From Jason Turner

[cpp.subst]

C++17 → C++20 5.1 KB 124 lines
Current Draft | LaTeX Source
Archived: C++17 | C++20
Markdown: C++17 | C++20
Explore in Adventure Game

Diff to HTML by rtfpessoa

Files changed (1) hide show
  1. tmp/tmpmoj9oz6p/{from.md → to.md} +114 -7
tmp/tmpmoj9oz6p/{from.md → to.md} RENAMED
@@ -1,15 +1,122 @@
1
  ### Argument substitution <a id="cpp.subst">[[cpp.subst]]</a>
2
 
 
 
 
 
 
3
  After the arguments for the invocation of a function-like macro have
4
- been identified, argument substitution takes place. A parameter in the
5
- replacement list, unless preceded by a `#` or `##` preprocessing token
6
- or followed by a `##` preprocessing token (see below), is replaced by
7
- the corresponding argument after all macros contained therein have been
8
- expanded. Before being substituted, each argument’s preprocessing tokens
9
- are completely macro replaced as if they formed the rest of the
10
- preprocessing file; no other preprocessing tokens are available.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
11
 
12
  An identifier `__VA_ARGS__` that occurs in the replacement list shall be
13
  treated as if it were a parameter, and the variable arguments shall form
14
  the preprocessing tokens used to replace it.
15
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1
  ### Argument substitution <a id="cpp.subst">[[cpp.subst]]</a>
2
 
3
+ ``` bnf
4
+ va-opt-replacement:
5
+ '__VA_OPT__ (' pp-tokensₒₚₜ ')'
6
+ ```
7
+
8
  After the arguments for the invocation of a function-like macro have
9
+ been identified, argument substitution takes place. For each parameter
10
+ in the replacement list that is neither preceded by a `#` or `##`
11
+ preprocessing token nor followed by a `##` preprocessing token, the
12
+ preprocessing tokens naming the parameter are replaced by a token
13
+ sequence determined as follows:
14
+
15
+ - If the parameter is of the form *va-opt-replacement*, the replacement
16
+ preprocessing tokens are the preprocessing token sequence for the
17
+ corresponding argument, as specified below.
18
+ - Otherwise, the replacement preprocessing tokens are the preprocessing
19
+ tokens of corresponding argument after all macros contained therein
20
+ have been expanded. The argument’s preprocessing tokens are completely
21
+ macro replaced before being substituted as if they formed the rest of
22
+ the preprocessing file with no other preprocessing tokens being
23
+ available.
24
+
25
+ [*Example 1*:
26
+
27
+ ``` cpp
28
+ #define LPAREN() (
29
+ #define G(Q) 42
30
+ #define F(R, X, ...) __VA_OPT__(G R X) )
31
+ int x = F(LPAREN(), 0, <:-); // replaced by int x = 42;
32
+ ```
33
+
34
+ — *end example*]
35
 
36
  An identifier `__VA_ARGS__` that occurs in the replacement list shall be
37
  treated as if it were a parameter, and the variable arguments shall form
38
  the preprocessing tokens used to replace it.
39
 
40
+ [*Example 2*:
41
+
42
+ ``` cpp
43
+ #define debug(...) fprintf(stderr, __VA_ARGS__)
44
+ #define showlist(...) puts(#__VA_ARGS__)
45
+ #define report(test, ...) ((test) ? puts(#test) : printf(__VA_ARGS__))
46
+ debug("Flag");
47
+ debug("X = %d\n", x);
48
+ showlist(The first, second, and third items.);
49
+ report(x>y, "x is %d but y is %d", x, y);
50
+ ```
51
+
52
+ results in
53
+
54
+ ``` cpp
55
+ fprintf(stderr, "Flag");
56
+ fprintf(stderr, "X = %d\n", x);
57
+ puts("The first, second, and third items.");
58
+ ((x>y) ? puts("x>y") : printf("x is %d but y is %d", x, y));
59
+ ```
60
+
61
+ — *end example*]
62
+
63
+ The identifier `__VA_OPT__` shall always occur as part of the
64
+ preprocessing token sequence *va-opt-replacement*; its closing `)` is
65
+ determined by skipping intervening pairs of matching left and right
66
+ parentheses in its *pp-tokens*. The *pp-tokens* of a
67
+ *va-opt-replacement* shall not contain `__VA_OPT__`. If the *pp-tokens*
68
+ would be ill-formed as the replacement list of the current function-like
69
+ macro, the program is ill-formed. A *va-opt-replacement* is treated as
70
+ if it were a parameter, and the preprocessing token sequence for the
71
+ corresponding argument is defined as follows. If the substitution of
72
+ `__VA_ARGS__` as neither an operand of `#` nor `##` consists of no
73
+ preprocessing tokens, the argument consists of a single placemarker
74
+ preprocessing token ([[cpp.concat]], [[cpp.rescan]]). Otherwise, the
75
+ argument consists of the results of the expansion of the contained
76
+ *pp-tokens* as the replacement list of the current function-like macro
77
+ before removal of placemarker tokens, rescanning, and further
78
+ replacement.
79
+
80
+ [*Note 1*: The placemarker tokens are removed before stringization
81
+ [[cpp.stringize]], and can be removed by rescanning and further
82
+ replacement [[cpp.rescan]]. — *end note*]
83
+
84
+ [*Example 3*:
85
+
86
+ ``` cpp
87
+ #define F(...) f(0 __VA_OPT__(,) __VA_ARGS__)
88
+ #define G(X, ...) f(0, X __VA_OPT__(,) __VA_ARGS__)
89
+ #define SDEF(sname, ...) S sname __VA_OPT__(= { __VA_ARGS__ })
90
+ #define EMP
91
+
92
+ F(a, b, c) // replaced by f(0, a, b, c)
93
+ F() // replaced by f(0)
94
+ F(EMP) // replaced by f(0)
95
+
96
+ G(a, b, c) // replaced by f(0, a, b, c)
97
+ G(a, ) // replaced by f(0, a)
98
+ G(a) // replaced by f(0, a)
99
+
100
+ SDEF(foo); // replaced by S foo;
101
+ SDEF(bar, 1, 2); // replaced by S bar = { 1, 2 }
102
+
103
+ #define H1(X, ...) X __VA_OPT__(##) __VA_ARGS__ // error: ## may not appear at
104
+ // the beginning of a replacement list[cpp.concat]
105
+
106
+ #define H2(X, Y, ...) __VA_OPT__(X ## Y,) __VA_ARGS__
107
+ H2(a, b, c, d) // replaced by ab, c, d
108
+
109
+ #define H3(X, ...) #__VA_OPT__(X##X X##X)
110
+ H3(, 0) // replaced by ""
111
+
112
+ #define H4(X, ...) __VA_OPT__(a X ## X) ## b
113
+ H4(, 1) // replaced by a b
114
+
115
+ #define H5A(...) __VA_OPT__()/**/__VA_OPT__()
116
+ #define H5B(X) a ## X ## b
117
+ #define H5C(X) H5B(X)
118
+ H5C(H5A()) // replaced by ab
119
+ ```
120
+
121
+ — *end example*]
122
+