Using member variable in lambda capture list inside a member function

Question

The following code compiles with gcc 4.5.1 but not with VS2010 SP1:

#include 
#include 
#include 

Answer 1

I believe VS2010 to be right this time, and I'd check if I had the standard handy, but currently I don't.

Now, it's exactly like the error message says: You can't capture stuff outside of the enclosing scope of the lambda.^† grid is not in the enclosing scope, but this is (every access to grid actually happens as this->grid in member functions). For your usecase, capturing this works, since you'll use it right away and you don't want to copy the grid

auto lambda = [this](){ std::cout << grid[0][0] << "\n"; }

If however, you want to store the grid and copy it for later access, where your puzzle object might already be destroyed, you'll need to make an intermediate, local copy:

vector<vector<int> > tmp(grid);
auto lambda = [tmp](){}; // capture the local copy per copy

---

† I'm simplifying - Google for "reaching scope" or see §5.1.2 for all the gory details.

Answer 2

I believe, you need to capture this.

Answer 3

Summary of the alternatives:

capture this:

auto lambda = [this](){};

use a local reference to the member:

auto& tmp = grid;
auto lambda = [ tmp](){}; // capture grid by (a single) copy
auto lambda = [&tmp](){}; // capture grid by ref

C++14:

auto lambda = [ grid = grid](){}; // capture grid by copy
auto lambda = [&grid = grid](){}; // capture grid by ref

example: https://godbolt.org/g/dEKVGD

Answer 4

An alternate method that limits the scope of the lambda rather than giving it access to the whole this is to pass in a local reference to the member variable, e.g.

auto& localGrid = grid;
int i;
for_each(groups.cbegin(),groups.cend(),[localGrid,&i](pair<int,set<int>> group){
            i++;
            cout<<i<<endl;
   });