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  lt iostream gt   include  lt vector gt   include  lt map gt   include  lt utility gt   include  lt set gt   include  lt algorithm gt   using namespace std  class puzzle           vector lt vector lt int gt  gt  grid          map lt int set lt int gt  gt  groups  public          int member function        int puzzle  member function             int i          for each groups cbegin   groups cend    grid  amp i  pair lt int set lt int gt  gt  group                   i                    cout lt  lt i lt  lt endl                int main             return 0      This is the error   error C3480   puzzle  grid   a lambda capture variable must be from an enclosing function scope warning C4573  the usage of  puzzle  grid  requires the compiler to capture  this  but the current default capture mode does not allow it   So   1  which compiler is right   2  How can I use member variables inside a lambda in VS2010

User · Accepted Answer

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- gt 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  lt  lt  grid 0  0   lt  lt    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 lt vector lt int gt   gt  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

User · Answer

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 amp  localGrid   grid  int i  for each groups cbegin   groups cend    localGrid  amp i  pair lt int set lt int gt  gt  group               i                cout lt  lt i lt  lt endl

User · Answer

Summary of the alternatives   capture this   auto lambda    this         use a local reference to the member   auto amp  tmp   grid  auto lambda     tmp          capture grid by  a single  copy auto lambda     amp tmp          capture grid by ref   C  14   auto lambda     grid   grid          capture grid by copy auto lambda     amp grid   grid          capture grid by ref   example  https   godbolt org g dEKVGD

User · Answer

I believe  you need to capture this

[c++] Using member variable in lambda capture list inside a member function

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