C++程序员如何向一个java工程师解释何为reference引用？

谁借莪１个温暖的怀抱￠ 2022-08-20 15:11 40阅读 0赞

继昨天某Android程序员问我extern “c”的作用后，她今天又问我什么是引用？

我回答他，引用就是别名，我引用你，我改变了，你也跟着改变。

那就再写个博客吧！

可能没学过C++的人更多的疑问就是，有了指针为何又要有引用呢？二者的区别何在？

A pointer can be re-assigned any number of times while a reference can not be re-seated after binding.  
指针可以被重新赋值，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.  
指针可以指向空，引用不能为空

You can’t take the address of a reference like you can with pointers.  
你不能获得一个引用的地址

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.  
可以指针指向指针，不能引用引用引用。(绕口令，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.  
函数的参数最好使用引用，如果返回值有意义，最好也返回引用。但是不要返回局部变量的引用

Use pointers to implement algorithms and data structures  
在算法和数据结构中最好使用指针