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1 //===- FormatVariadic.h - Efficient type-safe string formatting --*- C++-*-===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the formatv() function which can be used with other LLVM
11 // subsystems to provide printf-like formatting, but with improved safety and
12 // flexibility.  The result of `formatv` is an object which can be streamed to
13 // a raw_ostream or converted to a std::string or llvm::SmallString.
14 //
15 //   // Convert to std::string.
16 //   std::string S = formatv("{0} {1}", 1234.412, "test").str();
17 //
18 //   // Convert to llvm::SmallString
19 //   SmallString<8> S = formatv("{0} {1}", 1234.412, "test").sstr<8>();
20 //
21 //   // Stream to an existing raw_ostream.
22 //   OS << formatv("{0} {1}", 1234.412, "test");
23 //
24 //===----------------------------------------------------------------------===//
25 
26 #ifndef LLVM_SUPPORT_FORMATVARIADIC_H
27 #define LLVM_SUPPORT_FORMATVARIADIC_H
28 
29 #include "llvm/ADT/Optional.h"
30 #include "llvm/ADT/SmallString.h"
31 #include "llvm/ADT/STLExtras.h"
32 #include "llvm/ADT/StringRef.h"
33 #include "llvm/Support/FormatCommon.h"
34 #include "llvm/Support/FormatProviders.h"
35 #include "llvm/Support/FormatVariadicDetails.h"
36 #include "llvm/Support/raw_ostream.h"
37 #include <cstddef>
38 #include <string>
39 #include <tuple>
40 #include <utility>
41 #include <vector>
42 
43 namespace llvm {
44 
45 enum class ReplacementType { Empty, Format, Literal };
46 
47 struct ReplacementItem {
48   ReplacementItem() = default;
ReplacementItemReplacementItem49   explicit ReplacementItem(StringRef Literal)
50       : Type(ReplacementType::Literal), Spec(Literal) {}
ReplacementItemReplacementItem51   ReplacementItem(StringRef Spec, size_t Index, size_t Align, AlignStyle Where,
52                   char Pad, StringRef Options)
53       : Type(ReplacementType::Format), Spec(Spec), Index(Index), Align(Align),
54         Where(Where), Pad(Pad), Options(Options) {}
55 
56   ReplacementType Type = ReplacementType::Empty;
57   StringRef Spec;
58   size_t Index = 0;
59   size_t Align = 0;
60   AlignStyle Where = AlignStyle::Right;
61   char Pad;
62   StringRef Options;
63 };
64 
65 class formatv_object_base {
66 protected:
67   // The parameters are stored in a std::tuple, which does not provide runtime
68   // indexing capabilities.  In order to enable runtime indexing, we use this
69   // structure to put the parameters into a std::vector.  Since the parameters
70   // are not all the same type, we use some type-erasure by wrapping the
71   // parameters in a template class that derives from a non-template superclass.
72   // Essentially, we are converting a std::tuple<Derived<Ts...>> to a
73   // std::vector<Base*>.
74   struct create_adapters {
75     template <typename... Ts>
operatorcreate_adapters76     std::vector<detail::format_adapter *> operator()(Ts &... Items) {
77       return std::vector<detail::format_adapter *>{&Items...};
78     }
79   };
80 
81   StringRef Fmt;
82   std::vector<detail::format_adapter *> Adapters;
83   std::vector<ReplacementItem> Replacements;
84 
85   static bool consumeFieldLayout(StringRef &Spec, AlignStyle &Where,
86                                  size_t &Align, char &Pad);
87 
88   static std::pair<ReplacementItem, StringRef>
89   splitLiteralAndReplacement(StringRef Fmt);
90 
91 public:
formatv_object_base(StringRef Fmt,std::size_t ParamCount)92   formatv_object_base(StringRef Fmt, std::size_t ParamCount)
93       : Fmt(Fmt), Replacements(parseFormatString(Fmt)) {
94     Adapters.reserve(ParamCount);
95   }
96 
format(raw_ostream & S)97   void format(raw_ostream &S) const {
98     for (auto &R : Replacements) {
99       if (R.Type == ReplacementType::Empty)
100         continue;
101       if (R.Type == ReplacementType::Literal) {
102         S << R.Spec;
103         continue;
104       }
105       if (R.Index >= Adapters.size()) {
106         S << R.Spec;
107         continue;
108       }
109 
110       auto W = Adapters[R.Index];
111 
112       FmtAlign Align(*W, R.Where, R.Align);
113       Align.format(S, R.Options);
114     }
115   }
116   static std::vector<ReplacementItem> parseFormatString(StringRef Fmt);
117 
118   static Optional<ReplacementItem> parseReplacementItem(StringRef Spec);
119 
str()120   std::string str() const {
121     std::string Result;
122     raw_string_ostream Stream(Result);
123     Stream << *this;
124     Stream.flush();
125     return Result;
126   }
127 
sstr()128   template <unsigned N> SmallString<N> sstr() const {
129     SmallString<N> Result;
130     raw_svector_ostream Stream(Result);
131     Stream << *this;
132     return Result;
133   }
134 
135   template <unsigned N> operator SmallString<N>() const { return sstr<N>(); }
136 
string()137   operator std::string() const { return str(); }
138 };
139 
140 template <typename Tuple> class formatv_object : public formatv_object_base {
141   // Storage for the parameter adapters.  Since the base class erases the type
142   // of the parameters, we have to own the storage for the parameters here, and
143   // have the base class store type-erased pointers into this tuple.
144   Tuple Parameters;
145 
146 public:
formatv_object(StringRef Fmt,Tuple && Params)147   formatv_object(StringRef Fmt, Tuple &&Params)
148       : formatv_object_base(Fmt, std::tuple_size<Tuple>::value),
149         Parameters(std::move(Params)) {
150     Adapters = apply_tuple(create_adapters(), Parameters);
151   }
152 };
153 
154 // \brief Format text given a format string and replacement parameters.
155 //
156 // ===General Description===
157 //
158 // Formats textual output.  `Fmt` is a string consisting of one or more
159 // replacement sequences with the following grammar:
160 //
161 // rep_field ::= "{" [index] ["," layout] [":" format] "}"
162 // index     ::= <non-negative integer>
163 // layout    ::= [[[char]loc]width]
164 // format    ::= <any string not containing "{" or "}">
165 // char      ::= <any character except "{" or "}">
166 // loc       ::= "-" | "=" | "+"
167 // width     ::= <positive integer>
168 //
169 // index   - A non-negative integer specifying the index of the item in the
170 //           parameter pack to print.  Any other value is invalid.
171 // layout  - A string controlling how the field is laid out within the available
172 //           space.
173 // format  - A type-dependent string used to provide additional options to
174 //           the formatting operation.  Refer to the documentation of the
175 //           various individual format providers for per-type options.
176 // char    - The padding character.  Defaults to ' ' (space).  Only valid if
177 //           `loc` is also specified.
178 // loc     - Where to print the formatted text within the field.  Only valid if
179 //           `width` is also specified.
180 //           '-' : The field is left aligned within the available space.
181 //           '=' : The field is centered within the available space.
182 //           '+' : The field is right aligned within the available space (this
183 //                 is the default).
184 // width   - The width of the field within which to print the formatted text.
185 //           If this is less than the required length then the `char` and `loc`
186 //           fields are ignored, and the field is printed with no leading or
187 //           trailing padding.  If this is greater than the required length,
188 //           then the text is output according to the value of `loc`, and padded
189 //           as appropriate on the left and/or right by `char`.
190 //
191 // ===Special Characters===
192 //
193 // The characters '{' and '}' are reserved and cannot appear anywhere within a
194 // replacement sequence.  Outside of a replacement sequence, in order to print
195 // a literal '{' or '}' it must be doubled -- "{{" to print a literal '{' and
196 // "}}" to print a literal '}'.
197 //
198 // ===Parameter Indexing===
199 // `index` specifies the index of the paramter in the parameter pack to format
200 // into the output.  Note that it is possible to refer to the same parameter
201 // index multiple times in a given format string.  This makes it possible to
202 // output the same value multiple times without passing it multiple times to the
203 // function. For example:
204 //
205 //   formatv("{0} {1} {0}", "a", "bb")
206 //
207 // would yield the string "abba".  This can be convenient when it is expensive
208 // to compute the value of the parameter, and you would otherwise have had to
209 // save it to a temporary.
210 //
211 // ===Formatter Search===
212 //
213 // For a given parameter of type T, the following steps are executed in order
214 // until a match is found:
215 //
216 //   1. If the parameter is of class type, and contains a method
217 //      void format(raw_ostream &Stream, StringRef Options)
218 //      Then this method is invoked to produce the formatted output.  The
219 //      implementation should write the formatted text into `Stream`.
220 //   2. If there is a suitable template specialization of format_provider<>
221 //      for type T containing a method whose signature is:
222 //      void format(const T &Obj, raw_ostream &Stream, StringRef Options)
223 //      Then this method is invoked as described in Step 1.
224 //
225 // If a match cannot be found through either of the above methods, a compiler
226 // error is generated.
227 //
228 // ===Invalid Format String Handling===
229 //
230 // In the case of a format string which does not match the grammar described
231 // above, the output is undefined.  With asserts enabled, LLVM will trigger an
232 // assertion.  Otherwise, it will try to do something reasonable, but in general
233 // the details of what that is are undefined.
234 //
235 template <typename... Ts>
236 inline auto formatv(const char *Fmt, Ts &&... Vals) -> formatv_object<decltype(
237     std::make_tuple(detail::build_format_adapter(std::forward<Ts>(Vals))...))> {
238   using ParamTuple = decltype(
239       std::make_tuple(detail::build_format_adapter(std::forward<Ts>(Vals))...));
240   return formatv_object<ParamTuple>(
241       Fmt,
242       std::make_tuple(detail::build_format_adapter(std::forward<Ts>(Vals))...));
243 }
244 
245 } // end namespace llvm
246 
247 #endif // LLVM_SUPPORT_FORMATVARIADIC_H
248