Objective-C ++ 11-为什么我们不能为lambda分配一个块? [英] Objective-C++ 11 - Why can't we assign a block to a lambda?

问题描述

因此，我刚刚升级到Xcode 4.4，并且在变更日志中注意到了:

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

Apple LLVM编译器支持其他C ++ 11功能，包括lambdas

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

哪个很棒！所以我开始编码，然后发现了一些东西:

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

1. Lambda可分配给Objective-C块:

1. Lambdas are assignable to Objective-C blocks:

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

block();

std::function可以容纳一个Objective-C块:

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

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

func();

我们无法为Lambda分配一个Objective-C块:

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

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

lambda = ^{ // error!
    NSLog(@"Block!");
};

lambda();

这是为什么?鉴于我们上面已经看到的内容，两者在语义上是否应该等效吗?

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

推荐答案

明确禁用了C ++ 11的lambda的复制分配运算符 ¹ .这不是语义上等效"的问题.它甚至无法分配回自己.更不用说无关的类型了.

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可以容纳一个Objective-C块.

---

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

• std::function可以保存任何可以作为f(a,b,c,...)调用的类型.由于块支持"invoke运算符"，因此它也可以由std::function保留.但是请注意，Objective-C和C ++遵循不同的内存管理方案，因此长时间将块存储在std::function中可能会导致引用悬空.

• 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.

Lambda可分配给Objective-C块:

Lambdas are assignable to Objective-C blocks:

• 责备 SAHChandler ² :).但是，尚未记录.

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

¹ :C ++ 11§5.1.2/19:

¹: C++11 §5.1.2/19:

与 lambda-expression 关联的闭包类型具有已删除的(8.4.3)默认构造函数和已删除的副本分配运算符.

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

这篇关于Objective-C ++ 11-为什么我们不能为lambda分配一个块?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！