path: root/absl/strings/str_format.h

//
// 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 {

// 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()`.
//
// In particular, a successful call to `absl::SNPrintF()` writes at most `size`
// bytes of the formatted output to `output`, including a null terminator, and
// returns the number of bytes that would have been written if truncation did
// not occur. In the event of an error, a negative value is returned and `errno`
// is set.
//
// 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);
}

}  // namespace absl

#endif  // ABSL_STRINGS_STR_FORMAT_H_