std::unique_ptr with lambda

Question

I am trying to access a unique_ptr in a lambda but I get the following compile errors:

error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'

Code looks like this:

#include "stdafx.h"
#include <memory>
#include <vector>
#include <algorithm>

class Message {
public:
     Message() {}
};

int main(int argc, char* argv[])
{
     std::vector<std::unique_ptr<Message>>messages;
	
     for (int i = 0; i < 1000; 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;
}

Answer 1

The way you wrote the lambda will cause a copy of the unique_ptr to be made, that's not possible, it's supposed to be unique after all. Use a reference instead:

 std::for_each(messages.begin(), messages.end(),
                [](const std::unique_ptr<Message>& testMess) {
// do something stupid
     });

Answer 2

The way you wrote the lambda will cause a copy of the unique_ptr to be made, that's not possible, it's supposed to be unique after all. Use a reference instead:

 std::for_each(messages.begin(), messages.end(),
                [](const std::unique_ptr<Message>& testMess) {
// do something stupid
     });

Thanks a lot.

Answer 3

On 12/14/2011 12:13 PM, Anonymous7520 wrote:

I am trying to access a unique_ptr in a lambda but I get the following compile errors:

error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'

      std::for_each(messages.begin(), messages.end(),
                 [](std::unique_ptr<Message>  testMess) {    

You are passing unique_ptr by value. That would require copying it, but it can't be copied.

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Igor Tandetnik