ostringstream.h
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1 // Copyright 2017 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 
15 #ifndef ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
16 #define ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
17 
18 #include <cassert>
19 #include <ostream>
20 #include <streambuf>
21 #include <string>
22 
23 #include "absl/base/port.h"
24 
25 namespace absl {
26 namespace strings_internal {
27 
28 // The same as std::ostringstream but appends to a user-specified std::string,
29 // and is faster. It is ~70% faster to create, ~50% faster to write to, and
30 // completely free to extract the result std::string.
31 //
32 // std::string s;
33 // OStringStream strm(&s);
34 // strm << 42 << ' ' << 3.14; // appends to `s`
35 //
36 // The stream object doesn't have to be named. Starting from C++11 operator<<
37 // works with rvalues of std::ostream.
38 //
39 // std::string s;
40 // OStringStream(&s) << 42 << ' ' << 3.14; // appends to `s`
41 //
42 // OStringStream is faster to create than std::ostringstream but it's still
43 // relatively slow. Avoid creating multiple streams where a single stream will
44 // do.
45 //
46 // Creates unnecessary instances of OStringStream: slow.
47 //
48 // std::string s;
49 // OStringStream(&s) << 42;
50 // OStringStream(&s) << ' ';
51 // OStringStream(&s) << 3.14;
52 //
53 // Creates a single instance of OStringStream and reuses it: fast.
54 //
55 // std::string s;
56 // OStringStream strm(&s);
57 // strm << 42;
58 // strm << ' ';
59 // strm << 3.14;
60 //
61 // Note: flush() has no effect. No reason to call it.
62 class OStringStream : private std::basic_streambuf<char>, public std::ostream {
63  public:
64  // The argument can be null, in which case you'll need to call str(p) with a
65  // non-null argument before you can write to the stream.
66  //
67  // The destructor of OStringStream doesn't use the std::string. It's OK to
68  // destroy the std::string before the stream.
69  explicit OStringStream(std::string* s) : std::ostream(this), s_(s) {}
70 
71  std::string* str() { return s_; }
72  const std::string* str() const { return s_; }
73  void str(std::string* s) { s_ = s; }
74 
75  private:
76  using Buf = std::basic_streambuf<char>;
77 
78  Buf::int_type overflow(int c) override;
79  std::streamsize xsputn(const char* s, std::streamsize n) override;
80 
81  std::string* s_;
82 };
83 
84 } // namespace strings_internal
85 } // namespace absl
86 
87 #endif // ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
std::streamsize xsputn(const char *s, std::streamsize n) override
Definition: algorithm.h:29
Buf::int_type overflow(int c) override
const std::string * str() const
Definition: ostringstream.h:72
std::basic_streambuf< char > Buf
Definition: ostringstream.h:76


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autogenerated on Mon Feb 28 2022 21:31:19