指向包含向量的實例類的指針 <int> 問題

Question

我有這樣的課：

class largeInt{
  vector<int> myVector;
  largeInt  operator*  (const largeInt &arg);

}

在我的主要工作，我無法避免使用指針時的副本：

void main(){

    //this works but there are multiple copies: I return a copy of the calculated
    //largeInt from the multiplication and then i create a new largeInt from that copy.
    largeInt testNum = 10;
    largeInt *pNum = new HugeInt( testNum*10);

    //i think this code avoid one copy but at the end hI points to a largeInt that has
    // myVector = 0 (seems to be a new instance = 0 ). With simple ints this works  great.
    largeInt i = 10;
    largeInt *hI;
    hI = &(i*10);

}

我想我在矢量設計中缺少/不管理某些東西。即使不實例化新的largeInt，我也可以實現指針的無復制分配嗎？ 謝謝高手！

Answer 1

hI = &(i*10); 接受一個臨時 largeInt的地址，該地址在';'之后立即被銷毀 -所以hI指向無效的內存。

當您將兩個largeInt相乘時，您會得到一個新實例-這就是乘法的作用。 也許您打算改成operator*= ？ 那應該修改一個現有實例而不是創建一個新實例。

考慮：

int L = 3, R = 5;

int j = L*R; // You *want* a new instance - L and R shouldn't change
L*=R; // You don't want a new instance - L is modified, R is unchanged

另外，您不應該使用new來創建largeInt的堆-就像這樣：

largeInt i = 10; 
largeInt hi = i*10; // Create a temporary, copy construct from the temporary

要么：

largeInt i = 10;
largeInt hi = i; // Copy construction
hi *= 10; // Modify hi directly