非法调用非静态成员函数（C ++）？

Question

I'm developing a game which is based around the user controlling a ball which moves between areas on the screen. The 'map' for the screen is defined in the file ThreeDCubeGame.cpp:

char m_acMapData[MAP_WIDTH][MAP_HEIGHT];

The ThreeDCubeGame.cpp handles most of the stuff to do with the map, but the player (and keyboard input) is controlled by ThreeDCubePlayer.cpp. When a player moves into a new map cell, the game will have to check the contents of that cell and act accordingly. This function in ThreeDCubeGame.cpp is what I am trying to use:

inline char GetMapEntry( int iMapX, int iMapY ) { return m_acMapData[iMapX][iMapY]; }

So, in order to check whether the player is allowed to move into a map cell I use this function call from ThreeDCubePlayer.cpp:

if (ThreeDCubeGame::GetMapEntry(m_iMapX+MAP_OFF_X, m_iMapY+MAP_OFF_Y) == ' ')
{
// do stuff
}

But, when I compile this, I get the warning "error C2352: 'ThreeDCubeGame::GetMapEntry' : illegal call of non-static member function". Is this something to do with the scope of the variables? Is it fixable without redesigning all the code?

Answer 1

class A {
  int i;
public:
  A(): i(0) {}
  int get() const { return i; }
};

int main() {
  A a;
  a.get();  // works
  A::get(); // error C2352
}

There's no object to call the function with.

Answer 2

GetMapEntry is not static so you can't call it without an object of the type ThreeDCubeGame.

Alternatives:
-Make GetMapEntry static: static inline char GetMapEntry
-Create an instance of ThreeDCubeGame and do instance.GetMapEntry(

Answer 3

ThreeDCubeGame is a class, not an instance, thus you can only use it to access static members (that is, member function with the keyword static ) You have to instantiate an object of this class to use non-static members

ThreeDCubeGame map;
...
map.GetMapEntry(iMapX, iMapY).

Answer 4

You are trying to call a class method. Is that what you intend? Or do you mean for GetMapEntry to be an instance method? If it's a class method, it needs to be marked static. If it's an instance method, you need to call it with an instance of ThreeDCubeGame . Also, is GetMapEntry even a member of a class?

Answer 5

The error indicates that your are calling the GetMapEntry function as a static one whereas you have declare it as a member function. You need to:

• call it via an instance of ThreeDCubeGame: threedcubegameinstance.GetMapEntry() ,
• declare the GetMapEntry function as static (add a static before inline and make m_acMapData static too).

Answer 6

You're missing the "static" keyword.

// .h
class Playfield
{
public: 
  static char GetTile( int x, int y ); 
  // static on a method means no 'this' is involved
};

// .cpp
static char tiles[10][10] = {}; 
// static on vars in .cpp prevents access from outside this .cpp

char Playfield::GetTile( int x, int y )
{
  // handle invalid args

  // return tile
  return tiles[x][y];
}

There's other options if you want only one unique playfield: You can make Playfield a singleton, turn it into a namespace or use global functions. The result is the same from the caller's point of view.

On a side note: Since all of these use a static and/or global variable it's inherently not thread-safe.

If you require multiple playfields and/or want to play safe with multi-threadding and/or want to absolutely do it in an OOP fashion, you will need an instance of Playfield to call the function on (the 'this' pointer):

class Playfield
{
public:
   char GetTile( int x, int y ) const { return this->tiles[x][y]; } 
   // you can omit 'this->', but it's inherently present because
   // the method is not marked as static 
public:
   Playfield() 
   { /*you will have to initialize 'this->tiles' here because 
       you cannot use the struct initializer '= {}' on member vars*/ }

private:
   char tiles[10][10];
};

The calling code would use Playfield like this:

void main()
{
  // static version 
  char tile11 = Playfield::GetTile( 1, 1 );

  // non-static version
  Playfield myPlayfield;
  char tile12 = myPlayfield.GetTile( 1, 2 );
}

Answer 7

It can be useful to have a class containing a collection of functions, without any data members, if you don't want to expose the helper-functions.
Otherwise it would be more practical to use a namespace to collect these functions in.
Example:

class Solvers
{
public:
  void solve_a(std::vector<int> data);
  void solve_b(std::vector<int> data, int value);
private:
  int helper_a(int a, int b);
}

But a class needs to be initialised before use.
The simplest way to make these functions usable would be to mark them static in the class:
static void solve_a(std::vector<int> data);
Then the member-functions can be used as:
Solver::solve_a(my_vector);

Another way would be to initialise the class before using:
Solver solver;
solver.solve_a(my_vector);

And the third method, not mentioned before, is by default initialising it during use:
Solver().solve_a(my_vector);