Related
I have a rather simple question about variable scope.
I am familiar with the Enhanced For-Loops but I do not get why I should declare a new variable to keep each element. One example might clarify my question:
int[] ar = {1, 2, 3};
int i = 0;
for(i : ar) { // this causes an error if I do not declare a new variable: int i
// for(int i : ar) // this works fine
System.out.println(i);
}
So why I should declare this new variable? After all i is accessible inside the for loop. I did not want to use any previous value of i, just did not want to declare a new variable. (I guessed for other iterable items it might be faster using the same variable).
I guess that's how Enhanced For-Loops were built but does not this break the whole scope idea?
There is a question rising from the above behavior. Whether the compiler uses the same variable for the whole for loop and just updates its value or it creates a new variable for each iteration?
An interesting part is that if I keep both declaration of int i (before and inside the for loop) I even get a compiler error about
Duplicate local variable i
which makes (at least for me) things a bit more strange. So I cannot use the previous declared variable i inside the for loop but neither can I declare a new one inside it with the same name.
So why I should declare this new variable?
Because that's the way the syntax is defined.
After all i is accessible inside the for loop.
That's semantics. It's irrelevant to syntax.
I did not want to use any previous value of i, just did not want to declare a new variable. (I guessed for other iterable items it might be faster using the same variable).
Don 't guess about performance. Test and measure. But in this case there's nothing to measure, because any working code is faster than any non-working code.
Does this means that I have a local variable that gets different values or a different variable in each loop?
From a language point of view you have a different variable in each iteration. That’s why you can write:
for(final ItemType item: iterable) {
…
}
which makes a great difference as you can create inner class instances within the loop referring to the current element. With Java 8 you can use lambdas as well and even omit the final modifier but the semantic does not change: you don’t get the surprising results like in C#.
I guessed for other iterable items it might be faster using the same variable
That’s nonsense. As long as you don’t have a clue of how the produced code looks like you shouldn’t even guess.
But if you are interested in the details of Java byte code: within a stack frame local variables are addressed by a number rather than by a name. And the local variables of your program are mapped to these storage locations by reusing the storage of local variables that went out of scope. It makes no difference whether the variable exists during the entire loop or is “recreated” on every iteration. It will still occupy just one slot within the stack frame. Hence, trying to “reuse local variables” on a source code level makes no sense at all. It just makes your program less readable.
Just to have the reference here: The JLS Section 14.14.2, The enhanced for statement defines the enhanced for-loop to have the following structure (relevant for this question):
EnhancedForStatement:
for ( {VariableModifier} UnannType VariableDeclaratorId : Expression ) Statement
where UnannType can be summarized to be "a type" (primitive, reference...). So giving the type of the loop variable is simply obligatory according to the language specification - causing the (admittedly: somewhat confusing) observations described in the question.
The int i in the program is visible to the for loop and maybe other for loops beneath it (if present) under the same scope. But the i inside the for(int i : ar) is local to the for loop. Hence ending once the execution of loop is over. Thats the syntax defined for foreach loop that "you have to use a variable with scope limited to the loop".
So why I should declare this new variable? After all i is accessible inside the for loop. I did not want to use any previous value of i, just did not want to declare a new variable. (I guessed for other iterable items it might be faster using the same variable).
Why would there be any considerable performance benefit if you use the same variable tiny primitive variable over and over versus creating a one only when needed and which gets destroyed after loop ends.
I don't think anyone has answered the original question beyond just declaring that that is the syntax. We all know that that is the syntax. The question is, logically speaking, why?
After all, you can use a variable defined just before a loop as the loop variable, as long as the loop is a non-enhanced for loop!
I'm trying to write a very simple piece of code and can't figure out an elegant solution to do it:
int count = 0;
jdbcTemplate.query(readQuery, new RowCallbackHandler() {
#Override
public void processRow(ResultSet rs) throws SQLException {
realProcessRow(rs);
count++;
}
});
This obviously doesn't compile. The 2 solutions that I'm aware of both stink:
I don't want to make count a class field because it's really a local variable that I just need for logging purposes.
I don't want to make count an array because it is plain ugly.
This is just silly, there got to be a reasonable way to do it?
A third possibility is to use a final-mutable-int-object, for example:
final AtomicInteger count = new AtomicInteger(0);
....
count.incrementAndGet();
Apache Commons also have a MutableInteger I believe, but I have not used it.
You seem to already be aware of the solutions (they are different though); and you are probably aware of the reasons (it cannot capture local variables by reference because the variable might not exist by the time the closure is run, so it must capture by value (have multiple copies); it is bad to have the same variable refer to different copies in different scopes that each can be changed independently, so they cannot be changed).
If your closure does not need to share state back to the enclosing scope, then a field in the class is the right thing to do. I don't understand what your objection is. If the closure needs to be able to be called multiple times and it needs to increment each time, then it needs to maintain state in the object. A field (instance variable) properly expresses the storing of state in an object. The field can be initialized with the captured value from the outside scope.
If your closure needs to share state back to the enclosing scope (which is not a very common situation), then using a mutable structure (like an array) is the right thing to do, because it avoids the problem of the lifetime of the local variable.
I typically make count a class field but add a comment that it is only a field because it is used by an inner closure, Runnable etc...
For this Java code:
String var;
clazz.doSomething(var);
Why does the compiler report this error:
Variable 'var' might not have been initialized
I thought all variables or references were initialized to null. Why do you need to do:
String var = null;
??
Instance and class variables are initialized to null (or 0), but local variables are not.
See §4.12.5 of the JLS for a very detailed explanation which says basically the same thing:
Every variable in a program must have a value before its value is used:
Each class variable, instance variable, or array component is initialized with a default value when it is created:
[snipped out list of all default values]
Each method parameter is initialized to the corresponding argument value provided by the invoker of the method.
Each constructor parameter is initialized to the corresponding argument value provided by a class instance creation expression or explicit constructor invocation.
An exception-handler parameter is initialized to the thrown object representing the exception.
A local variable must be explicitly given a value before it is used, by either initialization or assignment, in a way that can be verified by the compiler using the rules for definite assignment.
It's because Java is being very helpful (as much as possible).
It will use this same logic to catch some very interesting edge-cases that you might have missed. For instance:
int x;
if(cond2)
x=2;
else if(cond3)
x=3;
System.out.println("X was:"+x);
This will fail because there was an else case that wasn't specified. The fact is, an else case here should absolutely be specified, even if it's just an error (The same is true of a default: condition in a switch statement).
What you should take away from this, interestingly enough, is don't ever initialize your local variables until you figure out that you actually have to do so. If you are in the habit of always saying "int x=0;" you will prevent this fantastic "bad logic" detector from functioning. This error has saved me time more than once.
Ditto on Bill K. I add:
The Java compiler can protect you from hurting yourself by failing to set a variable before using it within a function. Thus it explicitly does NOT set a default value, as Bill K describes.
But when it comes to class variables, it would be very difficult for the compiler to do this for you. A class variable could be set by any function in the class. It would be very difficult for the compiler to determine all possible orders in which functions might be called. At the very least it would have to analyze all the classes in the system that call any function in this class. It might well have to examine the contents of any data files or database and somehow predict what inputs users will make. At best the task would be extremely complex, at worst impossible. So for class variables, it makes sense to provide a reliable default. That default is, basically, to fill the field with bits of zero, so you get null for references, zero for integers, false for booleans, etc.
As Bill says, you should definitely NOT get in the habit of automatically initializing variables when you declare them. Only initialize variables at declaration time if this really make sense in the context of your program. Like, if 99% of the time you want x to be 42, but inside some IF condition you might discover that this is a special case and x should be 666, then fine, start out with "int x=42;" and inside the IF override this. But in the more normal case, where you figure out the value based on whatever conditions, don't initialize to an arbitrary number. Just fill it with the calculated value. Then if you make a logic error and fail to set a value under some combination of conditions, the compiler can tell you that you screwed up rather than the user.
PS I've seen a lot of lame programs that say things like:
HashMap myMap=new HashMap();
myMap=getBunchOfData();
Why create an object to initialize the variable when you know you are promptly going to throw this object away a millisecond later? That's just a waste of time.
Edit
To take a trivial example, suppose you wrote this:
int foo;
if (bar<0)
foo=1;
else if (bar>0)
foo=2;
processSomething(foo);
This will throw an error at compile time, because the compiler will notice that when bar==0, you never set foo, but then you try to use it.
But if you initialize foo to a dummy value, like
int foo=0;
if (bar<0)
foo=1;
else if (bar>0)
foo=2;
processSomething(foo);
Then the compiler will see that no matter what the value of bar, foo gets set to something, so it will not produce an error. If what you really want is for foo to be 0 when bar is 0, then this is fine. But if what really happened is that you meant one of the tests to be <= or >= or you meant to include a final else for when bar==0, then you've tricked the compiler into failing to detect your error. And by the way, that's way I think such a construct is poor coding style: Not only can the compiler not be sure what you intended, but neither can a future maintenance programmer.
I like Bill K's point about letting the compiler work for you- I had fallen into initializing every automatic variable because it 'seemed like the Java thing to do'. I'd failed to understand that class variables (ie persistent things that constructors worry about) and automatic variables (some counter, etc) are different, even though EVERYTHING is a class in Java.
So I went back and removed the initialization I'd be using, for example
List <Thing> somethings = new List<Thing>();
somethings.add(somethingElse); // <--- this is completely unnecessary
Nice. I'd been getting a compiler warning for
List<Thing> somethings = new List();
and I'd thought the problem was lack of initialization. WRONG. The problem was I hadn't understood the rules and I needed the <Thing> identified in the "new", not any actual items of type <Thing> created.
(Next I need to learn how to put literal less-than and greater-than signs into HTML!)
I don't know the logic behind it, but local variables are not initialized to null. I guess to make your life easy. They could have done it with class variables if it were possible. It doesn't mean you have to have it initialized in the beginning. This is fine :
MyClass cls;
if (condition) {
cls = something;
else
cls = something_else;
Sure, if you've really got two lines on top of each other as you show- declare it, fill it, no need for a default constructor. But, for example, if you want to declare something once and use it several or many times, the default constructor or null declaration is relevant. Or is the pointer to an object so lightweight that its better to allocate it over and over inside a loop, because the allocation of the pointer is so much less than the instantiation of the object? (Presumably there's a valid reason for a new object at each step of the loop).
Bill IV
Was there any reason why the designers of Java felt that local variables should not be given a default value? Seriously, if instance variables can be given a default value, then why can't we do the same for local variables?
And it also leads to problems as explained in this comment to a blog post:
Well this rule is most frustrating when trying to close a resource in a finally block. If I instantiate the resource inside a try, but try to close it within the finally, I get this error. If I move the instantiation outside the try, I get another error stating that a it must be within a try.
Very frustrating.
Local variables are declared mostly to do some calculation. So it's the programmer's decision to set the value of the variable and it should not take a default value.
If the programmer, by mistake, did not initialize a local variable and it takes a default value, then the output could be some unexpected value. So in case of local variables, the compiler will ask the programmer to initialize it with some value before they access the variable to avoid the usage of undefined values.
The "problem" you link to seems to be describing this situation:
SomeObject so;
try {
// Do some work here ...
so = new SomeObject();
so.DoUsefulThings();
} finally {
so.CleanUp(); // Compiler error here
}
The commenter's complaint is that the compiler balks at the line in the finally section, claiming that so might be uninitialized. The comment then mentions another way of writing the code, probably something like this:
// Do some work here ...
SomeObject so = new SomeObject();
try {
so.DoUsefulThings();
} finally {
so.CleanUp();
}
The commenter is unhappy with that solution because the compiler then says that the code "must be within a try." I guess that means some of the code may raise an exception that isn't handled anymore. I'm not sure. Neither version of my code handles any exceptions, so anything exception-related in the first version should work the same in the second.
Anyway, this second version of code is the correct way to write it. In the first version, the compiler's error message was correct. The so variable might be uninitialized. In particular, if the SomeObject constructor fails, so will not be initialized, and so it will be an error to attempt to call so.CleanUp. Always enter the try section after you have acquired the resource that the finally section finalizes.
The try-finally block after the so initialization is there only to protect the SomeObject instance, to make sure it gets cleaned up no matter what else happens. If there are other things that need to run, but they aren't related to whether the SomeObject instance was property allocated, then they should go in another try-finally block, probably one that wraps the one I've shown.
Requiring variables to be assigned manually before use does not lead to real problems. It only leads to minor hassles, but your code will be better for it. You'll have variables with more limited scope, and try-finally blocks that don't try to protect too much.
If local variables had default values, then so in the first example would have been null. That wouldn't really have solved anything. Instead of getting a compile-time error in the finally block, you'd have a NullPointerException lurking there that might hide whatever other exception could occur in the "Do some work here" section of the code. (Or do exceptions in finally sections automatically chain to the previous exception? I don't remember. Even so, you'd have an extra exception in the way of the real one.)
Moreover, in the example below, an exception may have been thrown inside the SomeObject construction, in which case the 'so' variable would be null and the call to CleanUp will throw a NullPointerException
SomeObject so;
try {
// Do some work here ...
so = new SomeObject();
so.DoUsefulThings();
} finally {
so.CleanUp(); // Compiler error here
}
What I tend to do is this:
SomeObject so = null;
try {
// Do some work here ...
so = new SomeObject();
so.DoUsefulThings();
} finally {
if (so != null) {
so.CleanUp(); // safe
}
}
The actual answer to your question is because method variables are instantiated by simply adding a number to the stack pointer. To zero them would be an extra step. For class variables they are put into initialized memory on the heap.
Why not take the extra step? Take a step back--Nobody mentioned that the "warning" in this case is a Very Good Thing.
You should never initialize your variable to zero or null on the first pass (when you are first coding it). Either assign it to the actual value or don't assign it at all because if you don't then Java can tell you when you really screw up. Take Electric Monk's answer as a great example. In the first case, it's actually amazingly useful that it's telling you that if the try() fails because SomeObject's constructor threw an exception, then you would end up with an NPE in the finally. If the constructor can't throw an exception, it shouldn't be in the try.
This warning is an awesome multi-path bad programmer checker that has saved me from doing stupid stuff since it checks every path and makes sure that if you used the variable in some path then you had to initialize it in every path that lead up to it. I now never explicitly initialize variables until I determine that it is the correct thing to do.
On top of that, isn't it better to explicitly say "int size=0" rather than "int size" and make the next programmer go figure out that you intend it to be zero?
On the flip side I can't come up with a single valid reason to have the compiler initialize all uninitialized variables to 0.
Notice that the final instance/member variables don't get initialized by default. Because those are final and can't be changed in the program afterwards. That's the reason that Java doesn't give any default value for them and force the programmer to initialize it.
On the other hand, non-final member variables can be changed later. Hence, the compiler doesn't let them remain uninitialised; precisely, because those can be changed later. Regarding local variables, the scope of local variables is much narrower; and compiler knows when it's getting used. Hence, forcing the programmer to initialize the variable, makes sense.
For me, the reason comes down to this this: The purpose of local variables is different than the purpose of instance variables. Local variables are there to be used as part of a calculation; instance variables are there to contain state. If you use a local variable without assigning it a value, that's almost certainly a logic error.
That said, I could totally get behind requiring that instance variables were always explicitly initialized; the error would occur on any constructor where the result allows an uninitialized instance variable (e.g., not initialized at declaration and not in the constructor). But that's not the decision Gosling, et. al., took in the early 90's, so here we are. (And I'm not saying they made the wrong call.)
I could not get behind defaulting local variables, though. Yes, we shouldn't rely on compilers to double-check our logic, and one doesn't, but it's still handy when the compiler catches one out. :-)
I think the primary purpose was to maintain similarity with C/C++. However the compiler detects and warns you about using uninitialized variables which will reduce the problem to a minimal point. From a performance perspective, it's a little faster to let you declare uninitialized variables since the compiler will not have to write an assignment statement, even if you overwrite the value of the variable in the next statement.
It is more efficient not to initialize variables, and in the case of local variables it is safe to do so, because initialization can be tracked by the compiler.
In cases where you need a variable to be initialized you can always do it yourself, so it is not a problem.
The idea behind local variables is they only exist inside the limited scope for which they are needed. As such, there should be little reason for uncertainty as to the value, or at least, where that value is coming from. I could imagine many errors arising from having a default value for local variables.
For example, consider the following simple code... (N.B. let us assume for demonstration purposes that local variables are assigned a default value, as specified, if not explicitly initialized)
System.out.println("Enter grade");
int grade = new Scanner(System.in).nextInt(); // I won't bother with exception handling here, to cut down on lines.
char letterGrade; // Let us assume the default value for a char is '\0'
if (grade >= 90)
letterGrade = 'A';
else if (grade >= 80)
letterGrade = 'B';
else if (grade >= 70)
letterGrade = 'C';
else if (grade >= 60)
letterGrade = 'D';
else
letterGrade = 'F';
System.out.println("Your grade is " + letterGrade);
When all is said and done, assuming the compiler assigned a default value of '\0' to letterGrade, this code as written would work properly. However, what if we forgot the else statement?
A test run of our code might result in the following
Enter grade
43
Your grade is
This outcome, while to be expected, surely was not the coder's intent. Indeed, probably in a vast majority of cases (or at least, a significant number, thereof), the default value wouldn't be the desired value, so in the vast majority of cases the default value would result in error. It makes more sense to force the coder to assign an initial value to a local variable before using it, since the debugging grief caused by forgetting the = 1 in for(int i = 1; i < 10; i++) far outweighs the convenience in not having to include the = 0 in for(int i; i < 10; i++).
It is true that try-catch-finally blocks could get a little messy (but it isn't actually a catch-22 as the quote seems to suggest), when for example an object throws a checked exception in its constructor, yet for one reason or another, something must be done to this object at the end of the block in finally. A perfect example of this is when dealing with resources, which must be closed.
One way to handle this in the past might be like so...
Scanner s = null; // Declared and initialized to null outside the block. This gives us the needed scope, and an initial value.
try {
s = new Scanner(new FileInputStream(new File("filename.txt")));
int someInt = s.nextInt();
} catch (InputMismatchException e) {
System.out.println("Some error message");
} catch (IOException e) {
System.out.println("different error message");
} finally {
if (s != null) // In case exception during initialization prevents assignment of new non-null value to s.
s.close();
}
However, as of Java 7, this finally block is no longer necessary using try-with-resources, like so.
try (Scanner s = new Scanner(new FileInputStream(new File("filename.txt")))) {
...
...
} catch(IOException e) {
System.out.println("different error message");
}
That said, (as the name suggests) this only works with resources.
And while the former example is a bit yucky, this perhaps speaks more to the way try-catch-finally or these classes are implemented than it speaks about local variables and how they are implemented.
It is true that fields are initialized to a default value, but this is a bit different. When you say, for example, int[] arr = new int[10];, as soon as you've initialized this array, the object exists in memory at a given location. Let's assume for a moment that there is no default values, but instead the initial value is whatever series of 1s and 0s happens to be in that memory location at the moment. This could lead to non-deterministic behavior in a number of cases.
Suppose we have...
int[] arr = new int[10];
if(arr[0] == 0)
System.out.println("Same.");
else
System.out.println("Not same.");
It would be perfectly possible that Same. might be displayed in one run and Not same. might be displayed in another. The problem could become even more grievous once you start talking reference variables.
String[] s = new String[5];
According to definition, each element of s should point to a String (or is null). However, if the initial value is whatever series of 0s and 1s happens to occur at this memory location, not only is there no guarantee you'll get the same results each time, but there's also no guarantee that the object s[0] points to (assuming it points to anything meaningful) even is a String (perhaps it's a Rabbit, :p)! This lack of concern for type would fly in the face of pretty much everything that makes Java Java. So while having default values for local variables could be seen as optional at best, having default values for instance variables is closer to a necessity.
Flip this around and ask: why are fields initialised to default values? If the Java compiler required you to initialise fields yourself instead of using their default values, that would be more efficient because there would be no need to zero out memory before you used it. So it would be a sensible language design if all variables were treated like local variables in this regard.
The reason is not because it's more difficult to check this for fields than for local variables. The Java compiler already knows how to check whether a field is definitely initialised by a constructor, because it has to check this for final fields. So it would be little extra work for the compiler to apply the same logic to other fields to ensure they are definitely assigned in the constructor.
The reason is that, even for final fields where the compiler proves that the field is definitely assigned in the constructor, its value before assignment can still be visible from other code:
class A {
final int x;
A() {
this.x = calculate();
}
int calculate() {
System.out.println(this.x);
return 1;
}
}
In this code, the constructor definitely assigns to this.x, but even so, the field's default initial value of 0 is visible in the calculate method at the point where this.x is printed. If the field wasn't zeroed out before the constructor was invoked, then the calculate method would be able to observe the contents of uninitialised memory, which would be non-deterministic behaviour and have potential security concerns.
The alternative would be to forbid the method call calculate() at this point in the code where the field isn't yet definitely assigned. But that would be inconvenient; it is useful to be able to call methods from the constructor like this. The convenience of being able to do that is worth more than the tiny performance cost of zeroing out the memory for the fields before invoking the constructor.
Note that this reasoning does not apply to local variables, because a method's uninitialised local variables are not visible from other methods; because they are local.
Eclipse even gives you warnings of uninitialized variables, so it becomes quite obvious anyway. Personally I think it's a good thing that this is the default behaviour, otherwise your application may use unexpected values, and instead of the compiler throwing an error it won't do anything (but perhaps give a warning) and then you'll be scratching your head as to why certain things don't quite behave the way they should.
Instance variable will have default values but the local variables could not have default values. Since local variables basically are in methods/behavior, its main aim is to do some operations or calculations. Therefore, it is not a good idea to set default values for local variables. Otherwise, it is very hard and time-consuming to check the reasons of unexpected answers.
The local variables are stored on a stack, but instance variables are stored on the heap, so there are some chances that a previous value on the stack will be read instead of a default value as happens in the heap.
For that reason the JVM doesn't allow to use a local variable without initializing it.
Memory stack for methods is created at execution time. The method stack order is decided at execution time.
There might be a function that may not be called at all. So to instantiate local variables at the time of object instantiation would be a complete wastage of memory. Also, Object variables remain in memory for a complete object lifecycle of a class whereas, local variables and their values become eligible for garbage collection the moment they are popped from the memory stack.
So, To give memory to the variables of methods that might not even be called or even if called, will not remain inside memory for the lifecycle of an object, would be a completely illogical and memory-waste-worthy
The answer is instance variables can be initialized in the class constructor or any class method. But in case of local variables, once you defined whatever in the method, that remains forever in the class.
I could think of the following two reasons
As most of the answers said, by putting the constraint of initialising the local variable, it is ensured that the local variable gets assigned a value as the programmer wants and ensures the expected results are computed.
Instance variables can be hidden by declaring local variables (same name) - to ensure the expected behaviour, local variables are forced to be initialised to a value (I would totally avoid this, though).
I have some Java code that uses curly braces in two ways
// Curly braces attached to an 'if' statement:
if(node.getId() != null)
{
node.getId().apply(this);
}
// Curly braces by themselves:
{
List<PExp> copy = new ArrayList<PExp>(node.getArgs());
for(PExp e : copy)
{
e.apply(this);
}
}
outAMethodExp(node);
What do those stand-alone curly braces after the first if statement mean?
The only purpose of the extra braces is to provide scope-limit. The List<PExp> copy will only exist within those braces, and will have no scope outside of them.
If this is generated code, I assume the code-generator does this so it can insert some code (such as this) without having to worry about how many times it has inserted a List<PExp> copy and without having to worry about possibly renaming the variables if this snippet is inserted into the same method more than once.
I second what matt b wrote, and I'll add that another use I've seen of anonymous braces is to declare an implicit constructor in anonymous classes. For example:
List<String> names = new ArrayList<String>() {
// I want to initialize this ArrayList instace in-line,
// but I can't define a constructor for an anonymous class:
{
add("Adam");
add("Eve");
}
};
Some unit-testing frameworks have taken this syntax to another level, which does allow some slick things which look totally uncompilable to work. Since they look unfamiliar, I am not such a big fan myself, but it is worthwhile to at least recognize what is going on if you run across this use.
I agree with the scope limit answer, but would add one thing.
Sometimes you see a construct like that in the code of people who like to fold sections of their code and have editors that will fold braces automatically. They use it to fold up their code in logical sections that don't fall into a function, class, loop, etc. that would usually be folded up.
I'd actually guess that someone forgot an else statement.
There's rarely a good reason to even bother with creating additional block scopes. In this, and most cases, it's far more likely someone may have forgotten to type their control statement than it is that they were doing something clever.
They make an inner scope. Variable declared inside these braces is not visible outside of them. This also applies to C/C++.
Braces are also useful to reduce the scope in switch/case statements.
switch(foo) {
case BAR:
int i = ...
...
case BAZ:
int i = ... // error, "i" already defined in scope
}
But you can write
switch(foo) {
case BAR:{
int i = ...
...
}
case BAZ:{
int i = ... // OK
}
}
It is also used for initialization blocks.
They define a new scope which means that everything declared in this scope is not visible outside the curly braces.
As an interesting note: the braces actually enable a class of statements: declarations.
This is illegal: if(a) int f;
but this is legal: if(a) { int f; }
I think they just define an unnamed level of scope.
The bring a scope, copy will not be visible outside of it, so you can declare another variable with same name later. And it can be gathered by the garbage collector right after you exit that scope. In this case copy serves as a temporary variable, so it is a good example.