两者的区别是什么?
请给我一些例子,好吗?
这是一种如何向函数传递参数的方法。通过引用传递意味着被调用的函数的参数将与调用者传递的参数相同(不是值,而是身份--变量本身)。通过值传递意味着被调用的函数'参数将是调用者'传递参数的副本。值将是相同的,但身份--变量--是不同的。因此,在一种情况下,被调用函数对参数的改变会改变所传递的参数,而在另一种情况下,只是改变被调用函数中参数的值(这只是一个副本)。匆匆忙忙中。
ref
用在调用者和被调用函数上)。Jon Skeet对此也有一个很好的解释here。代码
由于我的语言是C++,我将在这里使用它
// passes a pointer (called reference in java) to an integer
void call_by_value(int *p) { // :1
p = NULL;
}
// passes an integer
void call_by_value(int p) { // :2
p = 42;
}
// passes an integer by reference
void call_by_reference(int & p) { // :3
p = 42;
}
// this is the java style of passing references. NULL is called "null" there.
void call_by_value_special(int *p) { // :4
*p = 10; // changes what p points to ("what p references" in java)
// only changes the value of the parameter, but *not* of
// the argument passed by the caller. thus it's pass-by-value:
p = NULL;
}
int main() {
int value = 10;
int * pointer = &value;
call_by_value(pointer); // :1
assert(pointer == &value); // pointer was copied
call_by_value(value); // :2
assert(value == 10); // value was copied
call_by_reference(value); // :3
assert(value == 42); // value was passed by reference
call_by_value_special(pointer); // :4
// pointer was copied but what pointer references was changed.
assert(value == 10 && pointer == &value);
}
还有一个Java的例子也无妨。
class Example {
int value = 0;
// similar to :4 case in the c++ example
static void accept_reference(Example e) { // :1
e.value++; // will change the referenced object
e = null; // will only change the parameter
}
// similar to the :2 case in the c++ example
static void accept_primitive(int v) { // :2
v++; // will only change the parameter
}
public static void main(String... args) {
int value = 0;
Example ref = new Example(); // reference
// note what we pass is the reference, not the object. we can't
// pass objects. The reference is copied (pass-by-value).
accept_reference(ref); // :1
assert ref != null && ref.value == 1;
// the primitive int variable is copied
accept_primitive(value); // :2
assert value == 0;
}
}
维基百科
http://en.wikipedia.org/wiki/Pass_by_reference#Call_by_value
http://en.wikipedia.org/wiki/Pass_by_reference#Call_by_reference
这个人几乎钉住了它。
下面是一个例子。
#include <iostream>
void by_val(int arg) { arg += 2; }
void by_ref(int&arg) { arg += 2; }
int main()
{
int x = 0;
by_val(x); std::cout << x << std::endl; // prints 0
by_ref(x); std::cout << x << std::endl; // prints 2
int y = 0;
by_ref(y); std::cout << y << std::endl; // prints 2
by_val(y); std::cout << y << std::endl; // prints 2
}