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15.1 End points and reachability

王朝other·作者佚名  2006-05-04
窄屏简体版  字體: |||超大  

Every statement has an end point. In intuitive terms, the end point of a

statement is the location that

immediately follows the statement. The execution rules for composite

statements (statements that contain

embedded statements) specify the action that is taken when control reaches

the end point of an embedded

statement. [Example: For example, when control reaches the end point of a

statement in a block, control is

transferred to the next statement in the block. end example]

If a statement can possibly be reached by execution, the statement is said

to be reachable. Conversely, if

there is no possibility that a statement will be executed, the statement is

said to be unreachable.

[Example: In the example

void F() {

Console.WriteLine("reachable");

goto Label;

Console.WriteLine("unreachable");

Label:

Console.WriteLine("reachable");

}

C# LANGUAGE SPECIFICATION

178

the second invocation of Console.WriteLine is unreachable because there is

no possibility that the

statement will be executed. end example]

A warning is reported if the compiler determines that a statement is

unreachable. It is specifically not an

error for a statement to be unreachable.

[Note: To determine whether a particular statement or end point is

reachable, the compiler performs flow

analysis according to the reachability rules defined for each statement.

The flow analysis takes into account

the values of constant expressions (§14.15) that control the behavior of

statements, but the possible values of

non-constant expressions are not considered. In other words, for purposes

of control flow analysis, a nonconstant

expression of a given type is considered to have any possible value of that

type.

In the example

void F() {

const int i = 1;

if (i == 2) Console.WriteLine("unreachable");

}

the boolean expression of the if statement is a constant expression because

both operands of the

== operator are constants. As the constant expression is evaluated at

compile-time, producing the value

false, the Console.WriteLine invocation is considered unreachable. However,

if i is changed to be a

local variable

void F() {

int i = 1;

if (i == 2) Console.WriteLine("reachable");

}

the Console.WriteLine invocation is considered reachable, even though, in

reality, it will never be

executed. end note]

The block of a function member is always considered reachable. By

successively evaluating the reachability

rules of each statement in a block, the reachability of any given statement

can be determined.

[Example: In the example

void F(int x) {

Console.WriteLine("start");

if (x < 0) Console.WriteLine("negative");

}

the reachability of the second Console.WriteLine is determined as follows:

? The first Console.WriteLine expression statement is reachable because the

block of the F method is

reachable (§15.2).

? The end point of the first Console.WriteLine expression statement is

reachable15.2 because that

statement is reachable (§15.6 and §15.2).

? The if statement is reachable because the end point of the first

Console.WriteLine expression

statement is reachable (§15.6 and §15.2).

? The second Console.WriteLine expression statement is reachable because

the boolean expression of

the if statement does not have the constant value false.

end example]

There are two situations in which it is a compile-time error for the end

point of a statement to be reachable:

? Because the switch statement does not permit a switch section to ?fall

through? to the next switch

section, it is a compile-time error for the end point of the statement list

of a switch section to be

reachable. If this error occurs, it is typically an indication that a break

statement is missing.

? It is a compile-time error for the end point of the block of a function

member that computes a value to be

reachable. If this error occurs, it typically is an indication that a

return statement is missing.

 
 
 
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