damus/damus-c/typesafe_cb.h

/* CC0 (Public domain) - see LICENSE file for details */
#ifndef CCAN_TYPESAFE_CB_H
#define CCAN_TYPESAFE_CB_H
#include "config.h"

#if HAVE_TYPEOF && HAVE_BUILTIN_CHOOSE_EXPR && HAVE_BUILTIN_TYPES_COMPATIBLE_P
/**
 * typesafe_cb_cast - only cast an expression if it matches a given type
 * @desttype: the type to cast to
 * @oktype: the type we allow
 * @expr: the expression to cast
 *
 * This macro is used to create functions which allow multiple types.
 * The result of this macro is used somewhere that a @desttype type is
 * expected: if @expr is exactly of type @oktype, then it will be
 * cast to @desttype type, otherwise left alone.
 *
 * This macro can be used in static initializers.
 *
 * This is merely useful for warnings: if the compiler does not
 * support the primitives required for typesafe_cb_cast(), it becomes an
 * unconditional cast, and the @oktype argument is not used.  In
 * particular, this means that @oktype can be a type which uses the
 * "typeof": it will not be evaluated if typeof is not supported.
 *
 * Example:
 *    // We can take either an unsigned long or a void *.
 *    void _set_some_value(void *val);
 *    #define set_some_value(e)            \
 *        _set_some_value(typesafe_cb_cast(void *, unsigned long, (e)))
 */
#define typesafe_cb_cast(desttype, oktype, expr)            \
    __builtin_choose_expr(                        \
        __builtin_types_compatible_p(__typeof__(0?(expr):(expr)), \
                         oktype),            \
        (desttype)(expr), (expr))
#else
#define typesafe_cb_cast(desttype, oktype, expr) ((desttype)(expr))
#endif

/**
 * typesafe_cb_cast3 - only cast an expression if it matches given types
 * @desttype: the type to cast to
 * @ok1: the first type we allow
 * @ok2: the second type we allow
 * @ok3: the third type we allow
 * @expr: the expression to cast
 *
 * This is a convenient wrapper for multiple typesafe_cb_cast() calls.
 * You can chain them inside each other (ie. use typesafe_cb_cast()
 * for expr) if you need more than 3 arguments.
 *
 * Example:
 *    // We can take either a long, unsigned long, void * or a const void *.
 *    void _set_some_value(void *val);
 *    #define set_some_value(expr)                    \
 *        _set_some_value(typesafe_cb_cast3(void *,,        \
 *                        long, unsigned long, const void *,\
 *                        (expr)))
 */
#define typesafe_cb_cast3(desttype, ok1, ok2, ok3, expr)        \
    typesafe_cb_cast(desttype, ok1,                    \
             typesafe_cb_cast(desttype, ok2,        \
                      typesafe_cb_cast(desttype, ok3, \
                               (expr))))

/**
 * typesafe_cb - cast a callback function if it matches the arg
 * @rtype: the return type of the callback function
 * @atype: the (pointer) type which the callback function expects.
 * @fn: the callback function to cast
 * @arg: the (pointer) argument to hand to the callback function.
 *
 * If a callback function takes a single argument, this macro does
 * appropriate casts to a function which takes a single atype argument if the
 * callback provided matches the @arg.
 *
 * It is assumed that @arg is of pointer type: usually @arg is passed
 * or assigned to a void * elsewhere anyway.
 *
 * Example:
 *    void _register_callback(void (*fn)(void *arg), void *arg);
 *    #define register_callback(fn, arg) \
 *        _register_callback(typesafe_cb(void, (fn), void*, (arg)), (arg))
 */
#define typesafe_cb(rtype, atype, fn, arg)            \
    typesafe_cb_cast(rtype (*)(atype),            \
             rtype (*)(__typeof__(arg)),        \
             (fn))

/**
 * typesafe_cb_preargs - cast a callback function if it matches the arg
 * @rtype: the return type of the callback function
 * @atype: the (pointer) type which the callback function expects.
 * @fn: the callback function to cast
 * @arg: the (pointer) argument to hand to the callback function.
 *
 * This is a version of typesafe_cb() for callbacks that take other arguments
 * before the @arg.
 *
 * Example:
 *    void _register_callback(void (*fn)(int, void *arg), void *arg);
 *    #define register_callback(fn, arg)                   \
 *        _register_callback(typesafe_cb_preargs(void, void *,       \
 *                   (fn), (arg), int),               \
 *                   (arg))
 */
#define typesafe_cb_preargs(rtype, atype, fn, arg, ...)            \
    typesafe_cb_cast(rtype (*)(__VA_ARGS__, atype),            \
             rtype (*)(__VA_ARGS__, __typeof__(arg)),    \
             (fn))

/**
 * typesafe_cb_postargs - cast a callback function if it matches the arg
 * @rtype: the return type of the callback function
 * @atype: the (pointer) type which the callback function expects.
 * @fn: the callback function to cast
 * @arg: the (pointer) argument to hand to the callback function.
 *
 * This is a version of typesafe_cb() for callbacks that take other arguments
 * after the @arg.
 *
 * Example:
 *    void _register_callback(void (*fn)(void *arg, int), void *arg);
 *    #define register_callback(fn, arg) \
 *        _register_callback(typesafe_cb_postargs(void, (fn), void *, \
 *                   (arg), int),                    \
 *                   (arg))
 */
#define typesafe_cb_postargs(rtype, atype, fn, arg, ...)        \
    typesafe_cb_cast(rtype (*)(atype, __VA_ARGS__),            \
             rtype (*)(__typeof__(arg), __VA_ARGS__),    \
             (fn))
#endif /* CCAN_CAST_IF_TYPE_H */