5.12.5.2.2.2.2. 缺省实参
如果耗尽了 arg ,但 parm 还没有, parm 必然包含了缺省实参,该形参列表应该以特殊节点 void_list_node 来结尾。那么在对应的节点中, TREE_VALUE 保存了该类型,而 TREE_PURPOSE 是缺省实参的表达式。
4258 tree
4259 convert_default_arg (tree type, tree arg, tree fn, int parmnum) in call.c
4260 {
4261 /* If the ARG is an unparsed default argument expression, the
4262 conversion cannot be performed. */
4263 if (TREE_CODE (arg) == DEFAULT_ARG)
4264 {
4265 error ("the default argument for parameter %d of `%D' has "
4266 "not yet been parsed",
4267 parmnum, fn);
4268 return error_mark_node;
4269 }
4270
4271 if (fn && DECL_TEMPLATE_INFO (fn))
4272 arg = tsubst_default_argument (fn, type, arg);
4273
4274 arg = break_out_target_exprs (arg);
4275
4276 if (TREE_CODE (arg) == CONSTRUCTOR)
4277 {
4278 arg = digest_init (type, arg, 0);
4279 arg = convert_for_initialization (0, type, arg, LOOKUP_NORMAL,
4280 "default argument", fn, parmnum);
4281 }
4282 else
4283 {
4284 /* This could get clobbered by the following call. */
4285 if (TREE_HAS_CONSTRUCTOR (arg))
4286 arg = copy_node (arg);
4287
4288 arg = convert_for_initialization (0, type, arg, LOOKUP_NORMAL,
4289 "default argument", fn, parmnum);
4290 arg = convert_for_arg_passing (type, arg);
4291 }
4292
4293 return arg;
4294 }
节点 DEFAULT_ARG 是为未解析的缺省实参所构建的。记得在解析类定义的过程中,缺省实参被 DEFAULT_ARG 所缓存,它将在该解析结束后才解析。因此在这里不应该出现 DEFAULT_ARG (如果是这样,这可能是在类定义中缺少了“ }; ”),而且在这一点上,前端不知道如何处理这种节点。
那么在 4274 传递给下面函数的 arg ,是由该函数所有调用所共享的缺省实参。不过,正如【 3 】所定义的,“每次调用该函数都要评估缺省实参 ”,因此在真正评估这个缺省实参之前,需要 准备合适的 arg 的拷贝(我们不能直接改变 arg ),并如下所示地更新这个局部临时对象。
1259 tree
1260 break_out_target_exprs (tree t) in cp/tree.c
1261 {
1262 static int target_remap_count;
1263 static splay_tree target_remap;
1264
1265 if (!target_remap_count++)
1266 target_remap = splay_tree_new (splay_tree_compare_pointers,
1267 /*splay_tree_delete_key_fn=*/ NULL,
1268 /*splay_tree_delete_value_fn=*/ NULL);
1269 walk_tree (&t, bot_manip , target_remap, NULL);
1270 walk_tree (&t, bot_replace , target_remap, NULL);
1271
1272 if (!--target_remap_count)
1273 {
1274 splay_tree_delete (target_remap);
1275 target_remap = NULL;
1276 }
1277
1278 return t;
1279 }
上面, walk_tree 遍历以 t 为根的子树,并在以 t 的编码所选出的节点上执行给定的函数。在第一次遍历中,使用下面的函数。注意该函数总是返回 NULL (下面的 copy_tree_r 返回 NULL )来强制 walk_tree 执行深度优先的完整遍历(即, tp 可能是一个 tree_list ,其中的节点可以包含操作数,它们依次亦可能是 tree_list ,等等,访问将从底部开始向上);不过是否进入子树(即操作数)由局部变量 walk_subtrees 来控制(它在下面的函数中作为实参 walk_subtrees 传入)。在使用指定的函数处理树节点前, walk_subtrees 被设置为 1 ;是指定的函数来决定该节点是否是感兴趣的,并且需要进入。
1182 static tree
1183 bot_manip (tree* tp, int* walk_subtrees, void* data) in cp/tree.c
1184 {
1185 splay_tree target_remap = ((splay_tree) data);
1186 tree t = *tp;
1187
1188 if (TREE_CONSTANT (t))
1189 {
1190 /* There can't be any TARGET_EXPRs or their slot variables below
1191 this point. We used to check !TREE_SIDE_EFFECTS, but then we
1192 failed to copy an ADDR_EXPR of the slot VAR_DECL. */
1193 *walk_subtrees = 0;
1194 return NULL_TREE;
1195 }
1196 if (TREE_CODE (t) == TARGET_EXPR)
1197 {
1198 tree u;
1199
1200 if (TREE_CODE (TREE_OPERAND (t, 1)) == AGGR_INIT_EXPR)
1201 {
1202 mark_used (TREE_OPERAND (TREE_OPERAND (TREE_OPERAND (t, 1), 0), 0));
1203 u = build_cplus_new
1204 (TREE_TYPE (t), break_out_target_exprs (TREE_OPERAND (t, 1)));
1205 }
1206 else
1207 {
1208 u = build_target_expr_with_type
1209 (break_out_target_exprs (TREE_OPERAND (t, 1)), TREE_TYPE (t));
1210 }
1211
1212 /* Map the old variable to the new one. */
1213 splay_tree_insert (target_remap,
1214 (splay_tree_key) TREE_OPERAND (t, 0),
1215 (splay_tree_value) TREE_OPERAND (u, 0));
1216
1217 /* Replace the old expression with the new version. */
1218 *tp = u;
1219 /* We don't have to go below this point; the recursive call to
1220 break_out_target_exprs will have handled anything below this
1221 point. */
1222 *walk_subtrees = 0;
1223 return NULL_TREE;
1224 }
1225 else if (TREE_CODE (t) == CALL_EXPR)
1226 mark_used (TREE_OPERAND (TREE_OPERAND (t, 0), 0));
1227
1228 /* Make a copy of this node. */
1229 return copy_tree_r (tp, walk_subtrees, NULL);
1230 }
对于常量或 TRAGET_EXPR 以外的节点, copy_tree_r 拷贝这个节点,如果它是 *_CST (看到它其实被上面的 TREE_CONSTANT 滤掉了),或表达式,或 TREE_LIST ,或 TREE_VEC ,或 OVERLOAD (由下面的 C++ 的钩子 tree_chain_matters_p 来辨别,并注意到在这里 walk_subtrees 没有改变, walk_tree 将进入该节点的子节点,并继续拷贝其结构)。
1966 tree
1967 copy_tree_r (tree *tp, int *walk_subtrees, void *data ATTRIBUTE_UNUSED) in tree-inline.c
1968 {
1969 enum tree_code code = TREE_CODE (*tp);
1970
1971 /* We make copies of most nodes. */
1972 if (IS_EXPR_CODE_CLASS (TREE_CODE_CLASS (code))
1973 || TREE_CODE_CLASS (code) == 'c'
1974 || code == TREE_LIST
1975 || code == TREE_VEC
1976 || (*lang_hooks .tree_inlining.tree_chain_matters_p) (*tp))
1977 {
1978 /* Because the chain gets clobbered when we make a copy, we save it
1979 here. */
1980 tree chain = TREE_CHAIN (*tp);
1981
1982 /* Copy the node. */
1983 *tp = copy_node (*tp);
1984
1985 /* Now, restore the chain, if appropriate. That will cause
1986 walk_tree to walk into the chain as well. */
1987 if (code == PARM_DECL || code == TREE_LIST
1988 #ifndef INLINER_FOR_JAVA
1989 || (*lang_hooks .tree_inlining.tree_chain_matters_p) (*tp)
1990 || STATEMENT_CODE_P (code))
1991 TREE_CHAIN (*tp) = chain;
1992
1993 /* For now, we don't update BLOCKs when we make copies. So, we
1994 have to nullify all scope-statements. */
1995 if (TREE_CODE (*tp) == SCOPE_STMT)
1996 SCOPE_STMT_BLOCK (*tp) = NULL_TREE;
1997 #else /* INLINER_FOR_JAVA */
1998 || (*lang_hooks .tree_inlining.tree_chain_matters_p) (*tp))
1999 TREE_CHAIN (*tp) = chain;
2000 #endif /* INLINER_FOR_JAVA */
2001 }
2002 else if (TREE_CODE_CLASS (code) == 't')
2003 *walk_subtrees = 0;
2004
2005 return NULL_TREE;
2006 }
一个 TARGET_EXPR 代表一个临时对象。其第一个操作数是表示这个临时对象的一个 VAR_DECL 。其第二个操作数是该临时对象的初始值。该初始值被评估,然后拷贝入这个临时对象。
一个 AGGR_INIT_EXPR 代表,作为一个函数调用返回值,或一个构造函数结果的初始化。一个 AGGR_INIT_EXPR 将仅出现作为一个 TARGET_EXPR 的第二个操作数。该 AGGR_INIT_EXPR 的第一个操作数是所调用函数的地址,就像在一个 CALL_EXPR 那样。第二个操作数是以一个 TREE_LIST 形式传递给该函数的实参,同样类似于在一个 CALL_EXPR 里那样。这个表达式的值由这个函数返回。
那么如果 AGGR_INIT_EXPR 被用于 TRAGET_EXPR 里,递归 break_out_target_exprs 来拷贝这个节点。对于这个拷贝过来的表达式, build_cplus_new 为其初始化产生了代码。
2007 tree
2008 build_cplus_new (tree type, tree init) in cp/tree.c
2009 {
2010 tree fn;
2011 tree slot;
2012 tree rval;
2013 int is_ctor;
2014
2015 /* Make sure that we're not trying to create an instance of an
2016 abstract class. */
2017 abstract_virtuals_error (NULL_TREE, type);
2018
2019 if (TREE_CODE (init) != CALL_EXPR && TREE_CODE (init) != AGGR_INIT_EXPR)
2020 return convert (type, init);
2021
2022 fn = TREE_OPERAND (init, 0);
2023 is_ctor = (TREE_CODE (fn) == ADDR_EXPR
2024 && TREE_CODE (TREE_OPERAND (fn, 0)) == FUNCTION_DECL
2025 && DECL_CONSTRUCTOR_P (TREE_OPERAND (fn, 0)));
2026
2027 slot = build_local_temp (type);
2028
2029 /* We split the CALL_EXPR into its function and its arguments here.
2030 Then, in expand_expr, we put them back together. The reason for
2031 this is that this expression might be a default argument
2032 expression. In that case, we need a new temporary every time the
2033 expression is used. That's what break_out_target_exprs does; it
2034 replaces every AGGR_INIT_EXPR with a copy that uses a fresh
2035 temporary slot. Then, expand_expr builds up a call-expression
2036 using the new slot. */
2037
2038 /* If we don't need to use a constructor to create an object of this
2039 type, don't mess with AGGR_INIT_EXPR. */
2040 if (is_ctor || TREE_ADDRESSABLE (type))
2041 {
2042 rval = build (AGGR_INIT_EXPR, type, fn, TREE_OPERAND (init, 1), slot);
2043 TREE_SIDE_EFFECTS (rval) = 1;
2044 AGGR_INIT_VIA_CTOR_P (rval) = is_ctor;
2045 }
2046 else
2047 rval = init;
2048
2049 rval = build_target_expr (slot, rval);
2050
2051 return rval;
2052 }
现在需要更新在 TARGET_EXPR 中的临时对象,因为我们不是在产生原始的 TARGET_EXPR 的上下文中。看到这个临时对象成为了局部的,其 DECL_CONTEXT 被强制设置为 current_function_decl 。
253 static tree
254 build_local_temp (tree type) in cp/tree.c
255 {
256 tree slot = build_decl (VAR_DECL, NULL_TREE, type);
257 DECL_ARTIFICIAL (slot) = 1;
258 DECL_CONTEXT (slot) = current_function_decl ;
259 layout_decl (slot, 0);
260 return slot;
261 }
注意如果对于这个 AGGR_INIT_EXPR , AGGR_INIT_VIA_CTOR_P 成立,表示这个初始化是通过一个构造函数调用实现。在 2042 行所构建的 AGGR_INIT_EXPR ,其第三个操作数是这个临时对象,它总是一个 VAR_DECL 。而 init 是原来 TARGET_EXPR 中的 AGGR_INIT_EXPR (这个 AGGR_INIT_EXPR 的第三个操作数的地址被提取,并替换实参列表中的值。这个情况下,该表达式的值是提供给这个 AGGR_INIT_EXPR 的 VAR_DECL ;构造函数不返回值)。
最后,这个新构建的 TARGET_EXPR 被 build_cplus_new 返回。
而对于 TARGET_EXPR 的第二个操作数不是 AGGR_INIT_EXPR 的情况,该操作数由 build_target_expr_with_type 来处理。这里如果 init 是 TARGET_EXPR ,它必定是由在 bot_manip 中 1209 行的 break_out_target_exprs 所构建的,这个节点正是我们期望的。
320 tree
321 build_target_expr_with_type (tree init, tree type) in cp/tree.c
322 {
323 tree slot;
324
325 if (TREE_CODE (init) == TARGET_EXPR)
326 return init;
327 else if (CLASS_TYPE_P (type) && !TYPE_HAS_TRIVIAL_INIT_REF (type)
328 && TREE_CODE (init) != COND_EXPR
329 && TREE_CODE (init) != CONSTRUCTOR
330 && TREE_CODE (init) != VA_ARG_EXPR)
331 /* We need to build up a copy constructor call. COND_EXPR is a special
332 case because we already have copies on the arms and we don't want
333 another one here. A CONSTRUCTOR is aggregate initialization, which
334 is handled separately. A VA_ARG_EXPR is magic creation of an
335 aggregate; there's no additional work to be done. */
336 return force_rvalue (init);
337
338 slot = build_local_temp (type);
339 return build_target_expr (slot, init);
340 }
上面,如果 TYPE_HAS_TRIVIAL_INIT_REF 不是 0 ,表示该拷贝初始化可以使用按位拷贝。对于这样的情形,可以简单地构建 TARGET_EXPR ;否则,需要执行一个左值到右值的转换,包括如下所示的拷贝构造函数调用。
590 tree
591 force_rvalue (tree expr) in cvt.c
592 {
593 if (IS_AGGR_TYPE (TREE_TYPE (expr)) && TREE_CODE (expr) != TARGET_EXPR)
594 expr = ocp_convert (TREE_TYPE (expr), expr,
595 CONV_IMPLICIT|CONV_FORCE_TEMP, LOOKUP_NORMAL);
596 else
597 expr = decay_conversion (expr);
598
599 return expr;
600 }
不久之后我们将看到 ocp_convert 的细节。在这里总而言之,该函数将产生调用合适的拷贝构造函数代码,然后调用 build_cplus_new 来产生构建这个临时对象及其初始化。在离开 bot_manip 之前,看一下 build_target_expr 。
234 static tree
235 build_target_expr (tree decl, tree value) in cp/tree.c
236 {
237 tree t;
238
239 t = build (TARGET_EXPR, TREE_TYPE (decl), decl, value,
240 cxx_maybe_build_cleanup (decl), NULL_TREE);
241 /* We always set TREE_SIDE_EFFECTS so that expand_expr does not
242 ignore the TARGET_EXPR. If there really turn out to be no
243 side-effects, then the optimizer should be able to get rid of
244 whatever code is generated anyhow. */
245 TREE_SIDE_EFFECTS (t) = 1;
246
247 return t;
248 }
对于具有非平凡析构函数的临时对象,编译器需要产生代码,在其越出其作用域时,通过调用这个析构函数摧毁这个临时对象。因此在 240 行, cxx_maybe_build_cleanup 产生这些代码,如果需要的话。
现在在 bot_manip 的 1213 行, u 是对应于 t 的 TARGET_EXPR 。它把新旧版本的临时对象映射起来。然后,我们立即用新版本取代了旧版本。不过,在由 break_out_target_exprs 处理的节点的某些子节点中,可能仍然保留了这个旧版本的引用,它们需要如下的更新。
1236 static tree
1237 bot_replace (tree* t, in cp/tree.c
1238 int* walk_subtrees ATTRIBUTE_UNUSED ,
1239 void* data)
1240 {
1241 splay_tree target_remap = ((splay_tree) data);
1242
1243 if (TREE_CODE (*t) == VAR_DECL)
1244 {
1245 splay_tree_node n = splay_tree_lookup (target_remap,
1246 (splay_tree_key) *t);
1247 if (n)
1248 *t = (tree) n->value;
1249 }
1250
1251 return NULL_TREE;
1252 }
回到 convert_default_arg ,在 4274 行从 break_out_target_exprs 得到这个更新的 arg ,然后接下来的函数用于产生初始化代码。
5.12.5.2.2.2.3. 省略实参
最后一个的可能就是省略实参,注意到省略实参与缺省实参不可共存。在前端中,为了识别包含了省略实参的函数声明,形参列表由 NULL ,而不是 void_list_node 来结尾。
4161 tree
4162 convert_arg_to_ellipsis (tree arg) in call.c
4163 {
4164 /* [expr.call]
4165
4166 The lvalue-to-rvalue, array-to-pointer, and function-to-pointer
4167 standard conversions are performed. */
4168 arg = decay_conversion (arg);
4169 /* [expr.call]
4170
4171 If the argument has integral or enumeration type that is subject
4172 to the integral promotions (_conv.prom_), or a floating point
4173 type that is subject to the floating point promotion
4174 (_conv.fpprom_), the value of the argument is converted to the
4175 promoted type before the call. */
4176 if (TREE_CODE (TREE_TYPE (arg)) == REAL_TYPE
4177 && (TYPE_PRECISION (TREE_TYPE (arg))
4178 < TYPE_PRECISION (double_type_node)))
4179 arg = convert_to_real (double_type_node, arg);
4180 else if (INTEGRAL_OR_ENUMERATION_TYPE_P (TREE_TYPE (arg)))
4181 arg = perform_integral_promotions (arg);
4182
4183 arg = require_complete_type (arg);
4184
4185 if (arg != error_mark_node
4186 && !pod_type_p (TREE_TYPE (arg)))
4187 {
4188 /* Undefined behavior [expr.call] 5.2.2/7. We used to just warn
4189 here and do a bitwise copy, but now cp_expr_size will abort if we
4190 try to do that.
4191 If the call appears in the context of a sizeof expression,
4192 there is no need to emit a warning, since the expression won't be
4193 evaluated. We keep the builtin_trap just as a safety check. */
4194 if (!skip_evaluation )
4195 warning ("cannot pass objects of non-POD type `%#T' through `...'; "
4196 "call will abort at runtime", TREE_TYPE (arg));
4197 arg = call_builtin_trap ();
4198 arg = build (COMPOUND_EXPR, integer_type_node, arg,
4199 integer_zero_node);
4200 }
4201
4202 return arg;
4203 }
【 3 】的条文 5.2.2 “函数调用”,条款 7 如下定义了编译器在省略实参上的行为。
7. 当一个给定的实参没有对应的形参,该实参被以这样的方式传入——接受函数可以通过调用 va_arg ( 18.7 )来得到该实参值。左值到右值( 4.1 ),数组到指针( 4.2 ),及函数到指针( 4.3 )标准转换在该实参表达式上执行。在这些转换之后,如果该实参不是数字,枚举值,指针,成员的指针,或类类型,该程序非法。如果该实参具有一个 non-POD 类类型(条文 9 ),其行为是未定义的。如果该实参具有适用于整型提升( 4.5 )的整型或枚举类型,或适用于浮点提升( 4.6 )的浮点类型,在该调用之前,该实参的值被转换到提升后的类型。这些提升被称为缺省实参提升 。 |
函数 decay_conversion 执行在 exp 中的转换,它应用在当一个左值出现在一个右值上下文中时候。包括左值到右值,数组到指针,及函数到指针的转换。
1335 tree
1336 decay_conversion (tree exp) n typeck.c
1337 {
1338 tree type;
1339 enum tree_code code;
1340
1341 type = TREE_TYPE (exp);
1342 code = TREE_CODE (type);
1343
1344 if (code == REFERENCE_TYPE)
1345 {
1346 exp = convert_from_reference (exp);
1347 type = TREE_TYPE (exp);
1348 code = TREE_CODE (type);
1349 }
1350
1351 if (type == error_mark_node)
1352 return error_mark_node;
1353
1354 if (type_unknown_p (exp))
1355 {
1356 cxx_incomplete_type_error (exp, TREE_TYPE (exp));
1357 return error_mark_node;
1358 }
1359
1360 /* Constants can be used directly unless they're not loadable. */
1361 if (TREE_CODE (exp) == CONST_DECL)
1362 exp = DECL_INITIAL (exp);
1363 /* Replace a nonvolatile const static variable with its value. We
1364 don't do this for arrays, though; we want the address of the
1365 first element of the array, not the address of the first element
1366 of its initializing constant. */
1367 else if (code != ARRAY_TYPE)
1368 {
1369 exp = decl_constant_value (exp);
1370 type = TREE_TYPE (exp);
1371 }
1372
1373 /* build_c_cast puts on a NOP_EXPR to make the result not an lvalue.
1374 Leave such NOP_EXPRs, since RHS is being used in non-lvalue context. */
1375
1376 if (code == VOID_TYPE)
1377 {
1378 error ("void value not ignored as it ought to be");
1379 return error_mark_node;
1380 }
1381 if (invalid_nonstatic_memfn_p (exp))
1382 return error_mark_node;
1383 if (code == FUNCTION_TYPE || is_overloaded_fn (exp))
1384 return build_unary_op (ADDR_EXPR, exp, 0);
1385 if (code == ARRAY_TYPE)
1386 {
1387 tree adr;
1388 tree ptrtype;
1389
1390 if (TREE_CODE (exp) == INDIRECT_REF)
1391 return build_nop (build_pointer_type (TREE_TYPE (type)),
1392 TREE_OPERAND (exp, 0));
1393
1394 if (TREE_CODE (exp) == COMPOUND_EXPR)
1395 {
1396 tree op1 = decay_conversion (TREE_OPERAND (exp, 1));
1397 return build (COMPOUND_EXPR, TREE_TYPE (op1),
1398 TREE_OPERAND (exp, 0), op1);
1399 }
1400
1401 if (!lvalue_p (exp)
1402 && ! (TREE_CODE (exp) == CONSTRUCTOR && TREE_STATIC (exp)))
1403 {
1404 error ("invalid use of non-lvalue array");
1405 return error_mark_node;
1406 }
1407
1408 ptrtype = build_pointer_type (TREE_TYPE (type));
1409
1410 if (TREE_CODE (exp) == VAR_DECL)
1411 {
1412 if (!cxx_mark_addressable (exp))
1413 return error_mark_node;
1414 adr = build_nop (ptrtype, build_address (exp));
1415 TREE_SIDE_EFFECTS (adr) = 0; /* Default would be, same as EXP. */
1416 return adr;
1417 }
1418 /* This way is better for a COMPONENT_REF since it can
1419 simplify the offset for a component. */
1420 adr = build_unary_op (ADDR_EXPR, exp, 1);
1421 return cp_convert (ptrtype, adr);
1422 }
1423
1424 /* [basic.lval]: Class rvalues can have cv-qualified types; non-class
1425 rvalues always have cv-unqualified types. */
1426 if (! CLASS_TYPE_P (type))
1427 exp = cp_convert (TYPE_MAIN_VARIANT (type), exp);
1428
1429 return exp;
1430 }
首先对于 REFERENCE_TYPE ,它是一个左值,要把它转换为右值,就应该使用被引用的值,而不再保存引用。在前端中, INDIRECT_REF 为这个目标而构建(这是因为引用总是传入其地址就像指针那样;而在编译器中,引用的模式( mode )与指针的相同,都是 ptr_mode ,参见 build_reference_type )。
566 tree
567 convert_from_reference (tree val) in cvt.c
568 {
569 if (TREE_CODE (TREE_TYPE (val)) == REFERENCE_TYPE)
570 return build_indirect_ref (val, NULL);
571 return val;
572 }
记得指针是右值。当处理 ARRAY_TYPE 时,如果我们正在使用形如: int *a[i] ( a 是维度大于 1 的数组,例如, int A[2][2] )的表达式,满足 1390 行的条件, 因此构建了形如: int** 的类型,并且看到转换指针“ int *a[i] ”到“ int** ”不需要产生任何代码,因而为这个转换构建了 NOP_EXPR 。
而如果 exp 只是一个数组的声明,例如: int a[8] ,这个声明的右值是“ int* ”。可以直接构建这个右值。不过,对于其他的情形,例如:“ tempA.a ”(这是一棵以 SCOPE_REF 为根节点的树),就没有可以直接使用的简单规则,因此调用 build_unary_op 及 cp_convert 来执行合适的转换。
正如在 convert_arg_to_ellipsis 的 4188 行的注释所提到的,对于对应于 non-POD 类类型的未定义行为, GCC 以前使用按位拷贝,不过在当前版本中,这个行为在后面对 cp_expr_size 的调用将导致异常终止。这个终止由下面“ __builtin_trap ”的调用触发,在 4198 行它成为了 COMPOUND_EXPR 形式的 arg 的一部分。
4148 c tree
4149 call_builtin_trap (void) in call.c
4150 {
4151 tree fn = IDENTIFIER_GLOBAL_VALUE (get_identifier ("__builtin_trap"));
4152
4153 my_friendly_assert (fn != NULL, 20030927);
4154 fn = build_call (fn, NULL_TREE);
4155 return fn;
4156 }
内建陷阱的行为是:如果目标机器定义了陷阱指令,就使用它;否则编译器将调用 abort () (参考 expand_builtin_trap )。
下面是对 non-POD 类类型省略实参的一个有趣的测试:
#include <stdarg.h>
class A {
public : virtual void func() {}
};
int func (int a, ...) {
va_list ap;
va_start(ap, a) ;
va_arg(ap, A);
va_end(ap);
return 1;
}
int main() {
A a;
func (1, a); // sizeof (func (1, a))
}
编译器将给出以下警告:
test2.cpp: In function ‘int func(int, ...)’:
test2.cpp:11: warning: cannot receive objects of non-POD type ‘class A’ through ‘...’; call will abort at runtime
test2.cpp: In function ‘int main()’:
test2.cpp:19: warning: cannot pass objects of non-POD type ‘class A’ through ’...’; call will abort at runtime
当执行这个程序时,得到错误: Illegal instruction.
不过,如果我们使用注释中的语句,编译器将给出警告:
test2.cpp: In function ‘int func(int, ...)’:
test2.cpp:11: warning: cannot receive objects of non-POD type ‘class A’ through ‘...’; call will abort at runtime
而在执行时,没有产生错误,因为 func(1, a) 没有被评估,除了其返回值。