[英]C++ CRTP derived class object slicing
我有基類 Account 和派生類 Student 和 Teacher。 我在 CRTP 中實現了讀/寫,這樣我就不需要在每個類中編寫函數。
奇怪的是,read函數中有對象切片,函數中使用了*this,而write函數中沒有。
誰能解釋這里發生了什么? 以及如何修復它?
// Account.h
class Account {
public:
Account();
virtual void callMenu() = 0;
protected:
int ID;
// Some more data (Total of 148 bytes)
}
// AccountCRTP.h
template<class T>
class AccountCRTP : public Account {
public:
// To prevent object slicing
void callMenu() {
T account;
account.ID = this->ID;
cout << "Account size: " << sizeof(account) << " bytes\n"; pause(); // 268 bytes
if (account.readData())
account.menu();
}
bool readData() {
T account;
cout << "Account size : " << sizeof(account) << " bytes\n"; pause(); // 268 bytes
cout << "This size : " << sizeof(*this) << " bytes\n"; pause(); // 148 bytes??? Why?
while (reading data to account) {
if (this->ID == account.ID) {
*this = account; // Object slicing happens
return true;
}
}
return false;
}
bool writeData() {
T temp;
int position = 0l
while (reading data to temp) {
if (this->ID == account.ID)
break;
position++;
}
cout << "Account size : " << sizeof(temp) << " bytes\n"; pause(); // 268 bytes
cout << "This size : " << sizeof(*this) << " bytes\n"; pause(); // 148 bytes right?
datafile.seekp(position * sizeof(T));
// For unknown reason this also writes scores of the student
// Object slicing didn't happen here. Why?
datafile.write((char*)this, sizeof(T));
}
private:
// No additional data in this class (Still 148 bytes)
}
// Student.h
class Student :
public AccountCRTP<Student>
{
public:
void menu() {
// Student menu
}
private:
// Some more 120 bytes of data (Total of 268 bytes)
}
// main.cpp
int main() {
vector<unique_ptr<Account>> account;
account.push_back(unique_ptr<Account>(new Student));
account.push_back(unique_ptr<Account>(new Teacher));
account[0]->callMenu();
}
*this = account; // Object slicing happens
“對象切片發生”是的,您正在丟失account
任何特定屬性(成員變量)
你真正需要的是
*static_cast<T*>(this) = account;
保留派生T
。
static_cast<T*>(this)
技巧是使靜態多態起作用的類型安全的全部要點。
所有可能的類型檢查都將在編譯時應用。
讓我們在@Yakk 的評論中提到的更多語法糖:
template<typename T>
struct CRTPBase {
T const& self() const { return *static_cast<T const*>(this); }
T& self() { return *static_cast<T*>(this); }
};
然后線變成
self() = account;
對於這種情況,即使使用 c 預處理器宏似乎也是合適的:
#define DEFINE_CRTP_SELF(derived_param) \
derived_param const& self() const { return *static_cast<derived_param const*>(this); }
derived_param& self() { return *static_cast<derived_param*>(this); }
並像這樣使用它:
template<typename T>
class CRTP {
public:
DEFINE_CRTP_SELF(T);
};
注意:如果你在運行時有一些奇怪的東西(來自void*
指針左右),你就會迷失在地獄中。
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