As part of a unit test framework, I'm writing a function genArray that will generate NSArrays populated by a passed in generator block. So [ObjCheck genArray: genInt] would generate an NSArray of random integers, [ObjCheck genArray: genChar] would generate an NSArray of random characters, etc. In particular, I'm getting compiler errors in my implementation of genArray and genString, a wrapper around [ObjCheck genArray: genChar].
I believe Objective C can manipulate blocks this dynamically, but I don't have the syntax right.
ObjCheck.m
+ (id) genArray: (id) gen {
NSArray* arr = [NSMutableArray array];
int len = [self genInt] % 100;
int i;
for (i = 0; i < len; i++) {
id value = gen();
arr = [arr arrayByAddingObject: value];
}
return arr;
}
+ (id) genString {
NSString* s = #"";
char (^g)() = ^() {
return [ObjCheck genChar];
};
NSArray* arr = [self genArray: g];
s = [arr componentsJoinedByString: #""];
return s;
}
When I try to compile, gcc complains that it can't do gen(), because gen is not a function. This makes sense, since gen is indeed not a function but an id which must be cast to a function.
But when I rewrite the signatures to use id^() instead of id, I also get compiler errors. Can Objective C handle arbitrarily typed blocks (genArray needs this), or is that too dynamic?
Given that blocks are objects, you can cast between block types and id whenever you want, though if you cast the block to the wrong block type and call it, you're going to get unexpected results (since there's no way to dynamically check at runtime what the "real" type of the block is*).
BTW, id^() isn't a type. You're thinking of id(^)(). This may be a source of compiler error for you. You should be able to update +genArray: to use
id value = ((id(^)())(gen))();
Naturally, that's pretty ugly.
*There actually is a way, llvm inserts an obj-c type-encoded string representing the type of the block into the block's internal structure, but this is an implementation detail and would rely on you casting the block to its internal implementation structure in order to extract.
Blocks are a C-level feature, not an ObjC one - you work with them analogously to function pointers. There's an article with a very concise overview of the syntax. (And most everything else.)
In your example, I'd make the gen parameter an id (^gen)(). (Or possibly make it return a void*, using id would imply to me that gen generates ObjC objects and not completely arbitrary types.)
No matter how you declare your variables and parameters, your code won't work. There's a problem that runs through all your compiler errors and it would be a problem even if you weren't doing convoluted things with blocks.
You are trying to add chars to an NSArray. You can't do that. You will have to wrap them them as some kind of Objective C object. Since your only requirement for this example to work is that the objects can be inputs to componentsJoinedByString, you can return single-character NSStrings from g. Then some variety of signature like id^() will work for genArray. I'm not sure how you parenthesize it. Something like this:
+ (id) genArray: (id^()) gen;
+ (id) genString {
...
NSString * (^g)() = ^() {
return [NSString stringWithFormat:#"%c", [ObjCheck genChar]];
};
...
}
NSString * is an id. char is not. You can pass NSString * ^() to id ^(), but you get a compiler error when you try to pass a char ^() to an id ^(). If you gave up some generality of genArray and declared it to accept char ^(), it would compile your call to genArray, but would have an error within genArray when you tried to call arrayByAddingObject and the argument isn't typed as an id.
Somebody who understands the intricacies of block syntax feel free to edit my post if I got some subtle syntax errors.
Btw, use an NSMutableArray as your local variable in genArray. Calling arrayByAddingObject over and over again will have O(n^2) time performance I imagine. You can still declare the return type as NSArray, which is a superclass of NSMutableArray, and the callers of genArray won't know the difference.
Related
As I am learning objective C, my understanding is new and incomplete. The concept of a block is very similar to a function. They even look almost identical:
FUNCTION named 'multiply'
#import <Foundation/Foundation.h>
int multiply (int x, int y)
{
return x * y;
}
int main(int argc, char *argv[]) {
#autoreleasepool {
int result = multiply(7, 4); // Result is 28.
NSLog(#"this is the result %u",result);
}
}
BLOCK named 'Multiply'
#import <Foundation/Foundation.h>
int (^Multiply)(int, int) = ^(int num1, int num2) {
return num1 * num2;
};
int main(int argc, char *argv[]) {
#autoreleasepool {
int result = Multiply(7, 4); // Result is 28.
NSLog(#"this is the result %u",result);
}
}
I found various statements on the web like:
"Blocks are implemented as Objective-C objects, except they can be put on the stack, so they don't necessarily have to be malloc'd (if you retain a reference to a block, it will be copied onto the heap, though). "
Ray Wenderlich says:
"Blocks are first-class functions"
I have no clue what all this means. My example shows that the same thing is accomplished as a block or a function. Can someone show an example where blocks can do something functions cannot? or vice versa?
Or is it something more subtle, like the way the variable 'result' is handled in memory?
or is one faster/safer?
Can either of them be used as a method in a class definition?
Thank you.
Blocks are Objective-C objects, and functions aren't. In practice, this means you can pass around a block from one piece of code to another like so:
NSArray *names = #[#"Bob", #"Alice"];
[names enumerateObjectsUsingBlock:^(id name, NSUInteger idx, BOOL *stop) {
NSLog(#"Hello, %#", name);
}];
In C, you can achieve similar effects by passing around pointers to functions. The main difference between doing this and using blocks, however, is that blocks can capture values. For instance, in the example above, if we wanted to use a variable greeting:
NSString *greeting = #"Hello";
NSArray *names = #[#"Bob", #"Alice"];
[names enumerateObjectsUsingBlock:^(id name, NSUInteger idx, BOOL *stop) {
NSLog(#"%#, %#", greeting, name);
}];
In this example, the compiler can see that the block depends on the local variable greeting and will "capture" the value of greeting and store it along with the block (in this case, that means retaining and storing a pointer to an NSString). Wherever the block ends up getting used (in this case, within the implementation of [NSArray -enumerateObjectsUsingBlock:]), it will have access to the greetings variable as it was at the time the block was declared. This lets you use any local variables in the scope of your block without having to worry about passing them into the block.
To do the same using function pointers in C, greeting would have to be passed in as a variable. However, this can't happen because the caller (in this case, NSArray) can't know (especially at compile time) exactly which arguments it has to pass to your function. Even if it did, you'd need to somehow pass the value of greeting to NSArray, along with every other local variable you wanted to use, which would get hairy really quickly:
void greet(NSString *greeting, NSString *name) {
NSLog(#"%#, %#", greeting, name);
}
// NSArray couldn't actually implement this
NSString *greeting = #"Hello";
NSArray *names = #[#"Bob", #"Alice"];
[names enumerateObjectsUsingFunction:greet withGreeting:greeting];
Blocks are closures -- they can capture local variables from the surrounding scope. This is the big difference between blocks (and anonymous functions in other modern languages) and functions in C.
Here's an example of a higher-order function, makeAdder, which creates and returns an "adder", a function which adds a certain base number to its argument. This base number is set by the argument to makeAdder. So makeAdder can return different "adders" with different behavior:
typedef int (^IntFunc)(int);
IntFunc makeAdder(int x) {
return ^(int y) { return x + y; }
}
IntFunc adder3 = makeAdder(3);
IntFund adder5 = makeAdder(5);
adder3(4); // returns 7
adder5(4); // returns 9
adder3(2); // returns 5
This would not be possible to do with function pointers in C, because each function pointer must point to an actual function in the code, of which there is a finite number fixed at compile time, and each function's behavior is fixed at compile time. So the ability to create a virtually unlimited number of potential "adders" depending on a value at runtime, like makeAdder does, is not possible. You would instead need to create a structure to hold the state.
A block which does not capture local variables from the surrounding scope, like in your example, is not much different from a plain function, aside from the type.
I recently discovering these classes like NSMapTable and NSPointerArray, which work like the traditional collections, but also let you store weak references or plain old C pointers. Unfortunately it looks like you can't use the for...in syntax to iterate over non-NSObject pointers. For example:
typedef struct Segment {
CGPoint bottom, top;
} Segment;
...
NSPointerArray *segments = [[NSPointerArray alloc]
initWithOptions:NSPointerFunctionsOpaqueMemory];
...
Segment *s = malloc(sizeof(Segment));
[segments addPointer: s];
...
for (Segment *s in segments) { // nope...
The compiler does not like that last line. The error:
Selector element type 'Segment *' (aka 'struct Segment *') is not a valid object
So, do I need to do this?
for (int i=0, len=segments.count; i<len; i++) {
Segment *seg = [segments pointerAtIndex:i];
...
That's not the end of the world, but I just want to make sure.
(This might be more of theoretical interest.)
NSPointerArray does conform to the NSFastEnumeration protocol, it is only the
for (id object in collection) language construct that cannot be used with arbitrary pointers which
are not Objective-C pointers.
But you can get a whole bunch of pointers from the array by calling the NSFastEnumeration
method countByEnumeratingWithState:objects:count: directly. This is a bit tricky because
that method need not fill the supplied buffer (as explained here: How for in loop works internally - Objective C - Foundation).
Here is a simple example how this would work:
__unsafe_unretained id objs[10];
NSUInteger count = [segments countByEnumeratingWithState:&state
objects:objs count:10];
// Now state.itemsPtr points to an array of pointers:
for (NSUInteger i = 0; i < count; i++) {
Segment *s = (__bridge Segment *)state.itemsPtr[i];
NSLog(#"%p", s);
}
So this does not help to make the code simpler and you probably want to stick with
your explicit loop.
But for large arrays it might improve the performance because the pointers are "fetched"
in batches from the array instead of each pointer separately.
the for (... in ...) syntax won't work in this case because Segment is a struct, not an Objective C object. Your second for loop should work.
Is there any way to do the following at compile-time?
int anInteger = 0;
__if_object(anInteger) {
// send object some messages
}
__if_primitive(anInteger) {
// do something else
}
An dummy situation where this could be used is to define the __add_macro below.
#define __add_macro(var, val) __something_goes_here__
int i = 1;
MyInteger* num = [[MyNumber alloc] initWithValue:1]
__add_macro(i, 4);
__add_macro(num, 4);
// both should now hold 5
Clarification/Simplification
I guess there is no way to do this with one macro. But I still need it to warn if the macro is being used on the wrong datatype. Those two types are: object and non-object).
To check if it is an object, this works:
#define __warn_if_not_object(var) if(0){[(var) class];}
What I need:
#define _warn_if_object(var) if(0){__something_here__}
Again, I need this to happen at compile-time. And it can either throw an error or warning.
Thanks
When you declare an int variable you can really only put an int value in it.
While this is Objective-C, and hence C, so you can bypass just about every type protection mechanism that exists, this is not to be advised. Indeed there is no guarantee whatsoever that a, say, NSNumber reference will even fit into an int variable - and more than enough chance that if you try, and bypass any warnings, some bits will just get tossed making the reference invalid.
So, no, while you can tell what class an object reference refers to, you cannot in general tell whether a variable has an integer value or an object reference in it - you shouldn't even try to put these two very different things into the same variable.
Answer 2
Patrick, your comments and clarification seem to suggest you are not trying to do what the question starts out by asking (how do you determine if the value in an int is an object - answered above, you don't), but something rather different...
I think what you're after is function overloading, and as you seem to be trying to use macros, maybe inline functions as well. Clang supports function overloading, here is program fragment which may show you how to solve your problem:
// Clang likes prototypes so let's give it some
// The following declares two overloaded inline functions:
NS_INLINE void __attribute__((overloadable)) byType(int x);
NS_INLINE void __attribute__((overloadable)) byType(NSNumber *x);
// now some simple definitions:
NS_INLINE void __attribute__((overloadable)) byType(int x)
{
NSLog(#"int version called: %d", x);
}
NS_INLINE void __attribute__((overloadable)) byType(NSNumber *x)
{
NSLog(#"NSNumber version called: %#", x);
}
// now call them, automatically selecting the right function
// based on the argument type
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
int x = 5;
NSNumber *y = [NSNumber numberWithInt:42];
byType(x);
byType(y);
}
The above code when run outputs:
int version called: 5
NSNumber version called: 42
Clang 3 compiles the above code inlining the two calls, so you get the same code as using macros.
please don't mix between scalar values and pointers to objects... it will not end well.
if you insist you can do something with Objective-C++
something like
int sum(int,int);
NSNumber * sum(NSNumber *, NSNumber *);
Is there any easy way to convert an Objective-C holding class of NSStrings into parameters for a function accepting a variable list of char *? Specifically I have a function like:
-(void)someFunction:(NSSomething *) var
that I want to forward to a C function like
void someCFunction(char * var, ...)
Is there an easy way to go about this?
No, you can only do what you want if the number of arguments you're passing is known at compile time. If you just want to convert a single string, use the -UTF8String message:
// Example with two strings
NSString *str1 = ...;
NSString *str2 = ...;
someCFunction([str1 UTF8String], [str2 UTF8String]); // etc.
But if the number of strings will vary at runtime, you'll need to use a different API, if one is available. For example, if there's an API that took an array of strings, you could convert the Objective-C array into a C array:
// This function takes a variable number of strings. Note: in C/Objective-C
// (but not in C++/Objective-C++), it's not legal to convert 'char **' to
// 'char *const *', so you may sometimes need a cast to call this function
void someCFunction(const char *const *stringArray, int numStrings)
{
...
}
...
// Convert Objective-C array to C array
NSArray *objCArray = ...;
int numStrings = [objCArray count];
char **cStrArray = malloc(numStrings * sizeof(char*));
for (int i = 0; i < count; i++)
cStrArray[i] = [[objCArray objectAtIndex:i] UTF8String];
// Call the function; see comment above for note on cast
someCFunction((const char *const *)cStrArray, numStrings);
// Don't leak memory
free(cStrArray);
This would do the trick:
NSString *string = #"testing string"
const char * p1=[string UTF8String];
char * p2;
p2 = const_cast<char *>(p1);
Yes, this can be done, and is explained here:
How to create a NSString from a format string like #"xxx=%#, yyy=%#" and a NSArray of objects?
And here:
http://www.cocoawithlove.com/2009/05/variable-argument-lists-in-cocoa.html
With modifications for ARC here:
How to create a NSString from a format string like #"xxx=%#, yyy=%#" and a NSArray of objects?
Also, variable arguments are not statically or strongly typed, as the other poster seems to be suggesting. In fact, there is no clear indication in the callee of how many arguments you really have. Determining the number of arguments generally breaks down into having to either specify the number by an count parameter, using a null terminator, or inferring it from a format string a la (s)print* . This is frankly why the C (s)print* family of functions has been the source of many errors, now made much much safer by the XCode / Clang / GCC compiler that now warns.
As an aside, you can approach statically typed variable arguments in C++ by creating a template method that accepts an array of an unspecified size. This is generally considered bad form though as the compiler generates separate instances for each size of array seen by by the compiler (template bloat).
I want to write a function or a directive like NSLog() that takes any kind of variable, primitives and objects. In that function I want to distinguish those.
I know how it works for objects:
- (void)test:(id)object {
if ([object isKindOfClass:[NSString class]])
...
but how do I distinguish objects from structs or even integer or floats.
Something like:
"isKindOfStruct:CGRect" or "isInt"
for example?
Is this possible?
I thought since you can send everything to NSLog(#"...", objects, ints, structs) it must be possible?
Thanks for any help!
EDIT
My ultimate goal is to implement some kind of polymorphism.
I want to be able to call my function:
MY_FUNCTION(int)
MY_FUNCTION(CGRect)
MY_FUNCTION(NSString *)
...
or [self MYFUNCTION:int]...
and in MY_FUNCTION
-(void)MYFUNCTION:(???)value {
if ([value isKindOf:int])
...
else if ([value isKindOf:CGRect])
...
else if ([value isKindOfClass:[NSString class]])
...
}
I know that isKindOf doesn't exists and you can't even perform such methods on primitives. I'm also not sure about the "???" generic type of "value" in the function header.
Is that possible?
#define IS_OBJECT(T) _Generic( (T), id: YES, default: NO)
NSRect a = (NSRect){1,2,3,4};
NSString* b = #"whatAmI?";
NSInteger c = 9;
NSLog(#"%#", IS_OBJECT(a)?#"YES":#"NO"); // -> NO
NSLog(#"%#", IS_OBJECT(b)?#"YES":#"NO"); // -> YES
NSLog(#"%#", IS_OBJECT(c)?#"YES":#"NO"); // -> NO
Also, check out Vincent Gable's The Most Useful Objective-C Code I’ve Ever Written for some very handy stuff that uses the #encode() compiler directive (that) returns a string describing any type it’s given..."
LOG_EXPR(x) is a macro that prints out x, no matter what type x is, without having to worry about format-strings (and related crashes from eg. printing a C-string the same way as an NSString). It works on Mac OS X and iOS.
A function like NSLog() can tell what types to expect in its parameter list from the format string that you pass as the first parameter. So you don't query the parameter to figure out it's type -- you figure out what type you expect based on the format string, and then you interpret the parameter accordingly.
You can't pass a C struct or primitive as a parameter of type id. To do so, you'll have to wrap the primitive in an NSNumber or NSValue object.
e.g.
[self test: [NSNumber numberWithInt: 3.0]];
id is defined as a pointer to an Objective-C object.
#alex gray answer did not work(or at least did not work on iOS SDK 8.0). You can use #deepax11 answer, however I want to point how this 'magic macro' works. It relies on type encodings provided from the system. As per the Apple documentation:
To assist the runtime system, the compiler encodes the return and argument types for each method in a character string and associates the string with the method selector. The coding scheme it uses is also useful in other contexts and so is made publicly available with the #encode() compiler directive. When given a type specification, #encode() returns a string encoding that type. The type can be a basic type such as an int, a pointer, a tagged structure or union, or a class name—any type, in fact, that can be used as an argument to the C sizeof() operator.
To break the macro apart, we first get "typeOf" our variable, then call #encode() on that type, and finally compare returned value to 'object' and 'class' types from encoding table.
Full example should look like:
const char* myType = #encode(typeof(myVar));//myVar declared somewhere
if( [#"#" isEqualToString:#(myType)] || [#"#" isEqualToString:#(myType)] )
{
//myVar is object(id) or a Class
}
else if ( NSNotFound != [[NSString stringWithFormat:#"%s", myType] rangeOfCharacterFromSet:[NSCharacterSet characterSetWithCharactersInString:#"{}"]].location )
{
//myVar is struct
}
else if ( [#"i" isEqualToString:#(myType)] )
{
//my var is int
}
Please note that NSInteger will return int on 32-bit devices, and long on 64-bit devices. Full list of encodings:
‘c’ - char
‘i’ - int
’s’ - short
‘l’ - long
‘q’ - long long
‘C’ - unsigned char
‘I’ - unsigned int
’S’ - unsigned short
‘L’ - unsigned long
‘Q’ - unsigned long long
‘f’ - float
‘d’ - double
‘B’ - C++ bool or a C99 _Bool
‘v’ - void
‘*’ - character string(char *)
‘#’ - object(whether statically typed or typed id)
‘#’ - class object(Class)
‘:’ - method selector(SEL)
‘[<some-type>]’ - array
‘{<some-name>=<type1><type2>}’ - struct
‘bnum’ - bit field of <num> bits
‘^type’ - pointer to <type>
‘?’ - unknown type(may be used for function pointers)
Read more about Type Encodings at Apple
#define IS_OBJECT(x) ( strchr("##", #encode(typeof(x))[0]) != NULL )
This micro works which I got somewhere in stack overflow.
It's important to note that id represents any Objective-C object. And by Objective-C object, I mean one that is defined using #interface. It does not represent a struct or primitive type (int, char etc).
Also, you can only send messages (the [...] syntax) to Objective-C objects, so you cannot send the isKindOf: message to a normal struct or primitive.
But you can convert a integer etc to a NSNumber, a char* to a NSString and wrap a structure inside a NSObject-dervied class. Then they will be Objective-C objects.