mrmixer
Does anyone knows how Jonathan Blow's defer macro works ? Example of how he uses it at the end of this blog post.

1 2	auto name = filenames[i]; defer { gamelib_free_string(name); };

I found those two pages that show how to make defer macros, but I'm interested to know how Jonathan's curly braces syntax works.
http://the-witness.net/news/2012/11/scopeexit-in-c11/
http://www.gingerbill.org/article/defer-in-cpp.html

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struct DEFER_TAG {};
template< class Func >
ScopeExit< Func > operator+( DEFER_TAG, Func&& func )
{
    return MakeScopeExit< Func >( std::forward< Func >( func ) );
}

#define defer auto STRING_JOIN2(scope_exit_, __LINE__) = DEFER_TAG() + [&]()

I feel embarrassed replying to this thread. Firstly, because some forums don't like people raising old threads. Also because this is so basic, all the experienced programmers know it. But I think it will help a C beginner, so here it is.

The X Macro is mentioned above. The tutorials I found on the internet in the past said, it is an old trick that few know about. Then they give examples that aren't very interesting, like auto generate an enum and strings for debugging.

I was reading an old AI book by Buckland where the text is easy to understand, except he really bought into object oriented, C++ templates, and UML diagrams. So I thought I'd use Casey's method of using a fat struct and a switch statement to translate his C++ code to C.

What I found while converting his state machine example, was code that needs to be close together so you can reason about it, is far apart when using a big switch statement in different functions. Then it occurred to me to use X Macros to auto generate the switch statements with function calls, so I can put related code close together in functions.

This is the list of items the X Macros need to do their thing:

  #define ENTITY_STATE_LIST \
    X(ENTITY_STATE_NONE) \
    X(ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET) \
    X(ENTITY_STATE_VISIT_BANK_AND_DEPOSIT_GOLD) \
    X(ENTITY_STATE_GO_HOME_AND_SLEEP_TIL_RESTED) \
    X(ENTITY_STATE_QUENCH_THIRST) \
    X(ENTITY_STATE_EAT_STEW) \
    X(ENTITY_STATE_WIFES_GLOBAL) \
    X(ENTITY_STATE_DO_HOUSE_WORK) \
    X(ENTITY_STATE_VISIT_BATHROOM) \
    X(ENTITY_STATE_COOK_STEW)

The first X Macro generates an enum, the second makes strings that match the enum.

enum entity_state {
  #define X(name) name,
    ENTITY_STATE_LIST
  #undef X
  ENTITY_STATES_MAX
};


static const char *entity_state_str[] = {
  #define X(name) "name",
    ENTITY_STATE_LIST
  #undef X
  "ENTITY_STATES_MAX"
};

Which generates these:

enum entity_state {
        ENTITY_STATE_NONE,
        ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET,
        ENTITY_STATE_VISIT_BANK_AND_DEPOSIT_GOLD,
        ENTITY_STATE_GO_HOME_AND_SLEEP_TIL_RESTED,
        ENTITY_STATE_QUENCH_THIRST,
        ENTITY_STATE_EAT_STEW,
        ENTITY_STATE_WIFES_GLOBAL,
        ENTITY_STATE_DO_HOUSE_WORK,
        ENTITY_STATE_VISIT_BATHROOM,
        ENTITY_STATE_COOK_STEW,
        ENTITY_STATES_MAX
};

static const char *entity_state_str[] = {
        "ENTITY_STATE_NONE",
        "ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET",
        "ENTITY_STATE_VISIT_BANK_AND_DEPOSIT_GOLD",
        "ENTITY_STATE_GO_HOME_AND_SLEEP_TIL_RESTED",
        "ENTITY_STATE_QUENCH_THIRST",
        "ENTITY_STATE_EAT_STEW",
        "ENTITY_STATE_WIFES_GLOBAL",
        "ENTITY_STATE_DO_HOUSE_WORK",
        "ENTITY_STATE_VISIT_BATHROOM",
        "ENTITY_STATE_COOK_STEW",
        "ENTITY_STATES_MAX"
};

Here are two X Macros that make switch statements that call functions related to the enum above:

        switch (state) {
#define X(name) case name: name##_enter(entity); break;
                ENTITY_STATE_LIST
#undef X
        case ENTITY_STATES_MAX:
                break;
        }


        switch (state) {
#define X(name) case name: name##_execute(entity); break;
                ENTITY_STATE_LIST
#undef X
        case ENTITY_STATES_MAX:
                break;
        }

Which makes these:

switch (state) {
case ENTITY_STATE_NONE: ENTITY_STATE_NONE_enter(entity); break;
case ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET: ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET_enter(entity); break;
...
}

switch (state) {
case ENTITY_STATE_NONE: ENTITY_STATE_NONE_execute(entity); break;
case ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET: ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET_execute(entity); break;
...
}

Then in another file if you want, you write the functions the generated switch statements call.

internal void
ENTITY_STATE_NONE_enter(struct entity *entity)
{
 // Your code here
}

internal void
ENTITY_STATE_NONE_execute(struct entity *entity)
{
 // Your code here
}



internal void
ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET_enter(struct entity *entity)
{
 // Your code here
}

internal void
ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET_execute(struct entity *entity)
{
 // Your code here
}

When looking around this forum I found a blog by Cakelisp that mentioned them, then I found a link that actually has some good examples.

https://en.wikibooks.org/wiki/C_Programming/Preprocessor_directives_and_macros#X-Macros

So C has inheritance just as easy to read as C++, without using pointers. Well the X Macro part is ugly, but it is the same thing over and over.

Mods, the above post is messed up. Is there a page here that gives instructions as to what tags to use to mark code?

We support markdown, so you can do the usual triple-backtick. For example:

```c

#define ENTITY_STATE_LIST \
X(ENTITY_STATE_NONE) \
X(ENTITY_STATE_ENTER_MINE_AND_DIG_FOR_NUGGET) \
X(ENTITY_STATE_VISIT_BANK_AND_DEPOSIT_GOLD) \
X(ENTITY_STATE_GO_HOME_AND_SLEEP_TIL_RESTED) \
X(ENTITY_STATE_QUENCH_THIRST) \
X(ENTITY_STATE_EAT_STEW) \
X(ENTITY_STATE_WIFES_GLOBAL) \
X(ENTITY_STATE_DO_HOUSE_WORK) \
X(ENTITY_STATE_VISIT_BATHROOM) \
X(ENTITY_STATE_COOK_STEW) 

```

Thank you! :)