How to capture std::unique_ptr "by move" for a lambda in std::for_each

Question

I am learning the new features in c++11 and came across this problem. I'd like to capture an unique_ptr by moving it inside a lambda as an argument for for_each.

set up:

std::array<int,4> arr = {1,3,5,6};
std::unique_ptr<int> p(new int);  (*p) = 3;

attempt 1 - doesn't work because unique_ptr doesn't have a copy constructor. c++0x doesn't specify the pass by move syntax.

std::for_each(arr.begin(), arr.end(), [p](int& i) { i+=*p; });

attempt 2 - use bind to bind a moved copy of p to a function that takes int&:

std::for_each(arr.begin(), arr.end(),
     std::bind([](const unique_ptr<int>& p, int& i){
          i += (*p);
     }, std::move(p))
);

Compiler complains that 'result' : symbol is neither a class template nor a function template.

The main purpose of the exercise is to understand how a movable variable be captured in a lambda that's cached for later use.

This question has already been asked here. However, I don't vote to close since your question contains an alternative (though not-working) solution. — Luc Touraille

Anthony Williams Anthony Williams · Accepted Answer · 2012-04-23T13:05:05

Update: you can capture a movable variable in a lambda from C++14 onwards.

std::for_each(arr.begin(), arr.end(), [p=std::move(p)](int& i) { i+=*p; });

You cannot capture a movable variable into a lambda in any straightforward way in C++11.

Lambdas capture by copy or by reference. Thus, to capture a move-only variable, you have to wrap it in an object where copying => moving (such as std::auto_ptr). This is a nasty hack.

In your example, you can just capture by reference, but if this was just simplified code it may not do what you wanted with the real code:

std::for_each(arr.begin(), arr.end(), [&p](int& i) { i+=*p; });

Here's a copy-move-only wrapper:

template<typename T>
struct move_on_copy_wrapper
{
    mutable T value;

    move_on_copy_wrapper(T&& t):
        value(std::move(t))
    {}

    move_on_copy_wrapper(move_on_copy_wrapper const& other):
        value(std::move(other.value))
    {}

    move_on_copy_wrapper(move_on_copy_wrapper&& other):
        value(std::move(other.value))
    {}

    move_on_copy_wrapper& operator=(move_on_copy_wrapper const& other)
    {
        value=std::move(other.value);
        return *this;
    }

    move_on_copy_wrapper& operator=(move_on_copy_wrapper&& other)
    {
        value=std::move(other.value);
        return *this;
    }

};

You can then use it like this:

int main()
{
    std::unique_ptr<int> p(new int(3));
    move_on_copy_wrapper<std::unique_ptr<int>> mp(std::move(p));

    [mp]()
    {
        std::cout<<"*mp.value="<<*mp.value<<std::endl;
    }
    ();

    std::cout<<"p="<<p.get()<<", mp="<<mp.value.get()<<std::endl;
}

How to capture std::unique_ptr "by move" for a lambda in std::for_each

2 Answers