Smart pointer unique_ptr series in combat C + +-Unique_ptr with lambda error (especially capturing unique_ptr in lambda)

Lambda expression is a new introduction to C++11, which brings us a lot of convenience and makes the code simple and clear.

But when we combine unique_ptr and lambda expressions, errors tend to occur and are fatal.

Look directly at the following code:

#include "stdafx.h"#include <memory>#include <vector>#include <algorithm>classMessage { Public: Message () {}};intMainintargcChar* argv[]) {STD:: Vector<std::unique_ptr<message>>messages; for(inti =0; I < +; i++) {STD::unique_ptr<message> testmess; Messages.push_back (STD:: Move (testmess)); }STD:: For_each (Messages.begin (), Messages.end (), [] (STD::unique_ptr<message> testmess) {//do something stupid});return 0;}

Unfortunately, this code compilation produces errors, but it's also lucky:

d:  \program Files (x86) \ Microsoft Visual Studio 14.0  \vc\include  \ Algorithm (24 ) :  Error c2280  :  "STD:  :unique_ptr<message , STD:  :d Efault_delete<_ty>>  :  :unique_ptr  (const STD:  :unique_ptr<_ty , STD:  :d Efault_delete<_ty>>  &)"  attempt to reference a deleted function  

Simple analysis
What we use in lambda expressions is pass-by-value;
Passing by value produces a copy, which produces a unique_ptr copy;
But we know that this is obviously a mistake.

The workaround is simply to pass by reference instead by pointer:

#include <iostream>#include <memory>#include <vector>#include <algorithm>classMessage { Public: Message () {}};intMainintargcChar* argv[]) {STD:: Vector<std::unique_ptr<message>>messages; for(inti =0; I < +; i++) {STD::unique_ptr<message> testmess; Messages.push_back (STD:: Move (testmess)); }STD:: For_each (Messages.begin (), Messages.end (), [] (STD::unique_ptr<message> &testmess) {//do something stupid});return 0;}

The above content is just an appetizer, what if we want to capture unique_ptr in a lambda expression?

Write down the following code very gracefully:

#include<iostream>#include<memory>#include<string>int main(){    autostd::make_unique<std::string>("my string");    autostd::move(str)]{        std::coutstd::endl;    };    lambda();    return0;}

Everything is perfect, compile and run the output correctly.

Next, do something more:

#include <iostream>#include <memory>#include <string>intMain () {Autostr =STD::make_unique<STD::string> ("My string");AutoLambda = [CAPTUREDSTR =STD:: Move (str)] {STD::cout<< *capturedstr.get () <<STD:: Endl;AutoSTR2 =STD:: Move (CAPTUREDSTR);STD::cout<< *STR2 <<STD:: Endl;    }; Lambda ();return 0;}

Congratulations, compilation error.

What is the reason? Why Auto str2 = Std::move (CAPTUREDSTR);
This is the knowledge of lambda expressions:
The lambda expression is const by default, and we certainly cannot std::move a const object.

The solution is also very simple, is to add the keyword mutable
The program is as follows and is running intact:

#include <iostream>#include <memory>#include <string>intMain () {Autostr =STD::make_unique<STD::string> ("My string");AutoLambda = [CAPTUREDSTR =STD:: Move (str)] ()mutable{STD::cout<< *capturedstr.get () <<STD:: Endl;AutoSTR2 =STD:: Move (CAPTUREDSTR);STD::cout<< *STR2 <<STD:: Endl;    }; Lambda ();return 0;}

Smart pointer unique_ptr series in combat C + +-Unique_ptr with lambda error (especially capturing unique_ptr in lambda)