A major difference between them is that value-type variables store values, so specifying a value-type variable in a method call passes a copy of that variable's value to the method. Reference-type variables store references to objects, so specifying a reference-type variable as an argument passes the method a copy of the actual reference that refers to the object. Even though the reference itself is passed by value, the method can still use the reference it receives to interact with—and possibly modify—the original object. Similarly, when returning information from a method via a return statement, the method returns a copy of the value stored in a value-type variable or a copy of the reference stored in a reference-type variable. When a reference is returned, the calling method can use that reference to interact with the referenced object. So, in effect, objects are always passed by reference.

In c#, to pass a variable by reference so the called method can modify the variable's, C# provides keywords ref and out. Applying the ref keyword to a parameter declaration allows you to pass a variable to a method by reference—the called method will be able to modify the original variable in the caller. The ref keyword is used for variables that already have been initialized in the calling method. Normally, when a method call contains an uninitialized variable as an argument, the compiler generates an error. Preceding a parameter with keyword out creates an output parameter. This indicates to the compiler that the argument will be passed into the called method by reference and that the called method will assign a value to the original variable in the caller. If the method does not assign a value to the output parameter in every possible path of execution, the compiler generates an error. This also prevents the compiler from generating an error message for an uninitialized variable that is passed as an argument to a method. A method can return only one value to its caller via a return statement, but can return many values by specifying multiple output (ref and/or out) parameters.

请参阅c#讨论和示例链接文本

首先，CS理论中定义的“通过值传递与通过引用传递”的区别现在已经过时了，因为最初定义为“通过引用传递”的技术已经不再受欢迎，现在很少使用

较新的语言2倾向于使用不同(但相似)的技术来实现相同的效果(见下文)，这是混淆的主要来源。

第二个混淆的来源是，在“通过引用传递”中，“引用”的含义比一般术语“引用”更窄(因为这个短语比它更早)。

真实的定义是:

通过引用传递参数时，调用方和被调用方对参数使用相同的变量。如果被调用方修改了参数变量，则对调用方的变量可见。 当参数按值传递时，调用方和被调用方有两个具有相同值的自变量。如果被调用方修改了参数变量，则对调用方不可见。

在这个定义中需要注意的是:

"Variable" here means the caller's (local or global) variable itself -- i.e. if I pass a local variable by reference and assign to it, I'll change the caller's variable itself, not e.g. whatever it is pointing to if it's a pointer. This is now considered bad practice (as an implicit dependency). As such, virtually all newer languages are exclusively, or almost exclusively pass-by-value. Pass-by-reference is now chiefly used in the form of "output/inout arguments" in languages where a function cannot return more than one value. The meaning of "reference" in "pass by reference". The difference with the general "reference" term is that this "reference" is temporary and implicit. What the callee basically gets is a "variable" that is somehow "the same" as the original one. How specifically this effect is achieved is irrelevant (e.g. the language may also expose some implementation details -- addresses, pointers, dereferencing -- this is all irrelevant; if the net effect is this, it's pass-by-reference).

现在，在现代语言中，变量倾向于“引用类型”(另一个比“按引用传递”更晚发明的概念，并受到它的启发)，即实际的对象数据被单独存储在某个地方(通常是在堆上)，只有对它的“引用”被保存在变量中并作为参数传递

传递这样的引用属于值传递，因为从技术上讲，变量的值是引用本身，而不是被引用的对象。然而，对程序的最终影响可以是值传递或引用传递:

If a reference is just taken from a caller's variable and passed as an argument, this has the same effect as pass-by-reference: if the referred object is mutated in the callee, the caller will see the change. However, if a variable holding this reference is reassigned, it will stop pointing to that object, so any further operations on this variable will instead affect whatever it is pointing to now. To have the same effect as pass-by-value, a copy of the object is made at some point. Options include: The caller can just make a private copy before the call and give the callee a reference to that instead. In some languages, some object types are "immutable": any operation on them that seems to alter the value actually creates a completely new object without affecting the original one. So, passing an object of such a type as an argument always has the effect of pass-by-value: a copy for the callee will be made automatically if and when it needs a change, and the caller's object will never be affected. In functional languages, all objects are immutable.

正如你可能看到的，这对技术几乎与定义中的技术相同，只是在某种程度上间接地:只需将“变量”替换为“引用对象”。

它们没有统一的名称，这导致了一些扭曲的解释，比如“按值调用，其中值是引用”。1975年，Barbara Liskov提出了“按对象调用共享”(有时简称为“按对象调用共享”)这个术语，尽管它从未流行起来。此外，这两个短语都不能与原来的短语相提并论。难怪在没有更好的东西的情况下，旧的术语最终被重新使用，导致混乱

(对于新技术，我会使用术语“新的”或“间接的”值传递/引用传递。)

注:很长一段时间以来，这个答案都是这样说的:

Say I want to share a web page with you. If I tell you the URL, I'm passing by reference. You can use that URL to see the same web page I can see. If that page is changed, we both see the changes. If you delete the URL, all you're doing is destroying your reference to that page - you're not deleting the actual page itself. If I print out the page and give you the printout, I'm passing by value. Your page is a disconnected copy of the original. You won't see any subsequent changes, and any changes that you make (e.g. scribbling on your printout) will not show up on the original page. If you destroy the printout, you have actually destroyed your copy of the object - but the original web page remains intact.

这在很大程度上是正确的，除了狭义意义上的“引用”——它既是临时的又是隐式的(它不一定必须是临时的，但显式和/或持久性是额外的特性，不是引用传递语义的一部分，如上所述)。一个更接近的类比是给你一份文件的副本，而不是邀请你处理原件。

除非你用Fortran或Visual Basic编程，否则它不是默认行为，在现代使用的大多数语言中，真正的引用调用甚至是不可能的。

相当多的老年人也支持这一点

在一些现代语言中，所有类型都是引用类型。这种方法是由CLU语言在1975年首创的，后来被许多其他语言采用，包括Python和Ruby。还有更多的语言使用混合方法，其中一些类型是“值类型”，另一些是“引用类型”——其中包括c#、Java和JavaScript。

重复使用一个合适的旧术语本身并没有什么不好，但人们必须弄清楚每次使用的是什么意思。不这么做正是造成混乱的原因。

1. 按值传递/按值调用

   void printvalue(int x) 
   {
       x = x + 1 ;
       cout << x ;  // 6
   }

   int x = 5;
   printvalue(x);
   cout << x;    // 5

在按值调用中，当你将一个值传递给printvalue(x)时，即参数5，它被复制为void printvalue(int x)。现在，我们有两个不同的值5和复制的值5，这两个值存储在不同的内存位置。因此，如果你在void printvalue(int x)内做了任何更改，它不会反射回实参。

2. 引用传递/引用调用

   void printvalue(int &x) 
   {
      x = x + 1 ;
      cout << x ; // 6
   }

   int x = 5;
   printvalue(x);
   cout << x;   // 6

在引用调用中，只有一个区别。我们使用&，即地址操作符。通过做 Void printvalue(int &x)我们引用的是x的地址，这告诉我们它指向相同的位置。因此，在函数内部所做的任何更改都会在函数外部反映出来。

既然你来了，你也应该知道……

3.按指针传递/按地址调用

   void printvalue(int* x) 
   {
      *x = *x + 1 ;
      cout << *x ; // 6
   }

   int x = 5;
   printvalue(&x);
   cout << x;   // 6

在传递地址中，指针int* x保存传递给它的地址printvalue(&x)。因此，在函数内部所做的任何更改都会在函数外部反映出来。

问题是“vs”。

没有人指出一个重要的点。在传递值时，会占用额外的内存来存储传递的变量值。

在传递引用时，值不会占用额外的内存(在某些情况下内存是有效的)。

这里有一个例子:

#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
}

例子:

class Dog 
{ 
public:
    barkAt( const std::string& pOtherDog ); // const reference
    barkAt( std::string pOtherDog ); // value
};

Const &通常是最好的。你不会受到建造和破坏的惩罚。如果引用不是const，你的接口暗示它将改变传入的数据。