C++ 值，指针，引用的讨论

源自 stackoverflow 论坛，很有意义

第一个问题，引用传递和按值传递的场合

There are four main cases where you should use pass-by-reference over pass-by-value:

1. If you are calling a function that needs to modify its arguments, use pass-by-reference or pass-by-pointer. Otherwise, you’ll get a copy of the argument.
2. If you're calling a function that needs to take a large object as a parameter, pass it by const reference to avoid making an unnecessary copy of that object and taking a large efficiency hit.
3. If you're writing a copy or move constructor which by definition must take a reference, use pass by reference.
4. If you're writing a function that wants to operate on a polymorphic class, use pass by reference or pass by pointer to avoid slicing.

来源：Where should I prefer pass-by-reference or pass-by-value?

第二个问题，地址运算符（&）和引用运算符（&）的区别

代码解释，

int v = 5;

cout << v << endl; // prints 5
cout << &v << endl; // prints address of v

int* p;
p = &v; // stores address of v into p (p is a pointer to int)

int& r = v;

cout << r << endl; // prints 5

r = 6;

cout << r << endl; // prints 6
cout << v << endl; // prints 6 too because r is a reference to v

问题中更有意思的是对 *this 的解释，

Firstly, this is a pointer. The * dereferences the pointer, meaning return *this; returns the object, not a pointer to it.

Secondly, Test& is returning a reference to a Test instance. In your case, it is a reference to the object. To make it return a pointer, it should be Test*.

来源：Address-of operator (&) vs reference operator(&)

第三个问题，通过引用传递指针的原因

考虑这样的代码，

template<typename T>    
void moronic_delete(T*& p)
{
    delete p;
    p = nullptr;
}

回答：

如果你需要修改指针而不是指针指向的对象，你可能希望通过引用传递指针。（如果没有引用，你只会更改指针的本地副本）

这类似于为什么使用双指针；使用对指针的引用比使用指针稍微安全一些。

来源：Reason to Pass a Pointer by Reference in C++?

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