Block syntax error from Apple example 'Creating a Block'

Question

float (^oneFrom)(float);


oneFrom = ^(float aFloat) {

  float result = aFloat - 1.0;

  return result;

};

I get two errors:

Redefinition of 'oneFrom' with a different type: 'int' vs 'float(^)(float)'
Type specifier missing, defaults to 'int'

Also from the doc..

If you don’t explicitly declare the return value of a block expression, it can be automatically inferred from the contents of the block. If the return type is inferred and the parameter list is void, then you can omit the (void) parameter list as well. If or when multiple return statements are present, they must exactly match (using casting if necessary).

Nikolai Ruhe · Accepted Answer · 2013-07-03 14:30:31Z

3

You can't define blocks on file scope, only in functions. This works as expected:

void foo (void)
{
    float (^oneFrom)(float);
    oneFrom = ^(float aFloat) {
        float result = aFloat - 1.0;
        return result;
    };
}

edited Jul 3, 2013 at 14:30

answered Jul 3, 2013 at 14:20

Nikolai Ruhe

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6 Comments

estobbart Over a year ago

My understanding was that blocks are similar to C function pointers. Shouldn't I be able to declare it in the header file?

Nikolai Ruhe Over a year ago

@estobbart No, blocks are a much more complex concept than function pointers. Most important: they carry data, so they have to be dynamically allocated on the stack or heap. C does not allow for initialization of objects with static storage duration, so you can't define a block on file scope.

Nikolai Ruhe Over a year ago

@estobbart You can, of course, declare a block type variable using extern in a header file. You cannot define a block, though.

estobbart Over a year ago

Understood. There's a number of examples of declaring a block. Is there a situation where a block declaration is used? It looks like blocks are almost always used as block literals.

Nikolai Ruhe Over a year ago

One example would be a property of block type, for example NSFileHandle's readabilityHandler.

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Fonix · Accepted Answer · 2013-07-03 14:11:01Z

-1

that block doesnt have a return type, and the default return type is void, you will need to go

float (^oneFrom)(float);

oneFrom = ^ float (float aFloat) {

  float result = aFloat - 1.0;

  return result;

};

here is a better blocks example

answered Jul 3, 2013 at 14:11

Fonix

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4 Comments

estobbart Over a year ago

This example still reports the same errors. Am I missing something obvious?

Nikolai Ruhe Over a year ago

No. The block has a return type which is deducted from the actual return statement. It has the correct type of float.

Fonix Over a year ago

ah, yes my mistake, ive always implicitly done it for some reason. your answer is probably the real solution here as well, didnt think about where the block would be defined

Steph Sharp Over a year ago

@nikolai I think you mean "deduced" not "deducted". Perhaps "inferred" would be a better choice of words?

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