C++ lambda expression in std::find_if?

Question

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

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.

Answer 1

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 i and take a as an argument:

[i](LWItem a)->bool { return a->GetID()==i; } 

This is effectively a shorthand for creating a functor class with local variable i :

struct {
   LWItemID i;
   auto operator()(LWItem a) -> bool { return a->GetID()==i; } 
} lambda = {i};

Answer 2

From what i understand you code should look like this :

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.