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Diffstat (limited to 'absl/strings/string_view.h')
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diff --git a/absl/strings/string_view.h b/absl/strings/string_view.h new file mode 100644 index 000000000000..e2609f174f93 --- /dev/null +++ b/absl/strings/string_view.h @@ -0,0 +1,572 @@ +// +// Copyright 2017 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 +// +// http://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: string_view.h +// ----------------------------------------------------------------------------- +// +// This file contains the definition of the `absl::string_view` class. A +// `string_view` points to a contiguous span of characters, often part or all of +// another `std::string`, double-quoted std::string literal, character array, or even +// another `string_view`. +// +// This `absl::string_view` abstraction is designed to be a drop-in +// replacement for the C++17 `std::string_view` abstraction. +#ifndef ABSL_STRINGS_STRING_VIEW_H_ +#define ABSL_STRINGS_STRING_VIEW_H_ + +#include <algorithm> +#include "absl/base/config.h" + +#ifdef ABSL_HAVE_STD_STRING_VIEW + +#include <string_view> + +namespace absl { +using std::string_view; +}; + +#else // ABSL_HAVE_STD_STRING_VIEW + +#include <cassert> +#include <cstddef> +#include <cstring> +#include <iosfwd> +#include <iterator> +#include <limits> +#include <string> + +#include "absl/base/internal/throw_delegate.h" +#include "absl/base/macros.h" +#include "absl/base/port.h" + +namespace absl { + +// absl::string_view +// +// A `string_view` provides a lightweight view into the std::string data provided by +// a `std::string`, double-quoted std::string literal, character array, or even +// another `string_view`. A `string_view` does *not* own the std::string to which it +// points, and that data cannot be modified through the view. +// +// You can use `string_view` as a function or method parameter anywhere a +// parameter can receive a double-quoted std::string literal, `const char*`, +// `std::string`, or another `absl::string_view` argument with no need to copy +// the std::string data. Systematic use of `string_view` within function arguments +// reduces data copies and `strlen()` calls. +// +// Because of its small size, prefer passing `string_view` by value: +// +// void MyFunction(absl::string_view arg); +// +// If circumstances require, you may also pass one by const reference: +// +// void MyFunction(const absl::string_view& arg); // not preferred +// +// Passing by value generates slightly smaller code for many architectures. +// +// In either case, the source data of the `string_view` must outlive the +// `string_view` itself. +// +// A `string_view` is also suitable for local variables if you know that the +// lifetime of the underlying object is longer than the lifetime of your +// `string_view` variable. However, beware of binding a `string_view` to a +// temporary value: +// +// // BAD use of string_view: lifetime problem +// absl::string_view sv = obj.ReturnAString(); +// +// // GOOD use of string_view: str outlives sv +// std::string str = obj.ReturnAString(); +// absl::string_view sv = str; +// +// Due to lifetime issues, a `string_view` is sometimes a poor choice for a +// return value and usually a poor choice for a data member. If you do use a +// `string_view` this way, it is your responsibility to ensure that the object +// pointed to by the `string_view` outlives the `string_view`. +// +// A `string_view` may represent a whole std::string or just part of a std::string. For +// example, when splitting a std::string, `std::vector<absl::string_view>` is a +// natural data type for the output. +// +// +// When constructed from a source which is nul-terminated, the `string_view` +// itself will not include the nul-terminator unless a specific size (including +// the nul) is passed to the constructor. As a result, common idioms that work +// on nul-terminated strings do not work on `string_view` objects. If you write +// code that scans a `string_view`, you must check its length rather than test +// for nul, for example. Note, however, that nuls may still be embedded within +// a `string_view` explicitly. +// +// You may create a null `string_view` in two ways: +// +// absl::string_view sv(); +// absl::string_view sv(nullptr, 0); +// +// For the above, `sv.data() == nullptr`, `sv.length() == 0`, and +// `sv.empty() == true`. Also, if you create a `string_view` with a non-null +// pointer then `sv.data() != nullptr`. Thus, you can use `string_view()` to +// signal an undefined value that is different from other `string_view` values +// in a similar fashion to how `const char* p1 = nullptr;` is different from +// `const char* p2 = "";`. However, in practice, it is not recommended to rely +// on this behavior. +// +// Be careful not to confuse a null `string_view` with an empty one. A null +// `string_view` is an empty `string_view`, but some empty `string_view`s are +// not null. Prefer checking for emptiness over checking for null. +// +// There are many ways to create an empty string_view: +// +// const char* nullcp = nullptr; +// // string_view.size() will return 0 in all cases. +// absl::string_view(); +// absl::string_view(nullcp, 0); +// absl::string_view(""); +// absl::string_view("", 0); +// absl::string_view("abcdef", 0); +// absl::string_view("abcdef" + 6, 0); +// +// All empty `string_view` objects whether null or not, are equal: +// +// absl::string_view() == absl::string_view("", 0) +// absl::string_view(nullptr, 0) == absl:: string_view("abcdef"+6, 0) +class string_view { + public: + using traits_type = std::char_traits<char>; + using value_type = char; + using pointer = char*; + using const_pointer = const char*; + using reference = char&; + using const_reference = const char&; + using const_iterator = const char*; + using iterator = const_iterator; + using const_reverse_iterator = std::reverse_iterator<const_iterator>; + using reverse_iterator = const_reverse_iterator; + using size_type = size_t; + using difference_type = std::ptrdiff_t; + + static constexpr size_type npos = static_cast<size_type>(-1); + + // Null `string_view` constructor + constexpr string_view() noexcept : ptr_(nullptr), length_(0) {} + + // Implicit constructors + + template <typename Allocator> + string_view( // NOLINT(runtime/explicit) + const std::basic_string<char, std::char_traits<char>, Allocator>& + str) noexcept + : ptr_(str.data()), length_(str.size()) {} + + // Implicit constructor of a `string_view` from nul-terminated `str`. When + // accepting possibly null strings, use `absl::NullSafeStringView(str)` + // instead (see below). + constexpr string_view(const char* str) // NOLINT(runtime/explicit) + : ptr_(str), length_(StrLenInternal(str)) {} + + // Implicit consructor of a `string_view` from a `const char*` and length + constexpr string_view(const char* data, size_type len) + : ptr_(data), length_(CheckLengthInternal(len)) {} + + // NOTE(b/36227513): harmlessly omitted to work around gdb bug. + // constexpr string_view(const string_view&) noexcept = default; + // string_view& operator=(const string_view&) noexcept = default; + + // Iterators + + // string_view::begin() + // + // Returns an iterator pointing to the first character at the beginning of the + // `string_view`, or `end()` if the `string_view` is empty. + constexpr const_iterator begin() const noexcept { return ptr_; } + + // string_view::end() + // + // Returns an iterator pointing just beyond the last character at the end of + // the `string_view`. This iterator acts as a placeholder; attempting to + // access it results in undefined behavior. + constexpr const_iterator end() const noexcept { return ptr_ + length_; } + + // string_view::cbegin() + // + // Returns a const iterator pointing to the first character at the beginning + // of the `string_view`, or `end()` if the `string_view` is empty. + constexpr const_iterator cbegin() const noexcept { return begin(); } + + // string_view::cend() + // + // Returns a const iterator pointing just beyond the last character at the end + // of the `string_view`. This pointer acts as a placeholder; attempting to + // access its element results in undefined behavior. + constexpr const_iterator cend() const noexcept { return end(); } + + // string_view::rbegin() + // + // Returns a reverse iterator pointing to the last character at the end of the + // `string_view`, or `rend()` if the `string_view` is empty. + const_reverse_iterator rbegin() const noexcept { + return const_reverse_iterator(end()); + } + + // string_view::rend() + // + // Returns a reverse iterator pointing just before the first character at the + // beginning of the `string_view`. This pointer acts as a placeholder; + // attempting to access its element results in undefined behavior. + const_reverse_iterator rend() const noexcept { + return const_reverse_iterator(begin()); + } + + // string_view::crbegin() + // + // Returns a const reverse iterator pointing to the last character at the end + // of the `string_view`, or `crend()` if the `string_view` is empty. + const_reverse_iterator crbegin() const noexcept { return rbegin(); } + + // string_view::crend() + // + // Returns a const reverse iterator pointing just before the first character + // at the beginning of the `string_view`. This pointer acts as a placeholder; + // attempting to access its element results in undefined behavior. + const_reverse_iterator crend() const noexcept { return rend(); } + + // Capacity Utilities + + // string_view::size() + // + // Returns the number of characters in the `string_view`. + constexpr size_type size() const noexcept { + return length_; + } + + // string_view::length() + // + // Returns the number of characters in the `string_view`. Alias for `size()`. + constexpr size_type length() const noexcept { return size(); } + + // string_view::max_size() + // + // Returns the maximum number of characters the `string_view` can hold. + constexpr size_type max_size() const noexcept { return kMaxSize; } + + // string_view::empty() + // + // Checks if the `string_view` is empty (refers to no characters). + constexpr bool empty() const noexcept { return length_ == 0; } + + // std::string:view::operator[] + // + // Returns the ith element of an `string_view` using the array operator. + // Note that this operator does not perform any bounds checking. + constexpr const_reference operator[](size_type i) const { return ptr_[i]; } + + // string_view::front() + // + // Returns the first element of a `string_view`. + constexpr const_reference front() const { return ptr_[0]; } + + // string_view::back() + // + // Returns the last element of a `string_view`. + constexpr const_reference back() const { return ptr_[size() - 1]; } + + // string_view::data() + // + // Returns a pointer to the underlying character array (which is of course + // stored elsewhere). Note that `string_view::data()` may contain embedded nul + // characters, but the returned buffer may or may not be nul-terminated; + // therefore, do not pass `data()` to a routine that expects a nul-terminated + // std::string. + constexpr const_pointer data() const noexcept { return ptr_; } + + // Modifiers + + // string_view::remove_prefix() + // + // Removes the first `n` characters from the `string_view`, returning a + // pointer to the new first character. Note that the underlying std::string is not + // changed, only the view. + void remove_prefix(size_type n) { + assert(n <= length_); + ptr_ += n; + length_ -= n; + } + + // string_view::remove_suffix() + // + // Removes the last `n` characters from the `string_view`. Note that the + // underlying std::string is not changed, only the view. + void remove_suffix(size_type n) { + assert(n <= length_); + length_ -= n; + } + + // string_view::swap() + // + // Swaps this `string_view` with another `string_view`. + void swap(string_view& s) noexcept { + auto t = *this; + *this = s; + s = t; + } + + // Explicit conversion operators + + // Supports conversion to both `std::basic_string` where available. + template <typename A> + explicit operator std::basic_string<char, traits_type, A>() const { + if (!data()) return {}; + return std::basic_string<char, traits_type, A>(data(), size()); + } + + // string_view::copy() + // + // Copies the contents of the `string_view` at offset `pos` and length `n` + // into `buf`. + size_type copy(char* buf, size_type n, size_type pos = 0) const; + + // string_view::substr() + // + // Returns a "substring" of the `string_view` (at offset `post` and length + // `n`) as another std::string views. This function throws `std::out_of_bounds` if + // `pos > size'. + string_view substr(size_type pos, size_type n = npos) const { + if (ABSL_PREDICT_FALSE(pos > length_)) + base_internal::ThrowStdOutOfRange("absl::string_view::substr"); + n = std::min(n, length_ - pos); + return string_view(ptr_ + pos, n); + } + + // string_view::compare() + // + // Performs a lexicographical comparison between the `string_view` and + // another `absl::string_view), returning -1 if `this` is less than, 0 if + // `this` is equal to, and 1 if `this` is greater than the passed std::string + // view. Note that in the case of data equality, a further comparison is made + // on the respective sizes of the two `string_view`s to determine which is + // smaller, equal, or greater. + int compare(string_view x) const noexcept { + auto min_length = std::min(length_, x.length_); + if (min_length > 0) { + int r = memcmp(ptr_, x.ptr_, min_length); + if (r < 0) return -1; + if (r > 0) return 1; + } + if (length_ < x.length_) return -1; + if (length_ > x.length_) return 1; + return 0; + } + + // Overload of `string_view::compare()` for comparing a substring of the + // 'string_view` and another `absl::string_view`. + int compare(size_type pos1, size_type count1, string_view v) const { + return substr(pos1, count1).compare(v); + } + + // Overload of `string_view::compare()` for comparing a substring of the + // `string_view` and a substring of another `absl::string_view`. + int compare(size_type pos1, size_type count1, string_view v, size_type pos2, + size_type count2) const { + return substr(pos1, count1).compare(v.substr(pos2, count2)); + } + + // Overload of `string_view::compare()` for comparing a `string_view` and a + // a different C-style std::string `s`. + int compare(const char* s) const { return compare(string_view(s)); } + + // Overload of `string_view::compare()` for comparing a substring of the + // `string_view` and a different std::string C-style std::string `s`. + int compare(size_type pos1, size_type count1, const char* s) const { + return substr(pos1, count1).compare(string_view(s)); + } + + // Overload of `string_view::compare()` for comparing a substring of the + // `string_view` and a substring of a different C-style std::string `s`. + int compare(size_type pos1, size_type count1, const char* s, + size_type count2) const { + return substr(pos1, count1).compare(string_view(s, count2)); + } + + // Find Utilities + + // string_view::find() + // + // Finds the first occurrence of the substring `s` within the `string_view`, + // returning the position of the first character's match, or `npos` if no + // match was found. + size_type find(string_view s, size_type pos = 0) const noexcept; + + // Overload of `string_view::find()` for finding the given character `c` + // within the `string_view`. + size_type find(char c, size_type pos = 0) const noexcept; + + // string_view::rfind() + // + // Finds the last occurrence of a substring `s` within the `string_view`, + // returning the position of the first character's match, or `npos` if no + // match was found. + size_type rfind(string_view s, size_type pos = npos) const + noexcept; + + // Overload of `string_view::rfind()` for finding the given character `c` + // within the `string_view`. + size_type rfind(char c, size_type pos = npos) const noexcept; + + // string_view::find_first_of() + // + // Finds the first occurrence of any of the characters in `s` within the + // `string_view`, returning the start position of the match, or `npos` if no + // match was found. + size_type find_first_of(string_view s, size_type pos = 0) const + noexcept; + + // Overload of `string_view::find_first_of()` for finding a character `c` + // within the `string_view`. + size_type find_first_of(char c, size_type pos = 0) const + noexcept { + return find(c, pos); + } + + // string_view::find_last_of() + // + // Finds the last occurrence of any of the characters in `s` within the + // `string_view`, returning the start position of the match, or `npos` if no + // match was found. + size_type find_last_of(string_view s, size_type pos = npos) const + noexcept; + + // Overload of `string_view::find_last_of()` for finding a character `c` + // within the `string_view`. + size_type find_last_of(char c, size_type pos = npos) const + noexcept { + return rfind(c, pos); + } + + // string_view::find_first_not_of() + // + // Finds the first occurrence of any of the characters not in `s` within the + // `string_view`, returning the start position of the first non-match, or + // `npos` if no non-match was found. + size_type find_first_not_of(string_view s, size_type pos = 0) const noexcept; + + // Overload of `string_view::find_first_not_of()` for finding a character + // that is not `c` within the `string_view`. + size_type find_first_not_of(char c, size_type pos = 0) const noexcept; + + // string_view::find_last_not_of() + // + // Finds the last occurrence of any of the characters not in `s` within the + // `string_view`, returning the start position of the last non-match, or + // `npos` if no non-match was found. + size_type find_last_not_of(string_view s, + size_type pos = npos) const noexcept; + + // Overload of `string_view::find_last_not_of()` for finding a character + // that is not `c` within the `string_view`. + size_type find_last_not_of(char c, size_type pos = npos) const + noexcept; + + private: + static constexpr size_type kMaxSize = + std::numeric_limits<size_type>::max() / 2 + 1; + + static constexpr size_type StrLenInternal(const char* str) { + return str ? +// check whether __builtin_strlen is provided by the compiler. +// GCC doesn't have __has_builtin() +// (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66970), +// but has __builtin_strlen according to +// https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Other-Builtins.html. +#if ABSL_HAVE_BUILTIN(__builtin_strlen) || \ + (defined(__GNUC__) && !defined(__clang__)) + __builtin_strlen(str) +#else + strlen(str) +#endif + : 0; + } + + static constexpr size_type CheckLengthInternal(size_type len) { + return ABSL_ASSERT(len <= kMaxSize), len; + } + + const char* ptr_; + size_type length_; +}; + +// This large function is defined inline so that in a fairly common case where +// one of the arguments is a literal, the compiler can elide a lot of the +// following comparisons. +inline bool operator==(string_view x, string_view y) noexcept { + auto len = x.size(); + if (len != y.size()) { + return false; + } + return x.data() == y.data() || len <= 0 || + memcmp(x.data(), y.data(), len) == 0; +} + +inline bool operator!=(string_view x, string_view y) noexcept { + return !(x == y); +} + +inline bool operator<(string_view x, string_view y) noexcept { + auto min_size = std::min(x.size(), y.size()); + const int r = min_size == 0 ? 0 : memcmp(x.data(), y.data(), min_size); + return (r < 0) || (r == 0 && x.size() < y.size()); +} + +inline bool operator>(string_view x, string_view y) noexcept { return y < x; } + +inline bool operator<=(string_view x, string_view y) noexcept { + return !(y < x); +} + +inline bool operator>=(string_view x, string_view y) noexcept { + return !(x < y); +} + +// IO Insertion Operator +std::ostream& operator<<(std::ostream& o, string_view piece); + +} // namespace absl + +#endif // ABSL_HAVE_STD_STRING_VIEW + +namespace absl { + +// ClippedSubstr() +// +// Like `s.substr(pos, n)`, but clips `pos` to an upper bound of `s.size()`. +// Provided because std::string_view::substr throws if `pos > size()`, +// to support b/37991613. +inline string_view ClippedSubstr(string_view s, size_t pos, + size_t n = string_view::npos) { + pos = std::min(pos, static_cast<size_t>(s.size())); + return s.substr(pos, n); +} + +// NullSafeStringView() +// +// Creates an `absl::string_view` from a pointer `p` even if it's null-valued. +// This function should be used where an `absl::string_view` can be created from +// a possibly-null pointer. +inline string_view NullSafeStringView(const char* p) { + return p ? string_view(p) : string_view(); +} + +} // namespace absl + +#endif // ABSL_STRINGS_STRING_VIEW_H_ |