繼昨天某Android程序員問我extern “c”的作用後，她今天又問我什麼是引用？
我回答他，引用就是別名，我引用你，我改變了，你也跟著改變。
那就再寫個博客吧！
可能沒學過C++的人更多的疑問就是，有了指針為何又要有引用呢？二者的區別何在？
A pointer can be re-assigned any number of times while a reference can not be re-seated after binding.
指針可以被重新賦值，而引用不可以
int x = 5;
int y = 6;
int *p;
p = &x;
p = &y;
*p = 10;
assert(x == 5);
assert(y == 10);
int x = 5;
int y = 6;
int &r = x;
Pointers can point nowhere (NULL), whereas reference always refer to an object.
指針可以指向空，引用不能為空
You can’t take the address of a reference like you can with pointers.
你不能獲得一個引用的地址
A pointer has its own memory address and size on the stack (4 bytes on x86), whereas a reference shares the same memory address (with the original variable) but also takes up some space on the stack. Since a reference has the same address as the original variable itself, it is safe to think of a reference as another name for the same variable. Note: What a pointer points to can be on the stack or heap. Ditto a reference. My claim in this statement is not that a pointer must point to the stack. A pointer is just a variable that holds a memory address. This variable is on the stack. Since a reference has its own space on the stack, and since the address is the same as the variable it references. More on stack vs heap. This implies that there is a real address of a reference that the compiler will not tell you.
int x = 0;
int &r = x;
int *p = &x;
int *p2 = &r;
assert(p == p2);
You can have pointers to pointers to pointers offering extra levels of indirection. Whereas references only offer one level of indirection.
可以指針指向指針，不能引用引用引用。(繞口令，但我相信你能明白)
int x = 0;
int y = 0;
int *p = &x;
int *q = &y;
int **pp = &p;
pp = &q;//*pp = q
**pp = 4;
assert(y == 4);
assert(x == 0);
Const references can be bound to temporaries. Pointers cannot (not without some indirection):
const int &x = int(12); //legal C++
int *y = &int(12); //illegal to dereference a temporary
Use references in function parameters and return types to define useful and self-documenting interfaces.
函數的參數最好使用引用，如果返回值有意義，最好也返回引用。但是不要返回局部變量的引用
Use pointers to implement algorithms and data structures
在算法和數據結構中最好使用指針
