Capture and move a unique_ptr in a c++14 lambda expression

Question

I am capturing a unique_ptr in a lambda expression this way:

auto str = make_unique(\"my string\");
auto lambda = [ capturedStr = std::move(str)          


        

Answer 1

auto lambda = [ capturedStr = std::move(str) ] {
   cout << *capturedStr.get() << endl;
   auto str2 = std::move(capturedStr); // <--- Not working, why?
};

To give more detail the compiler is effectively making this transformation:

class NameUpToCompiler
{
    unique_ptr<string> capturedStr;  // initialized from move assignment in lambda capture expression

    void operator()() const
    {
        cout << *capturedStr.get() << endl;
        auto str2 = std::move(capturedStr);  // move will alter member 'captureStr' but can't because of const member function.
    }
}

The use of mutable on the lambda will remove the const from the operator() member function therefore allowing the members to be altered.

Answer 2

To make the advice more explicit: add mutable: http://coliru.stacked-crooked.com/a/a19897451b82cbbb

#include <memory>

int main()
{
    std::unique_ptr<int> pi(new int(42));

    auto ll = [ capturedInt = std::move(pi) ] () mutable { };
}

Answer 3

The operator () of a lambda is const by default, and you can't move from a const object.

Declare it mutable if you want to modify the captured variables.

auto lambda = [ capturedStr = std::move(str) ] () mutable {
//                                             ^^^^^^^^^^
    cout << *capturedStr.get() << endl;
    auto str2 = std::move(capturedStr);
};