Is it possible to create a block at runtime from a string in Objective-C?

Question

I am trying to create a block in Objective-C, at runtime, from a char *string. But I can't find the way to do it.

I think that the dynamic nature of Objective-C would allow to do something like

char *blockString = "^(int a, int b) {return a + b;};";
printf("%s\n", blockString);
int (^addBlock)(int a, int b) = (^)*blockString;
int result = addBlock(3, 6);

Obviously, the syntax of the third line is too wacky for the compiler to process, but I hope it gives you the idea of what I want to accomplish.

Is this possible?

You want to parse block literals into blocks... This is not going to end well. — CodaFi
– CodaFi, Commented May 17, 2014 at 0:23
Why would you have these strings? Where would you get them from? — newacct
– newacct, Commented May 17, 2014 at 21:21

Duncan C · Accepted Answer · 2014-05-17 00:22:34Z

8

No. Objective C is a compiled language, not interpreted.

answered May 17, 2014 at 0:22

Duncan C

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4 Comments

rmaddy Over a year ago

Though there are several ways to execute code dynamically at runtime using things like performSelector and NSSelectorFromString as well as various Objective-C runtime functions and NSInvocation.

eharo2 Over a year ago

Thx @Duncan C. I understand that part, but given that we can create/define classes dynamically at runtime, I was wondering if we could create a block (that is some sort of object) at runtime.

David Berry Over a year ago

@eharo2 The difference is that when we're dynamically altering/creating classes at run-time, we're not trying to compile code, just rearranging pointers to existing compiled code. What you're trying to do is to dynamically compile code, which is neither supported, nor allowed.

Duncan C Over a year ago

As david says, it's nether supported nor allowed, but with additional support from Apple it might be possible. LLVM now does on-the-fly compilation as part of the LLDB debugger. However, that's on a Mac with Xcode installed. The comelier is not present on iOS devices. It would be a major effort to port the compiler to iOS, and to allow just-in-time compilation. Apple has always banned the use of dynamically executed code like ActionScript, Java, etc as a matter of policy, so I think it VERY unlikely that they would commit the effort needed to make this possible.

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