90 lines
2.8 KiB
C++
90 lines
2.8 KiB
C++
// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
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#define ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
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#include <cassert>
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#include <ostream>
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#include <streambuf>
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#include <string>
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#include "absl/base/port.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace strings_internal {
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// The same as std::ostringstream but appends to a user-specified std::string,
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// and is faster. It is ~70% faster to create, ~50% faster to write to, and
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// completely free to extract the result std::string.
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//
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// std::string s;
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// OStringStream strm(&s);
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// strm << 42 << ' ' << 3.14; // appends to `s`
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//
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// The stream object doesn't have to be named. Starting from C++11 operator<<
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// works with rvalues of std::ostream.
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//
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// std::string s;
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// OStringStream(&s) << 42 << ' ' << 3.14; // appends to `s`
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//
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// OStringStream is faster to create than std::ostringstream but it's still
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// relatively slow. Avoid creating multiple streams where a single stream will
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// do.
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//
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// Creates unnecessary instances of OStringStream: slow.
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//
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// std::string s;
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// OStringStream(&s) << 42;
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// OStringStream(&s) << ' ';
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// OStringStream(&s) << 3.14;
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//
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// Creates a single instance of OStringStream and reuses it: fast.
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//
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// std::string s;
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// OStringStream strm(&s);
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// strm << 42;
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// strm << ' ';
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// strm << 3.14;
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//
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// Note: flush() has no effect. No reason to call it.
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class OStringStream : private std::basic_streambuf<char>, public std::ostream {
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public:
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// The argument can be null, in which case you'll need to call str(p) with a
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// non-null argument before you can write to the stream.
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//
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// The destructor of OStringStream doesn't use the std::string. It's OK to
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// destroy the std::string before the stream.
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explicit OStringStream(std::string* s) : std::ostream(this), s_(s) {}
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std::string* str() { return s_; }
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const std::string* str() const { return s_; }
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void str(std::string* s) { s_ = s; }
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private:
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using Buf = std::basic_streambuf<char>;
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Buf::int_type overflow(int c) override;
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std::streamsize xsputn(const char* s, std::streamsize n) override;
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std::string* s_;
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};
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} // namespace strings_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
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