std::find_if 中的 C++ lambda 表达式？
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C++ lambda expression in std::find_if?
提问by Anti-Distinctlyminty
I've got a std::map that contains a class and that class has an id. I have an id that I'm trying to find in the set
我有一个 std::map 包含一个类，并且该类有一个 id。我有一个我想在集合中找到的 ID
typedef std::set<LWItem> ItemSet;
ItemSet selectedItems;
LWItemID i = someID;
ItemSet::iterator isi;
isi = std::find_if(selectedItems.begin(), selectedItems.end(), [&a](LWItemID i)->bool { return a->GetID()==i; }
I get an error saying that the lambda capture variable is not found, but I have no idea what I'm supposed to do to get it to capture the container contents as it iterates through. Also, I know that I cant do this with a loop, but I'm trying to learn lambda functions.
我收到一条错误消息，说找不到 lambda 捕获变量，但我不知道我应该怎么做才能让它在迭代时捕获容器内容。另外，我知道我不能用循环来做到这一点，但我正在尝试学习 lambda 函数。
回答by ecatmur
You've got your capture and argument reversed. The bit inside the []is the capture; the bit inside ()is the argument list. Here you want to capture the local variable iand take aas an argument:
你已经把你的捕获和论点颠倒了。里面的位[]是捕获；里面的位()是参数列表。在这里，您要捕获局部变量i并将其a作为参数：
[i](LWItem a)->bool { return a->GetID()==i; }
This is effectively a shorthand for creating a functor class with local variable i:
这实际上是创建带有局部变量的函子类的简写i：
struct {
LWItemID i;
auto operator()(LWItem a) -> bool { return a->GetID()==i; }
} lambda = {i};
回答by Jiwan
From what i understand you code should look like this :
据我了解，您的代码应如下所示：
auto foundItem = std::find_if(selectedItems.begin(), selectedItems.end(),
[&i](LWItem const& item)
{
return item->GetID() == i;
});
This will capture the LWItem that have an ID equal to i, with i being a previosuly declared ID.
这将捕获 ID 等于 i 的 LWItem，其中 i 是先前声明的 ID。
