In the previous chapter, we covered weapons. Weapons are a special type of entity, but an entity nonetheless.
In this chapter, we will look at entities more generally, and see how they work. Later, we'll create two point entities (trigger_timer and trigger_conprint - prints to the console) and a brush entity (a "counting impulse" button - the more the player presses E, the more it'll progress).

entvars_t vs. edict_t vs. entity classes

Let's make this clear. The engine doesn't know the difference between a CBaseEntity and a CFuncWall. Instead, the engine only knows one form of entity. And that is edict_t.
It's basically an entity dictionary, containing data such as whether the entity's memory is free, its "serial number", its link to a BSP leaf, if it's a brush entity how many leaves it has, private entity data (to store member variables specific to the C++ classes of entities), and an entvars_t variable.
Generally, you won't use edict_t often.

In CBaseEntity, there's an entvars_t as well.

entvars_t *pev;

"pev" stands for "Pointer to Entity Variables", and it is essentially a set of variables that is common to all entities. entvars_t is defined in progdefs.h, and constants that can be used with variables from entvars_t are in const.h.
Some of the most important variables from entvars_t are:

• classname - entity's classname
• targetname - entity's name
• target - target entity's name
• origin - entity's current position
• velocity - entity's current velocity
• movedir - entity's current movement direction
• angles - entity's current angles
• avelocity - angular velocity
• model - path to the model
• nextthink - next time the entity will call its think callback
• movetype - movement type of the entity, which can be:
  • MOVETYPE_NONE - entity won't move
  • MOVETYPE_WALK - entity will use walk physics (players only)
  • MOVETYPE_STEP - entity will use walk physics for monsters
  • MOVETYPE_FLY - entity not be affected by gravity
  • MOVETYPE_TOSS - affected by gravity
  • MOVETYPE_PUSH - entity can get pushed
  • MOVETYPE_NOCLIP - not affected by gravity, passes through everything
  • MOVETYPE_FLYMISSILE - FLY but with extra size so it can hit monsters
  • MOVETYPE_BOUNCE - affected by gravity, bounces from the surface depending on pev->friction
  • MOVETYPE_BOUNCEMISSILE - combination of FLYMISSILE and BOUNCE
  • MOVETYPE_FOLLOW - follows pev->aiment
  • MOVETYPE_PUSHSTEP - BSP model that doesn't move on its own, but can collide with other entities

• solid - solidity mode of the entity, which can be:
  • SOLID_NOT - non-solid, everything passes through it
  • SOLID_TRIGGER - calls Touch, otherwise non-solid
  • SOLID_BBOX - uses its axis-aligned bounding box for collisions (this collision box cannot rotate, keep in mind), calls Touch when an entity touches one of its edges, can call Blocked
  • SOLID_SLIDEBOX - can only call Touch when on the ground, calls it when an entity touches one of its edges
  • SOLID_BSP - uses BSP cliphulls of the brush model for collisions, Touch on edge and can call Blocked

• rendermode - render mode of this entity, can be:
  • kRenderNormal - Normal
  • kRenderTransColor - Color
  • kRenderTransTexture - Texture
  • kRenderGlow - Glow
  • kRenderTransAlpha - Solid
  • kRenderTransAdd - Additive

• renderamt - render amount of this entity (typically wrongly named in FGDs as "FX Amount")
• rendercolor - render colour
• renderfx - render effect (look at const.h, it's got many of them)
• spawnflags - flags set in the map editor (in the Flags tab)
• flags - temporary entity flags

Certain utility functions will accept edict_t as a parameter, so you'll need to convert your CBaseEntity or CFuncWall or any entity class you're using into edict_t. While there's no direct conversion, you can use the ENT utility function to 'convert' an entvars_t into an edict_t. For example:

SET_MODEL(ENT(pev), STRING(pev->model));

We'll discuss utility functions later on.

string_t

You may have noticed several variables of the string_t type. This is not a C-string.

Beginners get very confused by string_t, since it's essentially an unsigned int, so I'll briefly explain it here.
The engine has a giant internal string. string_t is in fact just an offset into that giant string.

You can imagine it this way:

H  e  l  l  o     w  o  r  l  d \0 f  u  n  c  _  w  a  l  l  \0
0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21

If we wanted to allocate a new string there via ALLOC_STRING, like so:
string_t something = ALLOC_STRING( "something" );
...it'd return 22. If we had a string_t variable with a value of 11, STRING would return 11. That is essentially what string_t is. Just an offset into the internal string.

CBaseEntity

The mother of all entities.
As mentioned previously, every entity in the HL SDK inherits from CBaseEntity. We won't cover all of its methods and members, but we will go through the most important ones.

Spawn

Called for every entity in the map by the engine.

Precache

Entities override this so they can load model, sound and other files that they may use.
Files can be precached with the following macros:
PRECACHE_MODEL - precaches studio models
PRECACHE_SOUND - precaches audio files
PRECACHE_GENERIC - precaches any file, you can even precache a whole .bsp if you wanted to

KeyValue

Handles map keyvalues and assigns values from them to member variables. The engine calls this multiple times until it covers all keyvalues of every entity.

Parameters:

• pkvd - the data of one keyvalue

This is how keyvalues are typically handled:

if (FStrEq(pkvd->szKeyName, "myKeyvalue"))
{
    m_myVariable = atoi(pkvd->szValue);
    pkvd->fHandled = TRUE;
}

else
    CBaseEntity::KeyValue(pkvd);

We compare szKeyName to all possible keyvalues this entity may have, and then convert the szValue string into a value of the type we need.
If no match is found, then go back up to the KeyValue method of our superclass.

Save & Restore

Save and Restore are the key methods for transferring custom data fields across savefiles. If an entity has its own members, but doesn't save/restore, then it is most likely to break while loading a savefile, or across level transitions.

A save/restore is fully set up when an entity class has defined and implemented a save-restore table, as well as the save-restore methods.

Inside the entity class, you override the Save and Restore methods as well as declaring the save-restore table (TYPEDESCRIPTION) as m_SaveData like this:

virtual int        Save( CSave &save );
virtual int        Restore( CRestore &restore );
static    TYPEDESCRIPTION m_SaveData[];

Outside of the entity class, you define the save-restore table like this:

TYPEDESCRIPTION    CPendulum::m_SaveData[] =
{
    DEFINE_FIELD( CPendulum, m_accel, FIELD_FLOAT ),
    DEFINE_FIELD( CPendulum, m_distance, FIELD_FLOAT ),
    DEFINE_FIELD( CPendulum, m_time, FIELD_TIME ),
    DEFINE_FIELD( CPendulum, m_damp, FIELD_FLOAT ),
    DEFINE_FIELD( CPendulum, m_maxSpeed, FIELD_FLOAT ),
    DEFINE_FIELD( CPendulum, m_dampSpeed, FIELD_FLOAT ),
    DEFINE_FIELD( CPendulum, m_center, FIELD_VECTOR ),
    DEFINE_FIELD( CPendulum, m_start, FIELD_VECTOR ),
};

IMPLEMENT_SAVERESTORE( CPendulum, CBaseEntity );

Note: Valve generally placed this near either the class declaration or the LINK_ENTITY_TO_CLASS line, but it can be placed anywhere.

DEFINE_FIELD requires 3 parameters: the class being saved/restored, the attribute being saved/restored and its type. The following table contains all available types of variables you can save and restore. The types with an asterisk (*) are the ones you will see and likely use very often in the HL SDK :

Type	Description
FIELD_FLOAT*	A floating point value (0.2, 75.5) that is not related to time (use FIELD_TIME for that purpose)
FIELD_STRING*	A string ID (string_t - often the result of ALLOC_STRING)
FIELD_ENTITY	An entity offset (EOFFSET)
FIELD_CLASSPTR*	An entity class pointer (like CBaseEntity *)
FIELD_EHANDLE*	An entity handle (EHANDLE)
FIELD_EVARS	An entity variables pointer (EVARS *)
FIELD_EDICT	An entity dictionary pointer (edict_t *)
FIELD_VECTOR*	A vector (Vector, vec3_t, array of 3 float)
FIELD_POSITION_VECTOR	A world coordinate (fixed up across level transitions "automagically")
FIELD_POINTER	Arbitrary data pointer (scheduled to be removed by Valve but seems that isn't the case)
FIELD_INTEGER*	An integer (5, 3) or enum value.
FIELD_FUNCTION	A class function pointer (like Think, Touch, Use...)
FIELD_BOOLEAN*	A "boolean" value (see the warning below as to why boolean is between quotes)
FIELD_SHORT	A 2 byte integer value.
FIELD_CHARACTER	A single byte value.
FIELD_TIME*	Same as FIELD_FLOAT but for variables related to time (like weapons reload time), usually a floating point value that has a relation to the game time (gpGlobals->time)
FIELD_MODELNAME	An engine string that is a model's name (requires precaching).
FIELD_SOUNDNAME	Same as previous one but for sounds.

The last line, IMPLEMENT_SAVERESTORE requires 2 parameters: the class being saved and its parent. It tells the save/restore code to take care of your entity variables as well as the parent's ones. This is handy because it avoids having to redefine every variable from the parent(s).

ObjectCaps

A way of storing read-only flags for entities. Entities can override this to return certain constants that will make the entity usable by the player, or held down continuously, transferable between levels etc.

The constants can be:
FCAP_CUSTOMSAVE - unknown and unused
FCAP_ACROSS_TRANSITION - the entity will transfer across level transitions
FCAP_MUST_SPAWN - calls Spawn right after Restore, i.e. after loading a savefile
FCAP_DONT_SAVE - don't write into the savefile
FCAP_IMPULSE_USE - can be used by the player
FCAP_CONTINUOUS_USE - can be held by the player (such as levers and valves)
FCAP_ONOFF_USE - can be toggled by the player
FCAP_DIRECTIONAL_USE - receives +/- from the player, only used by func_tracktrain
FCAP_MASTER - entity can be used as a master (multisource has this cap)
FCAP_FORCE_TRANSITION - entity always goes across transitions

Active

Called on every entity after the server is activated. It is used, for example, by CFuncTrain/func_train to teleport itself to the first path_corner as soon as the server starts. It is rarely used.

Modular methods

These methods are actually callbacks.

void (CBaseEntity ::*m_pfnThink)(void);
void (CBaseEntity ::*m_pfnTouch)( CBaseEntity *pOther );
void (CBaseEntity ::*m_pfnUse)( CBaseEntity *pActivator, CBaseEntity *pCaller, USE_TYPE useType, float value );
void (CBaseEntity ::*m_pfnBlocked)( CBaseEntity *pOther );

This means that each can be changed at any time.

Think

Called every time period, or every tick, this method is often used by entities to perform real-time or otherwise periodic logic, e.g. check if a certain entity is within radius every 0.5 seconds.

To set a think callback, use the SetThink macro.

To set the next time an entity will "think", set the pev->nextthink variable.
Example:

pev->nextthink = gpGlobals->time + 0.5; // think every 0.5 seconds

Touch

Called every time the entity is touched by another entity. Entities such as trigger_once absolutely rely on this to work.

To set a touch callback, use the SetTouch macro.

Parameters:

• pOther - entity that touched this entity

Use

Called either by players when USE-ing an entity, or by other entities that have the ability to trigger other entities. For example, when a func_button triggers a func_door, it is calling the func_door's Use method.

To set the use callback, use the SetUse macro.

Parameters:

• pActivator - entity that started the trigger sequence
• pCaller - entity that actually fired this entity
• useType - type of usage: USE_TOGGLE, USE_ON, USE_OFF and USE_SET
• value - the value sent from pCaller, typically used only in func_tracktrain

Blocked

Called every time an entity blocks this entity.
This is essentially how doors and trains damage and gib things. They apply damage to any entity that blocked them.

Parameters:

• pOther - entity that blocked this entity

Static methods

These methods are useful in certain situations.

Create

Allocates a new entity of any classname and spawns it at given coordinates. Returns a CBaseEntity pointer to the newly spawned entity.

Parameters:

• szName - the classname itself, e.g. env_sprite, monster_zombie etc.
• vecOrigin - the position at which it'll spawn
• vecAngles - the angles it'll have after spawning
• pentOwner - owner of the newly created entity (NULL by default)

Instance

Retrieves private data from an existing entity. It is used to 'convert' edict_t*, entvars_t* or entity indices into CBaseEntity*.

Parameters:

• pent - entity dictionary/edict to convert from (edict_t* overload)
• pev - entity variables to convert from (entvars_t* overload)
• eoffset - entity index to retrieve from (int overload)

Dispatch functions

If you wonder how the engine actually calls these functions, it is done through the Dispatch functions:

DispatchSpawn
DispatchKeyValue
DispatchTouch
DispatchUse
DispatchThink
DispatchBlocked
DispatchSave
DispatchRestore
DispatchObjectCollsionBox

It locates their addresses when it loads hl.dll, and then calls them for each edict_t in its internal array of entities.

Other base classes and important entity classes

CWorld - worldspawn entity
CItem - base item class
CBaseDelay - generic delay entity, can be spawned when an entity tries to trigger another entity with a delay

• CBaseAnimating - entity that supports studio model animations, provides utilities for submodels, setting controllers etc.
  • CBasePlayerItem - generic player item
    • CBasePlayerWeapon - generic player weapon
  • CBaseToggle - generic toggleable entity, implements linear movement for trains, opening and closing mechanism for doors etc.
    • CBaseMonster - base monster class
      • CBasePlayer - player class

On the next page, you'll learn how to create a very basic entity that will print a mapper-defined message to the console.
Note: The next page isn't actually written yet.