c++類物件陣列初始化的幾種方法
類物件陣列初始化
如有一個如下類:
class EquipmentPiece {
private:
int IDNumber;
public:
EquipmentPiece(int IDNumber) : IDNumber(IDNumber) {};
};
以下列出幾種初始化的方法:
<一>、物件陣列
int ID1, ID2, ID3;
EquipmentPiece bestPieces[] = { EquipmentPiece(ID1), EquipmentPiece(ID2), EquipmentPiece(ID3) };
注意:
EquipmentPiece bestPieces[10]; //no appropriate default constructor available
EquipmentPiece *bestPieces = new EquipmentPiece[10]; //no appropriate default constructor available
當然,如果你將建構函式引數全都設了預設值,以上兩種寫法也成功,如將類中建構函式修改如下:
...
EquipmentPiece(int IDNumber = 0) : IDNumber(IDNumber) {};
...
<二>、指標陣列
typedef EquipmentPiece* PEP; //PEP是個指向EquipmentPiece的指標
PEP bestPieces[10]; //等同於 PEP *bestPieces = new PEP[10];
//然後初始化
for(int i = 0; i < 10; i++){
bestPieces[i] = new EquipmentPiece( IDNumber );
}
注意:
要記得將此陣列所指的所有物件刪除。如果忘了會產生資源洩露。還有就是該方法與物件陣列相比需要額外記憶體用於存放指標。(過度使用記憶體 這一問題可以避免,見第三種方法)
<三>、使用placement new
方法是:先為此陣列分配raw memory,然後使用"placement new"在這塊記憶體上構造EquipmentPiece objects;
//分配足夠的raw memory,給一個預備容納10個EquipmentPiece objects的陣列使用
void *rawMemory = operator new(10*sizeof(EquipmentPiece));
//讓bestPieces指向此記憶體,使這塊記憶體被視為一個EquipmentPiece陣列
EquipmentPiece *bestPieces = reinterpret_cast<EquipmentPiece*>(rawMemory);
//利用"placement new"構造這塊記憶體中的EquipmentPiece objects。
int IDNumber = 0;
for(int i = 0; i < 10; i++){
new (&bestPieces[i]) EquipmentPiece( IDNumber );
}
注意:該方法維護比較困難。在陣列內物件結束生命時,要以手動方式呼叫destructors,最後還得呼叫operator delete釋放raw memory。
//將bestPieces中物件以構造次序的反序析構掉
for(i = 0; i < 10; i++){
bestPieces[i].~EquipmentPiece();
}
//釋放raw memory
operator delete (rawMemory);
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