>* book, int a, long b){
PL temp;
temp.setQuantity(b);
book->insert(std::make_pair(a, temp));
}
std::unordered_map::insert
不不,如果一个使用相同的密钥已经存在插入一个新元素。
auto p = book->insert(std::make_pair(a, temp));
std::cout << std::boolalpha;
std::cout << "Did insert succeed? " << p.second << std::endl;
如果要更新现有元素(如果存在),请使用operator[]
:
(*book)[a] = temp;
注意:您不需要传递指针,除非您希望允许传递指针nullptr
。使用引用更简单:
void OB::x(std::unordered_map<int,PL>& book, int a, long b) { ... }
x(book_b, a, b);
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