C++11 lambda 返回 lambda [英] C++11 lambda returning lambda
问题描述
这段代码对于JS开发者来说并不是陌生的
this piece of code is not something unknown to JS developers
function get_counter()
{
return (
function() {
var c = 0;
return function() { return ++c; };
})();
}
它基本上创建了一个，它创建了不同的枚举器.所以我想知道是否可以使用新的 lambda 语义在 C++11 中完成同样的事情?我最终写了这段 C++ 不幸的是不能编译！
it basically creates a which creates different enumerators. So I was wondering if same thing can be done in C++11 with new lambda semantics? I ended up writing this piece of C++ which unfortunately does not compile!
int main()
{
int c;
auto a = [](){
int c = 0;
return [&](){
cout << c++;
};
};
return 0;
}
所以我想知道是否有解决方法可以编译它，如果有编译器如何使此代码正确运行?我的意思是它必须创建单独的枚举器，但它也应该收集垃圾(未使用的 c 变量).
so I was wondering if there is a workaround to get it compiled and if there is how can compiler make this code run correctly? I mean it has to create separate enumerators but it should also collect garbage (unused c variables).
顺便说一下，我使用的是 VS2012 编译器，它会生成此错误:
by the way I'm using VS2012 compiler and it generates this error:
Error 2 error C2440: 'return' : cannot convert from 'main::<lambda_10d109c73135f5c106ecbfa8ff6f4b6b>::()::<lambda_019decbc8d6cd29488ffec96883efe2a>' to 'void (__cdecl *)(void)' c:usersalidocumentsvisual studio 2012projects est estmain.cpp 25 1 Test
推荐答案
您的代码有一个错误，它包含一个悬空引用；c 引用将引用外部 lambda 中的局部变量，当外部 lambda 返回时该变量将被销毁.
Your code has a bug in that it contains a dangling reference; the c reference will refer to the local variable in the outer lambda, which will be destroyed when the outer lambda returns.
您应该使用 mutable 按值 lambda 捕获来编写它:
You should write it using a mutable by-value lambda capture:
auto a = []() {
int c = 0;
return [=]() mutable {
cout << c++;
};
};
这依赖于后标准扩展，以允许在返回类型推导的 lambda 中使用多个语句；如果 lambda 包含多个语句，是否有理由不允许它推导出返回类型? 最简单的修复方法是提供一个参数，以便 lambda 只包含一个语句:
This relies on a post-standard extension to allow multiple statements in a return-type-deducing lambda; Is there a reason on not allowing lambdas to deduce the return type if it contains more than one statement? The easiest way to fix it is to supply a parameter so that the lambda contains only a single statement:
auto a = [](int c) {
return [=]() mutable {
cout << c++;
};
};
不幸的是，lambdas 中不允许使用默认参数，因此您必须将其称为 a(0).或者，以可读性为代价，您可以使用嵌套的 lambda 调用:
Unfortunately default parameters aren't allowed in lambdas, so you'd have to call this as a(0). Alternatively at the cost of readability you could use a nested lambda call:
auto a = []() {
return ([](int c) {
return [=]() mutable {
cout << c++;
};
})(0);
};
这种工作方式是，当 a 执行内部 lambda 时，会将所有引用的变量复制到其闭包类型的一个实例中，如下所示:
The way this works is that when a executes the inner lambda copies all the referenced variables into an instance of its closure type, which here would be something like:
struct inner_lambda {
int c;
void operator()() { cout << c++; }
};
闭包类型的实例然后由外部 lambda 返回，并且可以被调用并在调用时修改它的 c 副本.
The instance of the closure type is then returned by the outer lambda, and can be invoked and will modify its copy of c when called.
总的来说，你的(固定的)代码被翻译成:
Overall, your (fixed) code is translated to:
struct outer_lambda {
// no closure
struct inner_lambda {
int c; // by-value capture
// non-const because "mutable"
void operator()() { cout << c++; }
}
// const because non-"mutable"
inner_lambda operator()(int c) const {
return inner_lambda{c};
}
};
如果您将 c 作为按引用捕获保留，则为:
If you left c as a by-reference capture, this would be:
struct outer_lambda {
// no closure
struct inner_lambda {
int &c; // by-reference capture
void operator()() const { cout << c++; } // const, but can modify c
}
inner_lambda operator()(int c) const {
return inner_lambda{c};
}
};
这里的 inner_lambda::c 是对局部参数变量 c 的悬空引用.
Here inner_lambda::c is a dangling reference to the local parameter variable c.
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