Question

I have an application that is creating maps for civilization V. As an interesting design choice I decided to create a couple of functions that would do the looping through the map for me. This way I could pass a function pointer or a lambda function to that function that goes through the whole map doing something to each tile. Reasoning behind this was if I or someone else would change the way the map is stored (from 2D array to a 2D vector or whatever) one would only need to change one function instead of the whole codebase.

Now the problem, here is some code first.

Error code.

    case ALL_SNOW:
        m.loop_through_limit([] (Tile* t) {
            t = new Snow(t->get_x(), t->get_y()); 
            return t;
        }, x, y, width, height);
        break;
    case PTN_ONE:
        m.loop_through_limit([&] (Tile* t) {
            int cur_x = t->get_x();
            int cur_y = t->get_y();
            t = new Plains(cur_x, cur_y);
            // if (y <= height/4 || y >= (height*3)/4) {
            //     Top quarter rows and bottom quarter rows
            //     t = new Ocean(cur_x, cur_y);
            // } else if (cur_x <= width/4) {
            //     Leftmost columns
            //     t = new Ocean(cur_x, cur_y);
            // } else if (cur_x >= (width*3)/4) {
            //     Rightmost columns
            //     t = new Desert(cur_x, cur_y);
            // } 
            return t;
        }, x, y, width, height);
        break;

Definitions from header file.

void loop_through(void (*)(Tile* t));
void loop_through_limit(Tile* (*)(Tile* t), int start_x, int start_y, int width, int height);

Now the difference in each case isn’t much apart from the commented out code. This works fine. When I comment out that if statement block, then this is my output.

c++ -c  -g -O3 -ffast-math -Wall -Weffc++ -std=c++0x -o tile_block.o tile_block.cpp 
tile_block.cpp: In static member function ‘static void TileBlock::write(Map&, TileBlock::Patterns, int, int, int, int)’:
tile_block.cpp:82:35: error: no matching function for call to ‘Map::loop_through_limit(TileBlock::write(Map&, TileBlock::Patterns, int, int, int, int)::<lambda(Tile*)>, int&, int&, int&, int&)’
tile_block.cpp:82:35: note: candidate is:
map.h:26:10: note: void Map::loop_through_limit(Tile* (*)(Tile*), int, int, int, int)
map.h:26:10: note:   no known conversion for argument 1 from ‘TileBlock::write(Map&, TileBlock::Patterns, int, int, int, int)::<lambda(Tile*)>’ to ‘Tile* (*)(Tile*)’

And I believe the problem comes when I start using the parameters I’m trying to capture by reference. Then it starts to turn into a “lambda” function instead of just a “function pointer”, maybe I’m just not getting it.

Any suggestions?

Answer 1

C++11 lambda are not function pointers if they capture variables. What you need is called std::function, especially for the second function, because the lambda for that capture variables.

So change these:

void loop_through(void (*)(Tile* t));
void loop_through_limit(Tile* (*)(Tile* t), /*...*/);

to these:

void loop_through(std::function<void(Tile*)>  fun);
void loop_through_limit(std::function<Tile*(Tile*)> fun, /*...*/);

Now you can pass lambda to the above functions.