c++ - 为什么我的赋值运算符不能用于自赋值？

Question

当我做类似 a = b 的事情时它工作正常，但如果我做 a = a，我得到 vector 中所有元素的 -1.255 +-67。这是我的复制构造函数和赋值运算符:

VecXd(const VecXd &source){
    dimension = source.dimension;
    vector = new T[dimension];
    for(int i=0; i < dimension; i++)
        vector[i] = source.vector[i];
}

VecXd operator=(const VecXd &source){
    dimension = source.dimension;
vector = new T[dimension];
for(int i=0; i < dimension; i++)
    vector[i] = source.vector[i];
return *this;
}

Answer 1

这是因为一旦您分配了一个新 vector ，您就失去了 source.vector 的先前值。这是一个自赋值，所以 source 和 *this 指的是同一个对象，因此 vector 和 source.vector 是同一个对象。

您可以修复此问题以及内存泄漏，如下所示:

VecXd operator=(const VecXd &source){
    dimension = source.dimension;
    T *temp = new T[dimension]; // Don't loose source.vector yet
    for(int i=0; i < dimension; i++)
        temp[i] = source.vector[i];
    delete [] vector; // Delete old vector
    vector = temp;
    return *this;
}

更好的是，您可以防止自赋值以防止这种讽刺:

VecXd operator=(const VecXd &source){
    if(this == &source)
        return *this; // This is a self-assignment, so there's nothing to do
    delete [] vector; // Delete old vector
    dimension = source.dimension;
    vector = new T[dimension]; // Now we are sure that vector and source.vector differ
    for(int i=0; i < dimension; i++)
        vector[i] = source.vector[i];
    return *this;
}