Objective-C++ 11 - Why can't we assign a block to a lambda?

问题

So, I just upgraded to Xcode 4.4, and I noticed in the changelog:

Apple LLVM compiler supports additional C++11 features, including lambdas

Which is awesome! So I got around to coding, and I found a few things out:

1. Lambdas are assignable to Objective-C blocks:

   void (^block)() = []() -> void { 
       NSLog(@"Inside Lambda called as block!");
   };
   
   block();
   

2. std::function can hold an Objective-C block:

   std::function<void(void)> func = ^{
       NSLog(@"Block inside std::function");
   };
   
   func();
   

3. We cant assign an Objective-C block to a lambda:

   auto lambda = []() -> {
       NSLog(@"Lambda!");
   };
   
   lambda = ^{ // error!
       NSLog(@"Block!");
   };
   
   lambda();
   

Why is this? Shouldn't the two be semantically equivalent, given what we've seen above?

回答1:

C++11's lambda's copy-assignment operator is explicitly disabled¹. This is not a matter of "semantically equivalent". It can't even assign back to itself. Not to mention an unrelated type.

#include <cstdio>
#include <type_traits>

int main() {
    auto lambda1 = []() -> void { printf("Lambda 1!\n"); };
    lambda1 = lambda1;  // error: use of deleted function ‘main()::<lambda()>& main()::<lambda()>::operator=(const main()::<lambda()>&)’
    return 0;
}

---

std::function can hold an Objective-C block.

• std::function can hold any types which can be invoked as f(a,b,c,...). Since blocks support "the invoke operator", it can also be held by a std::function. But notice that Objective-C and C++ follow different memory management scheme, so storing a block in a std::function for a long time may cause dangling reference.

---

Lambdas are assignable to Objective-C blocks:

• Blame SAHChandler² :). It's not documented yet, though.

---

¹: C++11 §5.1.2/19:

The closure type associated with a lambda-expression has a deleted (8.4.3) default constructor and a deleted copy assignment operator.

²: http://llvm.org/viewvc/llvm-project?view=rev&revision=150620

回答2:

Lambdas have their own, implementation-defined types which are specific to each lambda. The following code is also an error:

auto l1=[](){return 1;}
auto l2=[](){return 2;}
l1=l2; //Error

std::function is a wrapper which is designed to hold any callable type; you should use that to hold callables which may be of different types.

来源：https://stackoverflow.com/questions/11669292/objective-c-11-why-cant-we-assign-a-block-to-a-lambda