C＋＋庫研究筆記——賦值操作符operator=的正確過載方式（三個準則）
阿新 • • 發佈：2019-01-02
最終設計：
MyClass& MyClass::operator=(const MyClass &rhs) {
// Check for self-assignment!
if (this == &rhs) // Same object?
return *this; // Yes, so skip assignment, and just return *this.
... // Deallocate, allocate new space, copy values...
return *this;
}設計要求：
a, b, c, d, e;
a = b = c = d = e = 42;
This is interpreted by the compiler as:
a = (b = (c = (d = (e = 42))));
MyClass a, b, c;
...
(a = b) = c; // What??
// Take a const-reference to the right-hand side of the assignment.
// Return a non-const reference to the left-hand side.
//對應上條
MyClass& MyClass::operator=(const MyClass &rhs) {
... // Do the assignment operation!
return *this; // Return a reference to myself.
}
MyClass& MyClass::operator=(const MyClass &rhs) {
// 1. Deallocate any memory that MyClass is using internally
// 2. Allocate some memory to hold the contents of rhs
// 3. Copy the values from rhs into this instance
// 4. Return *this
}MyClass mc;
...
mc = mc; // BLAMMO.
CHECK FOR SELF-ASSIGNMENT.
所以正確且安全的設計如下：So, the correct and safe version of the MyClass assignment operator would be this:
MyClass& MyClass::operator=(const MyClass &rhs) {
// Check for self-assignment!
if (this == &rhs) // Same object?
return *this; // Yes, so skip assignment, and just return *this.
... // Deallocate, allocate new space, copy values...
return *this;
} 或者：
MyClass& MyClass::operator=(const MyClass &rhs) {
// Only do assignment if RHS is a different object from this.
if (this != &rhs) {
... // Deallocate, allocate new space, copy values...
}
return *this;
}1.右端值要為const 型別（不想改變他的值）Take a const-reference for the argument (the right-hand side of the assignment).
2.要返回一個引用，以便於實現（a=b=c…… (Do this by returning *this.)
3.檢查是否為自我賦值Check for self-assignment, by comparing the pointers (this to &rhs).
－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－
a<<(b<<c) 與上文一樣的
當然如果我們只允許a<<b非連鎖的操作（指a<<b<<c不允許），我們可以拋棄掉規則2
void MyClass::operator=(const MyClass &rhs) {
// Only do assignment if RHS is a different object from this.
if (this != &rhs) {
... // Deallocate, allocate new space, copy values...
}
return *this;
}