summaryrefslogtreecommitdiff
path: root/absl/strings/str_format.h
blob: b4d1b7bdb350fcb5d4212659363bdf046797bcf2 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
//
// Copyright 2018 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// File: str_format.h
// -----------------------------------------------------------------------------
//
// The `str_format` library is a typesafe replacement for the family of
// `printf()` string formatting routines within the `<cstdio>` standard library
// header. Like the `printf` family, the `str_format` uses a "format string" to
// perform argument substitutions based on types. See the `FormatSpec` section
// below for format string documentation.
//
// Example:
//
//   std::string s = absl::StrFormat(
//                      "%s %s You have $%d!", "Hello", name, dollars);
//
// The library consists of the following basic utilities:
//
//   * `absl::StrFormat()`, a type-safe replacement for `std::sprintf()`, to
//     write a format string to a `string` value.
//   * `absl::StrAppendFormat()` to append a format string to a `string`
//   * `absl::StreamFormat()` to more efficiently write a format string to a
//     stream, such as`std::cout`.
//   * `absl::PrintF()`, `absl::FPrintF()` and `absl::SNPrintF()` as
//     replacements for `std::printf()`, `std::fprintf()` and `std::snprintf()`.
//
//     Note: a version of `std::sprintf()` is not supported as it is
//     generally unsafe due to buffer overflows.
//
// Additionally, you can provide a format string (and its associated arguments)
// using one of the following abstractions:
//
//   * A `FormatSpec` class template fully encapsulates a format string and its
//     type arguments and is usually provided to `str_format` functions as a
//     variadic argument of type `FormatSpec<Arg...>`. The `FormatSpec<Args...>`
//     template is evaluated at compile-time, providing type safety.
//   * A `ParsedFormat` instance, which encapsulates a specific, pre-compiled
//     format string for a specific set of type(s), and which can be passed
//     between API boundaries. (The `FormatSpec` type should not be used
//     directly except as an argument type for wrapper functions.)
//
// The `str_format` library provides the ability to output its format strings to
// arbitrary sink types:
//
//   * A generic `Format()` function to write outputs to arbitrary sink types,
//     which must implement a `RawSinkFormat` interface. (See
//     `str_format_sink.h` for more information.)
//
//   * A `FormatUntyped()` function that is similar to `Format()` except it is
//     loosely typed. `FormatUntyped()` is not a template and does not perform
//     any compile-time checking of the format string; instead, it returns a
//     boolean from a runtime check.
//
// In addition, the `str_format` library provides extension points for
// augmenting formatting to new types. These extensions are fully documented
// within the `str_format_extension.h` header file.

#ifndef ABSL_STRINGS_STR_FORMAT_H_
#define ABSL_STRINGS_STR_FORMAT_H_

#include <cstdio>
#include <string>

#include "absl/strings/internal/str_format/arg.h"  // IWYU pragma: export
#include "absl/strings/internal/str_format/bind.h"  // IWYU pragma: export
#include "absl/strings/internal/str_format/checker.h"  // IWYU pragma: export
#include "absl/strings/internal/str_format/extension.h"  // IWYU pragma: export
#include "absl/strings/internal/str_format/parser.h"  // IWYU pragma: export

namespace absl {
inline namespace lts_2019_08_08 {

// UntypedFormatSpec
//
// A type-erased class that can be used directly within untyped API entry
// points. An `UntypedFormatSpec` is specifically used as an argument to
// `FormatUntyped()`.
//
// Example:
//
//   absl::UntypedFormatSpec format("%d");
//   std::string out;
//   CHECK(absl::FormatUntyped(&out, format, {absl::FormatArg(1)}));
class UntypedFormatSpec {
 public:
  UntypedFormatSpec() = delete;
  UntypedFormatSpec(const UntypedFormatSpec&) = delete;
  UntypedFormatSpec& operator=(const UntypedFormatSpec&) = delete;

  explicit UntypedFormatSpec(string_view s) : spec_(s) {}

 protected:
  explicit UntypedFormatSpec(const str_format_internal::ParsedFormatBase* pc)
      : spec_(pc) {}

 private:
  friend str_format_internal::UntypedFormatSpecImpl;
  str_format_internal::UntypedFormatSpecImpl spec_;
};

// FormatStreamed()
//
// Takes a streamable argument and returns an object that can print it
// with '%s'. Allows printing of types that have an `operator<<` but no
// intrinsic type support within `StrFormat()` itself.
//
// Example:
//
//   absl::StrFormat("%s", absl::FormatStreamed(obj));
template <typename T>
str_format_internal::StreamedWrapper<T> FormatStreamed(const T& v) {
  return str_format_internal::StreamedWrapper<T>(v);
}

// FormatCountCapture
//
// This class provides a way to safely wrap `StrFormat()` captures of `%n`
// conversions, which denote the number of characters written by a formatting
// operation to this point, into an integer value.
//
// This wrapper is designed to allow safe usage of `%n` within `StrFormat(); in
// the `printf()` family of functions, `%n` is not safe to use, as the `int *`
// buffer can be used to capture arbitrary data.
//
// Example:
//
//   int n = 0;
//   std::string s = absl::StrFormat("%s%d%n", "hello", 123,
//                       absl::FormatCountCapture(&n));
//   EXPECT_EQ(8, n);
class FormatCountCapture {
 public:
  explicit FormatCountCapture(int* p) : p_(p) {}

 private:
  // FormatCountCaptureHelper is used to define FormatConvertImpl() for this
  // class.
  friend struct str_format_internal::FormatCountCaptureHelper;
  // Unused() is here because of the false positive from -Wunused-private-field
  // p_ is used in the templated function of the friend FormatCountCaptureHelper
  // class.
  int* Unused() { return p_; }
  int* p_;
};

// FormatSpec
//
// The `FormatSpec` type defines the makeup of a format string within the
// `str_format` library. It is a variadic class template that is evaluated at
// compile-time, according to the format string and arguments that are passed to
// it.
//
// You should not need to manipulate this type directly. You should only name it
// if you are writing wrapper functions which accept format arguments that will
// be provided unmodified to functions in this library. Such a wrapper function
// might be a class method that provides format arguments and/or internally uses
// the result of formatting.
//
// For a `FormatSpec` to be valid at compile-time, it must be provided as
// either:
//
// * A `constexpr` literal or `absl::string_view`, which is how it most often
//   used.
// * A `ParsedFormat` instantiation, which ensures the format string is
//   valid before use. (See below.)
//
// Example:
//
//   // Provided as a string literal.
//   absl::StrFormat("Welcome to %s, Number %d!", "The Village", 6);
//
//   // Provided as a constexpr absl::string_view.
//   constexpr absl::string_view formatString = "Welcome to %s, Number %d!";
//   absl::StrFormat(formatString, "The Village", 6);
//
//   // Provided as a pre-compiled ParsedFormat object.
//   // Note that this example is useful only for illustration purposes.
//   absl::ParsedFormat<'s', 'd'> formatString("Welcome to %s, Number %d!");
//   absl::StrFormat(formatString, "TheVillage", 6);
//
// A format string generally follows the POSIX syntax as used within the POSIX
// `printf` specification.
//
// (See http://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html.)
//
// In specific, the `FormatSpec` supports the following type specifiers:
//   * `c` for characters
//   * `s` for strings
//   * `d` or `i` for integers
//   * `o` for unsigned integer conversions into octal
//   * `x` or `X` for unsigned integer conversions into hex
//   * `u` for unsigned integers
//   * `f` or `F` for floating point values into decimal notation
//   * `e` or `E` for floating point values into exponential notation
//   * `a` or `A` for floating point values into hex exponential notation
//   * `g` or `G` for floating point values into decimal or exponential
//     notation based on their precision
//   * `p` for pointer address values
//   * `n` for the special case of writing out the number of characters
//     written to this point. The resulting value must be captured within an
//     `absl::FormatCountCapture` type.
//
// Implementation-defined behavior:
//   * A null pointer provided to "%s" or "%p" is output as "(nil)".
//   * A non-null pointer provided to "%p" is output in hex as if by %#x or
//     %#lx.
//
// NOTE: `o`, `x\X` and `u` will convert signed values to their unsigned
// counterpart before formatting.
//
// Examples:
//     "%c", 'a'                -> "a"
//     "%c", 32                 -> " "
//     "%s", "C"                -> "C"
//     "%s", std::string("C++") -> "C++"
//     "%d", -10                -> "-10"
//     "%o", 10                 -> "12"
//     "%x", 16                 -> "10"
//     "%f", 123456789          -> "123456789.000000"
//     "%e", .01                -> "1.00000e-2"
//     "%a", -3.0               -> "-0x1.8p+1"
//     "%g", .01                -> "1e-2"
//     "%p", (void*)&value      -> "0x7ffdeb6ad2a4"
//
//     int n = 0;
//     std::string s = absl::StrFormat(
//         "%s%d%n", "hello", 123, absl::FormatCountCapture(&n));
//     EXPECT_EQ(8, n);
//
// The `FormatSpec` intrinsically supports all of these fundamental C++ types:
//
// *   Characters: `char`, `signed char`, `unsigned char`
// *   Integers: `int`, `short`, `unsigned short`, `unsigned`, `long`,
//         `unsigned long`, `long long`, `unsigned long long`
// *   Floating-point: `float`, `double`, `long double`
//
// However, in the `str_format` library, a format conversion specifies a broader
// C++ conceptual category instead of an exact type. For example, `%s` binds to
// any string-like argument, so `std::string`, `absl::string_view`, and
// `const char*` are all accepted. Likewise, `%d` accepts any integer-like
// argument, etc.

template <typename... Args>
using FormatSpec =
    typename str_format_internal::FormatSpecDeductionBarrier<Args...>::type;

// ParsedFormat
//
// A `ParsedFormat` is a class template representing a preparsed `FormatSpec`,
// with template arguments specifying the conversion characters used within the
// format string. Such characters must be valid format type specifiers, and
// these type specifiers are checked at compile-time.
//
// Instances of `ParsedFormat` can be created, copied, and reused to speed up
// formatting loops. A `ParsedFormat` may either be constructed statically, or
// dynamically through its `New()` factory function, which only constructs a
// runtime object if the format is valid at that time.
//
// Example:
//
//   // Verified at compile time.
//   absl::ParsedFormat<'s', 'd'> formatString("Welcome to %s, Number %d!");
//   absl::StrFormat(formatString, "TheVillage", 6);
//
//   // Verified at runtime.
//   auto format_runtime = absl::ParsedFormat<'d'>::New(format_string);
//   if (format_runtime) {
//     value = absl::StrFormat(*format_runtime, i);
//   } else {
//     ... error case ...
//   }
template <char... Conv>
using ParsedFormat = str_format_internal::ExtendedParsedFormat<
    str_format_internal::ConversionCharToConv(Conv)...>;

// StrFormat()
//
// Returns a `string` given a `printf()`-style format string and zero or more
// additional arguments. Use it as you would `sprintf()`. `StrFormat()` is the
// primary formatting function within the `str_format` library, and should be
// used in most cases where you need type-safe conversion of types into
// formatted strings.
//
// The format string generally consists of ordinary character data along with
// one or more format conversion specifiers (denoted by the `%` character).
// Ordinary character data is returned unchanged into the result string, while
// each conversion specification performs a type substitution from
// `StrFormat()`'s other arguments. See the comments for `FormatSpec` for full
// information on the makeup of this format string.
//
// Example:
//
//   std::string s = absl::StrFormat(
//       "Welcome to %s, Number %d!", "The Village", 6);
//   EXPECT_EQ("Welcome to The Village, Number 6!", s);
//
// Returns an empty string in case of error.
template <typename... Args>
ABSL_MUST_USE_RESULT std::string StrFormat(const FormatSpec<Args...>& format,
                                           const Args&... args) {
  return str_format_internal::FormatPack(
      str_format_internal::UntypedFormatSpecImpl::Extract(format),
      {str_format_internal::FormatArgImpl(args)...});
}

// StrAppendFormat()
//
// Appends to a `dst` string given a format string, and zero or more additional
// arguments, returning `*dst` as a convenience for chaining purposes. Appends
// nothing in case of error (but possibly alters its capacity).
//
// Example:
//
//   std::string orig("For example PI is approximately ");
//   std::cout << StrAppendFormat(&orig, "%12.6f", 3.14);
template <typename... Args>
std::string& StrAppendFormat(std::string* dst,
                             const FormatSpec<Args...>& format,
                             const Args&... args) {
  return str_format_internal::AppendPack(
      dst, str_format_internal::UntypedFormatSpecImpl::Extract(format),
      {str_format_internal::FormatArgImpl(args)...});
}

// StreamFormat()
//
// Writes to an output stream given a format string and zero or more arguments,
// generally in a manner that is more efficient than streaming the result of
// `absl:: StrFormat()`. The returned object must be streamed before the full
// expression ends.
//
// Example:
//
//   std::cout << StreamFormat("%12.6f", 3.14);
template <typename... Args>
ABSL_MUST_USE_RESULT str_format_internal::Streamable StreamFormat(
    const FormatSpec<Args...>& format, const Args&... args) {
  return str_format_internal::Streamable(
      str_format_internal::UntypedFormatSpecImpl::Extract(format),
      {str_format_internal::FormatArgImpl(args)...});
}

// PrintF()
//
// Writes to stdout given a format string and zero or more arguments. This
// function is functionally equivalent to `std::printf()` (and type-safe);
// prefer `absl::PrintF()` over `std::printf()`.
//
// Example:
//
//   std::string_view s = "Ulaanbaatar";
//   absl::PrintF("The capital of Mongolia is %s", s);
//
//   Outputs: "The capital of Mongolia is Ulaanbaatar"
//
template <typename... Args>
int PrintF(const FormatSpec<Args...>& format, const Args&... args) {
  return str_format_internal::FprintF(
      stdout, str_format_internal::UntypedFormatSpecImpl::Extract(format),
      {str_format_internal::FormatArgImpl(args)...});
}

// FPrintF()
//
// Writes to a file given a format string and zero or more arguments. This
// function is functionally equivalent to `std::fprintf()` (and type-safe);
// prefer `absl::FPrintF()` over `std::fprintf()`.
//
// Example:
//
//   std::string_view s = "Ulaanbaatar";
//   absl::FPrintF(stdout, "The capital of Mongolia is %s", s);
//
//   Outputs: "The capital of Mongolia is Ulaanbaatar"
//
template <typename... Args>
int FPrintF(std::FILE* output, const FormatSpec<Args...>& format,
            const Args&... args) {
  return str_format_internal::FprintF(
      output, str_format_internal::UntypedFormatSpecImpl::Extract(format),
      {str_format_internal::FormatArgImpl(args)...});
}

// SNPrintF()
//
// Writes to a sized buffer given a format string and zero or more arguments.
// This function is functionally equivalent to `std::snprintf()` (and
// type-safe); prefer `absl::SNPrintF()` over `std::snprintf()`.
//
// Example:
//
//   std::string_view s = "Ulaanbaatar";
//   char output[128];
//   absl::SNPrintF(output, sizeof(output),
//                  "The capital of Mongolia is %s", s);
//
//   Post-condition: output == "The capital of Mongolia is Ulaanbaatar"
//
template <typename... Args>
int SNPrintF(char* output, std::size_t size, const FormatSpec<Args...>& format,
             const Args&... args) {
  return str_format_internal::SnprintF(
      output, size, str_format_internal::UntypedFormatSpecImpl::Extract(format),
      {str_format_internal::FormatArgImpl(args)...});
}

// -----------------------------------------------------------------------------
// Custom Output Formatting Functions
// -----------------------------------------------------------------------------

// FormatRawSink
//
// FormatRawSink is a type erased wrapper around arbitrary sink objects
// specifically used as an argument to `Format()`.
// FormatRawSink does not own the passed sink object. The passed object must
// outlive the FormatRawSink.
class FormatRawSink {
 public:
  // Implicitly convert from any type that provides the hook function as
  // described above.
  template <typename T,
            typename = typename std::enable_if<std::is_constructible<
                str_format_internal::FormatRawSinkImpl, T*>::value>::type>
  FormatRawSink(T* raw)  // NOLINT
      : sink_(raw) {}

 private:
  friend str_format_internal::FormatRawSinkImpl;
  str_format_internal::FormatRawSinkImpl sink_;
};

// Format()
//
// Writes a formatted string to an arbitrary sink object (implementing the
// `absl::FormatRawSink` interface), using a format string and zero or more
// additional arguments.
//
// By default, `std::string` and `std::ostream` are supported as destination
// objects. If a `std::string` is used the formatted string is appended to it.
//
// `absl::Format()` is a generic version of `absl::StrFormat(), for custom
// sinks. The format string, like format strings for `StrFormat()`, is checked
// at compile-time.
//
// On failure, this function returns `false` and the state of the sink is
// unspecified.
template <typename... Args>
bool Format(FormatRawSink raw_sink, const FormatSpec<Args...>& format,
            const Args&... args) {
  return str_format_internal::FormatUntyped(
      str_format_internal::FormatRawSinkImpl::Extract(raw_sink),
      str_format_internal::UntypedFormatSpecImpl::Extract(format),
      {str_format_internal::FormatArgImpl(args)...});
}

// FormatArg
//
// A type-erased handle to a format argument specifically used as an argument to
// `FormatUntyped()`. You may construct `FormatArg` by passing
// reference-to-const of any printable type. `FormatArg` is both copyable and
// assignable. The source data must outlive the `FormatArg` instance. See
// example below.
//
using FormatArg = str_format_internal::FormatArgImpl;

// FormatUntyped()
//
// Writes a formatted string to an arbitrary sink object (implementing the
// `absl::FormatRawSink` interface), using an `UntypedFormatSpec` and zero or
// more additional arguments.
//
// This function acts as the most generic formatting function in the
// `str_format` library. The caller provides a raw sink, an unchecked format
// string, and (usually) a runtime specified list of arguments; no compile-time
// checking of formatting is performed within this function. As a result, a
// caller should check the return value to verify that no error occurred.
// On failure, this function returns `false` and the state of the sink is
// unspecified.
//
// The arguments are provided in an `absl::Span<const absl::FormatArg>`.
// Each `absl::FormatArg` object binds to a single argument and keeps a
// reference to it. The values used to create the `FormatArg` objects must
// outlive this function call. (See `str_format_arg.h` for information on
// the `FormatArg` class.)_
//
// Example:
//
//   std::optional<std::string> FormatDynamic(
//       const std::string& in_format,
//       const vector<std::string>& in_args) {
//     std::string out;
//     std::vector<absl::FormatArg> args;
//     for (const auto& v : in_args) {
//       // It is important that 'v' is a reference to the objects in in_args.
//       // The values we pass to FormatArg must outlive the call to
//       // FormatUntyped.
//       args.emplace_back(v);
//     }
//     absl::UntypedFormatSpec format(in_format);
//     if (!absl::FormatUntyped(&out, format, args)) {
//       return std::nullopt;
//     }
//     return std::move(out);
//   }
//
ABSL_MUST_USE_RESULT inline bool FormatUntyped(
    FormatRawSink raw_sink, const UntypedFormatSpec& format,
    absl::Span<const FormatArg> args) {
  return str_format_internal::FormatUntyped(
      str_format_internal::FormatRawSinkImpl::Extract(raw_sink),
      str_format_internal::UntypedFormatSpecImpl::Extract(format), args);
}

}  // inline namespace lts_2019_08_08
}  // namespace absl

#endif  // ABSL_STRINGS_STR_FORMAT_H_