问题
I know that a semicolon is required after a statement (I'm talking about Java, C++, and similar languages), but is not required after a curled bracket. Why so?
if (a > b)
printf("hello!"); // semicolon is mandatory
if (a > b) {
printf("hello!");
} // semicolon is not required
What is the reason? I mean, what is the theory behind this?
回答1:
Because curly brackets are used for grouping statements, but they are not statements themselves.
回答2:
Because the language is defined as:
statement:
labeled-statement
expression-statement
compound-statement
selection-statement
iteration-statement
jump-statement
declaration-statement
try-block
labeled-statement:
identifier : statement
case constant-expression : statement
default : statement
expression-statement:
expressionopt ;
compound-statement:
{ statement-seqopt }
statement-seq:
statement
statement-seq statement
selection-statement:
if ( condition ) statement
if ( condition ) statement else statement
switch ( condition ) statement
condition:
expression
type-specifier-seq declarator = assignment-expression
iteration-statement:
while ( condition ) statement
do statement while ( expression ) ;
for ( for-init-statement conditionopt ; expressionopt ) statement
for-init-statement:
expression-statement
simple-declaration
jump-statement:
break ;
continue ;
return expressionopt ;
goto identifier ;
declaration-statement:
block-declaration
All normal control statements are built recursively from each other. The real work is done by the expression-statement. If you notice the expression-statement is always terminate by the ;. The other statements to watch are the jump-statement.
But the main reason is that they are not needed after the {} block to allow the easy parsing of a statement.
回答3:
The philosphical reasoning aside, down under the hood it's essential that the compiler knows where to separate each command from the next. A bracket is in and of itself a separator, so a semi-colon is unnecessary.
回答4:
Most people think of statements as being a simple command, often with a keyword, and some parameters, such as "goto x", "a=y+2", etc. There has to be some indication of where one statement ends and another begins, much like English sentences need to end with a period. Traditionally the grammars of most langauges require semicolons after such statement as such indication.
A { ... } "curly brace pair" is a block, which is a special kind of a statement, but the semicolon isn't needed because the curly braces make the boundaries clear.
Many language also allow ";" by itself, to represent the empty statement. Why would you need one? For the same reason the natural number system requires "zero" instead of "one", and sets can be empty.
But it means you can write:
{ ... } ;
and most langauge compilers accept it without remark. But you should think of it as:
{ ... }
;
and generally there's no good reason to write that.
As a practical matter, languages that accept {} (e.g., "empty brackets") don't need the empty-statement ;, because these are semantically identical. But language designers seem stuck on tradition; have you noticed how every "modern" language seems to be a bad syntactic copy of C?
回答5:
This is a fair question. A block is a statement. It is natural to desire uniformity, and wonder why all statements are not terminated the same. There is no technical problem if we do require a ; after a block. But we are also lazy, since } can unambiguously mark the end of a statement, we don't want to have to type another marker.
A related observation: in C++, you must end class declaration with a ;
class A
{
...
}; // the semicolon is mandatory!
That annoys the heck of a lot of people. The semicolon is required because the language allows some other stuff after } so the } is not a reliable end marker.
In Java, that's not the case. The } ends the class declaration and that is it. So ; is not needed.
回答6:
Putting a semicolon in is the same effect as
if (a > b) {
printf("hello!");
}printf("Goodbye");
and leaving the printf("Goodbye") part out.
回答7:
If you are using the object initialization syntax in C# 3+ a semicolon comes after the bracket
var foo = new Foo
{
Bar = "Fizzbuzz"
};
回答8:
In this case curly brackets are defining a block of statements. Like any other block. while you are about to declare and initialize the array you must provide the ; because in this case you are writing a statement.
回答9:
Note : this answer is specific to the C++ language, not Java.
I would say that the semicolon is not required by the language grammar (2003). This is how the language defines the grammar.
The code which you've written is called Compound statement or block and the language specification (2003) defines the grammar of compound statement in the section §6.3/1 as,
So that several statements can be used where one is expected, the compound statement (also, and equivalently, called “block”) is provided.
compound-statement: { statement-seqopt } statement-seq: statement statement-seq statement
Do you see any semi-colon in the grammar shown above? No. That's why the semi-colon is not required after the curly bracket in your code.
回答10:
The only place you will need a semi-colon after a close curly bracket is after an array initialization as you could continue the line e.g.
int[] i= { 1,2,3}, j= {1};
The semi-colon is required because the '}' here doesn't tell the compiler where the end of the line is.
Similarly
Runnable r = new Runnable() {
public void run() {
}
}, r2 = new Runnable() {
public void run() {
}
}; // required as you could define another Runnable or an array of Runnable.
回答11:
Because curly is not the statement it is used to group the statements. It does not need a terminator.
回答12:
When you use curly braces to enclose a block of code, you don't need semicolon:
namespace Some
{
void fun(int i)
{
while(i != 0)
{
cout<<"i = "<<i<<endl;
i --;
} // semicolon not needed
} // semicolon not needed
} // semicolon not needed
In other cases, you need semicolon (like initialization, declaration etc.):
struct A {
// ... declare members
}; // declaration; put semicolon
int a[] = {0, 1, 2}; // initialization; put semicolon
来源：https://stackoverflow.com/questions/5561869/why-semicolon-is-not-required-after-a-curled-bracket