[英]Capture and move a unique_ptr in a c++14 lambda expression
I am capturing a unique_ptr in a lambda expression this way: 我正在以这种方式捕获lambda表达式中的unique_ptr:
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. 它很有效,直到我尝试将
capturedStr
移动到另一个unique_ptr。 For instance, the following is not working: 例如,以下内容不起作用:
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: 这是编译器的输出:
.../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
? 为什么不能移动
capturedStr
?
The operator ()
of a lambda is const
by default, and you can't move from a const
object. 默认情况下,lambda的
operator ()
是const
,并且不能从const
对象移动。
Declare it mutable
if you want to modify the captured variables. 如果要修改捕获的变量,请声明它是
mutable
。
auto lambda = [ capturedStr = std::move(str) ] () mutable {
// ^^^^^^^^^^
cout << *capturedStr.get() << endl;
auto str2 = std::move(capturedStr);
};
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: 为了提供更多细节,编译器正在有效地进行这种转换:
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. 在lambda上使用mutable将从operator()成员函数中删除const,从而允许更改成员。
To make the advice more explicit: add mutable
: http://coliru.stacked-crooked.com/a/a19897451b82cbbb 为了使建议更明确:添加
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 { };
}
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