class TUIManager
{ //Manager for the OpenGL-SDL-userinterface
  private: 

    ...

    vector<TTF_Font*> FontList;		//stores all fonts which are used in the GUI
    vector<string*> FontNameTable;	//stores pointers to the names of the fonts. The i-th string is the name of the font with ID=i.

    ...

  public:
    // Constructor
    TUIManager();

    // Destructor
    ~TUIManager();

    //----------------------Methodes------------------------------------

    ...

    // Returns true if the font was deleted, else false.
    bool deleteFont(int fontID);
};

// Returns true if the font was deleted, else false.
bool TUIManager::deleteFont(int fontID)
{
	if ( (fontID >= 0)&&( (unsigned int)fontID < FontList.size() ) )
	{
		if (FontList[fontID] == NULL) return true;

		TTF_CloseFont(FontList[fontID]);
		delete(FontList[fontID]);
		FontList[fontID] = NULL;
		delete(FontNameTable[fontID]);
		FontNameTable[fontID] = NULL;
		return true;
	}

	return false;
}

Beim aufruf der Methode deleteFont() bekomme ich bei

delete(FontList[fontID]);

folgenden Compiler-Fehler:

../src/TUIManager.cpp: In member function `bool TUIManager::deleteFont(int)':
../src/TUIManager.cpp:546: warning: possible problem detected in invocation of delete operator:
../src/TUIManager.cpp:546: warning: invalid use of undefined type `struct _TTF_Font'
C:/Programmieren/Imported_Libs/SDL_ttf-2.0.7/include/SDL_ttf.h:80: warning: forward declaration of `struct _TTF_Font'
../src/TUIManager.cpp:546: note: neither the destructor nor the class-specific operator delete will be called, 
  even if they are declared when the class is defined.

Im header ist die datei SDL_TTF includiert, welche die Klasse TTF_Font bereitstellt.

Was mach ich falsch?

Siehe auch: http://jcatki.no-ip.org/SDL_ttf/SDL_ttf.html#SEC18

Es sind immer die Einfachsten Dinge die einen am längsten aufhalten.