I am capturing a unique_ptr in a lambda expression this way:
auto str = make_unique<string>("my string");
auto lambda = [ capturedStr = std::move(str) ] {
cout << *capturedStr.get() << endl;
};
lambda();
It works great until I try to move capturedStr
to another unique_ptr. For instance, the following is not working:
auto str = make_unique<string>("my string");
auto lambda = [ capturedStr = std::move(str) ] {
cout << *capturedStr.get() << endl;
auto str2 = std::move(capturedStr); // <--- Not working, why?
};
lambda();
Here is the output from the compiler:
.../test/main.cpp:11:14: error: call to implicitly-deleted copy
constructor of 'std::__1::unique_ptr<std::__1::basic_string<char>,
std::__1::default_delete<std::__1::basic_string<char> > >'
auto str2 = std::move(capturedStr);
^ ~~~~~~~~~~~~~~~~~~~~~~ ../include/c++/v1/memory:2510:31: note: copy constructor is implicitly
deleted because 'unique_ptr<std::__1::basic_string<char>,
std::__1::default_delete<std::__1::basic_string<char> > >' has a
user-declared move constructor
_LIBCPP_INLINE_VISIBILITY unique_ptr(unique_ptr&& __u) _NOEXCEPT
^ 1 error generated.
Why isn't it possible to move capturedStr
?
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);
};