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Value and variable parameters

Most parameters are either value parameters (the default) or variable (var) parameters. Value parameters are passed by value, while variable parameters are passed by reference. To see what this means, consider the following functions.

function DoubleByValue(X: Integer): Integer; // X is a value parameter

begin

X := X * 2;

Result := X;

end;

function DoubleByRef(var X: Integer): Integer; // X is a variable parameter

begin

X := X * 2;

Result := X;

end;

These functions return the same result, but only the second one – DoubleByRef can change the value of a variable passed to it. Suppose we call the functions like this:

var

I, J, V, W: Integer;

begin

I := 4;

V := 4;

J := DoubleByValue(I); // J = 8, I = 4

W := DoubleByRef(V); // W = 8, V = 8

end;

After this code executes, the variable I, which was passed to DoubleByValue, has the same value we initially assigned to it. But the variable V, which was passed to DoubleByRef, has a different value.

A value parameter acts like a local variable that gets initialized to the value passed in the procedure or function call. If you pass a variable as a value parameter, the procedure or function creates a copy of it; changes made to the copy have no effect on the original variable and are lost when program execution returns to the caller.

A variable parameter, on the other hand, acts like a pointer rather than a copy. Changes made to the parameter within the body of a function or procedure persist after program execution returns to the caller and the parameter name itself has gone out of scope.

Even if the same variable is passed in two or more var parameters, no copies are made. This is illustrated in the following example.

procedure AddOne(var X, Y: Integer);

begin

X := X + 1;

Y := Y + 1;

end;

var I: Integer;

begin

I := 1;

AddOne(I, I);

end;

After this code executes, the value of I is 3.

If a routine’s declaration specifies a var parameter, you must pass an assignable expression, that is, a variable, typed constant (in the {$J+} state), dereferenced pointer, field, or indexed variable to the routine when you call it. To use our previous examples, DoubleByRef(7) produces an error, although DoubleByValue(7) is legal.

Indexes and pointer dereferences passed in var parameters. For example, DoubleByRef(MyArray[I]) are evaluated once, before execution of the routine.

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值参数和变量参数

大多数参数是值参数（缺省）或变量（var）参数。值参数传递值（by value），变量参数传递引用（by reference）。要明白这一点，可以考虑下面的函数，

function DoubleByValue(X: Integer): Integer; // X是一个值参数

begin

X := X * 2;

Result := X;

end;

function DoubleByRef(var X: Integer): Integer; // X是一个变量参数

begin

X := X * 2;

Result := X;

end;

上面的函数返回相同的结果，但只有第二个函数 – DoubleByRef可以改变传递给其的变量的值。假设如下调用这两个函数：

var

I, J, V, W: Integer;

begin

I := 4;

V := 4;

J := DoubleByValue(I); //执行后 J = 8, I = 4

W := DoubleByRef(V); //执行后 W = 8, V = 8

end;

这些代码执行后，传递给DoubleByValue的变量 I 与最初赋给它的值相同。而传递给DoubleByRef的变量 V 则具有不同的值。

值参数的表现就象一个具有初始化值并传递到过程或函数调用中的局部变量。如果把一个变量作为值参数传递，那么过程或函数会创建一个副本；对值参数的改变只改变副本而不影响原始变量，并且当程序执行返回给调用者时这些副本被自动销毁。

另一方面，变量参数的表现就象一个指针副本。在函数或过程主体中对变量参数的改变将一致持续，直到程序执行返回调用者并且变量参数名自身已经离开其作用域。

即使相同的变量作为两个或更多的变量（var）参数被传递，也不会创建任何副本。例如，

procedure AddOne(var X, Y: Integer);

begin

X := X + 1;

Y := Y + 1;

end;

var I: Integer;

begin

I := 1;

AddOne(I, I);

end;

这些代码执行后，变量 I 的值为3。

如果一个例程的声明指定了一个变量（var）参数，那么当调用该例程时必需向其传递可赋值的表达式，如变量、类型常量（在 {$J+} 状态下）、解除参照的指针、域，或者被索引的变量。对于前一个例子（DoubleByValue和DoubleByRef），DoubleByRef(7)将导致错误，而DoubleByValue(7)是合法的。

索引和指针解除参照可以作为变量（var）参数传递。例如，DoubleByRef(MyArray[I])在例程执行之前将被求值一次。

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Value and variable parameters

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译文

值参数和变量参数

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