Presentazione | std::cout è out

# std::cout is out
## Why iostreams must go

---
class: middle
# I/O in the standard

---
# C-style I/O

std::FILE* fp = std::fopen(strPath, "wb");
???
- API does not exploit type safety: fopen write-only and then fread is possible.
- Is fp binary or text, narrow or wide?
- Even worse: All open files access global locale state
--
        if(!fp) { /* error handling, check errno for error message */ }
???
- errno error reporting loses (platform-dependent) information. 
- C++ has better tools for this.
--

unsigned int n = ...; 
        std::string str = ...;
        if(std::fprintf(fp, "Dec: %d Hex: %x String: %s\n", n, n, str.c_str())<0) {
          // error handling
        }
???
- fprintf calls and friends are not type-safe either. Use unsafe variadic functions, compilers try to catch errors.

---
# C++ Streams

--
        std::basic_ofstream<char> ofs(strPath, std::ios_base::binary | std::ios_base::out);
???
- *Good:* Improves type safety. ostream vs istream, narrow or wide. 
- Still can't distinguish binary from text, or e.g. seekable files.
- Private locale
--
        if(!ofs) { /*error handling*/ }
???
- Error handling loses all specific information
--

unsigned int n = ...; 
        std::string str = ...;

ofs << "Dec: " << n 
            << " Hex: " << std::hex << n
            << " String: " << str << std::endl;
???
- Formatting is safe, but not perfect. This line has two problems. Who spots them?
---
# C++ Streams

std::basic_ofstream<char> ofs(strPath, std::ios_base::binary | std::ios_base::out);
        if(!ofs) { /*error handling*/ }

unsigned int n = ...; 
        std::string str = ...;

ofs << "Dec: " << n 
            << " Hex: " << std::hex << n << std::unsetf(std::ios_base::hex)
            << " String: " << str << "\n";
???
- The formatters are stateful. std::hex sets a formatting flag that you need to unset. All future numbers output has hex otherwise.
- std::endl does not only print EOL, also flushes. Kills performance.
--
        if(ofs.fail() || ofs.bad()) { 
          // error handling
        }
???
- The worst problem: error handling. What is the semantics of fail() and bad()? recoverable / not-recoverable? 
- That is a stupid distinction because the API cannot make this. 
  - *(Encountering eof while reading is certainly "recoverable", not being able to seek not so much)*
- If my algorithm requires a seekable stream, yet seek "only" sets failbit, that is a logic error in my
program and not recoverable. 
---
# So far, so bad
--

_How do you create a temporary file on Windows?_
???
- By themselves IO APIs aren't very good. 
- On top, they aren't powerful enough to support frequent use-cases
--

```cpp
HANDLE h = CreateFile(
      szFile,
      GENERIC_READ|GENERIC_WRITE,
*     FILE_SHARE_READ,
*     nullptr, // security descriptor, handle inheritance
      CREATE_NEW,
*     FILE_ATTRIBUTE_TEMPORARY, // attributes incl e.g. encryption, hidden flag
*     nullptr // template file
);
```
???
- Highlighted is everything std APIs cannot express
- similar options on POSIX: 
  - follow symlinks in szPath, 
  - block until file is available
  - exclusive or shared lock etc

- fopen/iostreams not flexible enough. Strict subset of platform-specific possibilities. 
- Creation from native handle is not specified (but exists on POSIX and Windows).
---
# What would we like to have?
--

## Better Files

- Platform-independent files, creatable from native handles 
- Use type-system to document file capabilities
- With sensible error handling

## Better Formatting

- Stateless formatting, extensible, usable for different types <br />
(Write to `std::string`, `std::vector<char>`, files, `IStream*`!) 
- Distinction between binary and text output
- Automatic encoding conversion UTF8 - UTF16

---

---
# Type Safety

--
    struct unmanaged_filebase_t {
      // ~unmanaged_filebase_t does not close file, use for stdin etc
      std::FILE* m_file;
    };

struct filebase_t : unmanaged_filebase_t {
      ~filebase_t(); // closes file
    };

???
- Uses FILE* internally, didn't want to reimplement buffering
- Use type system
--
    template<typename base>
    struct readfile_impl_t : base { /* read operations */ };

template<typename base>
    struct appendfile_impl_t : base { /* write operations */ };
---
# Type Safety

// Append-only, non-owned file, e.g. for writing to stdout
    using unmanaged_appendfile = appendfile_impl_t<unmanaged_filebase_t>;

// Append-only, owned file
    struct appendfile : appendfile_impl_t<filebase_t> { /* ... */ };

struct readwritefile = readfile_impl_t<appendfile_impl_t<filebase_t>> {
      // Inherits read, write operations on single file handle
    };
???
- Now we can play mix and match with the four building blocks 
---
# File Creation: POSIX
???
- Opening/creating files is platform-specific. 
- Constructors implemented differently for each OS
- Create FILE* from OS handle
--

```
appendfile(filechar const*  szFile, create_always_tag) 
    throw(std::ios_base::failure)
{
  int fd = open(szFile, O_WRONLY | O_APPEND | O_TRUNC | ..., ...); // may fail
  if(-1!=fd) throw std::ios_base::failure("");

// File handle conversion assumed not to fail
  std::FILE* file = VERIFY(fdopen(fd, "a"));
  ...
}
```
???
- VERIFY is a think-cell error handling macro, left in to document that we assume the conversion not to fail
---
# File Creation: Windows

```
appendfile(filechar const*  szFile, create_always_tag) 
    throw(std::ios_base::failure)
{
  HANDLE hfile = CreateFile(...); // may fail, error handling omitted
  if(INVALID_HANDLE_VALUE!=hfile) throw std::ios_base::failure("");

// File handle conversion assumed not to fail
  int fd = _open_osfhandle(reinterpret_cast<intptr_t>(hfile), _O_WRONLY);
  _ASSERT(-1 != fd);
  std::FILE* file = VERIFY(_fdopen(fd, "wb"))
  ...
}
```
---

# Error handling
???
- Uses exceptions, really useful for program flow in i/o code

```
template<typename base>
struct readfile_impl_t : base {
  // throws in case of error only:
  std::size_t try_read(void* pv, std::size_t cb) throw(std::ios_base::failure);

// throws in case of error or if less than cb bytes read:
  void read(void* pv, std::size_t cb) throw(std::ios_base::failure);

template<typename T>
  T read() throw(std::ios_base::failure);
};
```
???
- Different operations have different error cases
- Also: All reads (and writes) are binary

---
# Error handling

*Reading in chunks e.g. for parsing:*
```
auto cbRead = file.try_read(achBuffer, sizeof(achBuffer)); // may return < sizeof(achBuffer)
```
*Reading structured data*
```
int nLength = file.read<int>(); // must read sizeof(int)
// Now read nLength characters
```
---

---
# Better Formatting

// Stateless formatting, into arbitrary types, with encoding conversion:
      std::basic_string<char16_t> str;
      str << "Dec: " << as_dec(1000) << " Hex: " << as_hex(1000);
--

// We need that too (unfortunately):
      std::vector<char, some_custom_allocator> vecch;
      vecch << "Dec: " << as_dec(1000) << " Hex: " << as_hex(1000);
--

// And this of course:
      appendfile file(...);
      make_typed_stream<char16_t>(file) // write UTF16 to file
        << "Dec: " << as_dec(1000) << " Hex: " << as_hex(1000);
--

// Let's get crazy
      IStream* iStream = ...;
      make_typed_stream<char>(iStream)  // write UTF8 to iStream
        << "Dec: " << as_dec(1000) << " Hex: " << as_hex(1000);
--

_10 lines of code needed to adapt `IStream*` to formatting library._
---
# Make Everything Streamable

--
How to pipe characters into anything?
--

```
template<typename Sink>
typename std::enable_if< 
*  is_streamable<Sink>::value, 
  Sink&
>::type 
operator<<( Sink& os, stream_traits<Sink>::value_type ch) {
*  stream_traits<Sink>::append(os, ch);  
  return os;
}
```
--
## _We need traits of course!_
---

# Type Traits the Classic Way

template<typename T>
      struct stream_traits { // fall-back implementation
        using value_type = void; // with non-sensical value_type
      };

template<typename Char>
      struct stream_traits<std::vector<Char>> { // specialization for std::vector
        using value_type = Char; // provide needed typedefs and methods (omitted)
      }; 
--

template<typename T> // Define is_streamable<T> by looking up stream_traits<T>
      using is_streamable = std::integral_constant<
        bool, 
    *    !std::is_same<void, typename stream_traits<T>::value_type>::value
      >;
???
- Why is that not the best way? 
- Does not support traits for inherited types
- All traits must be in global namespace
---
# Type Traits using Function Lookup
       
    struct not_streamable {}; 
    not_streamable tc_stream_traits(...); // Fallback implemenation, always least preferred overload

template<typename Char>;
    struct vector_stream_traits { 
      using value_type = Char; // Again, required trait methods are omitted
    };    
    template<typename Char> // Overload tc_stream_traits for std::vector<Char>
    vector_stream_traits<Char> tc_stream_traits(std::vector<Char>&); 
--

template<typename T>
    using stream_traits = // Lookup return type of tc_stream_traits(T&) (with ADL and overloads!)
      decltype( tc_stream_traits( std::declval<std::decay_t<T>&>() ) );
--

template<typename T> // Again, is_streamable<T> is defined via stream_traits<T>
    using is_streamable = std::integral_constant<
          bool, 
    *     !std::is_same<not_streamable, stream_traits<T>>::value
        >;
---
# A Complete Traits Implementation
            
    struct istream_stream_traits {
      using value_type = unsigned char; // binary device, like a file

// Write a character
      static void append(ISequentialStream* iStream, unsigned char ch) noexcept {
        HRERR(iStream->Write(std::addressof(ch), 1, nullptr));
      }

// Write a range of characters
      static void append_range(ISequentialStream* iStream, Rng const& rng) noexcept {
        HRERR(iStream->Write(boost::begin(rng), size(rng), nullptr));
      }

// Flush device
      static void flush(ISequentialStream* iStream) noexcept {}
    };
    istream_stream_traits tc_stream_traits(ISequentialStream*);

_`IStream*` is convertible to `ISequentialStream*`_

---
# Converting Encodings

```
template<typename Sink, typename Rng>
typename std::enable_if< 
*  is_streamable<Sink>::value && is_char_range<Rng>::value, 
  Sink&
>::type 
operator<<( Sink& os, Rng&& rng) {
* ConvertEnc<char_t of Rng, char_t of Sink>::Append(os, std::forward<Rng>(rng));
  return os;
}
```
--
- `ConvertEnc<Char, Char>::Append(os, rng)` simply calls `stream_traits<Sink>::append_range`
- Specializations `ConvertEnc<char, char16_t>::Append` and `ConvertEnc<char16_t, char>::Append` convert encodings
- Compile-time error when no suitable conversion exists, e.g. `ConvertEnc<char16_t, unsigned char>`
---
# Converting Encodings

_Convert UTF-8 to UTF-16:_
```
std::basic_string<char16_t> str; 
str << "Hello World\n"; // UTF8 to UTF16
```
--
_Files are binary and have no encoding. This does not compile:_
```
appendfile file = ...;
file << "Hello World\n";
```
--
_Declare file to accept UTF-8 encoding:_
```
appendfile file = ...;
make_typed_stream<char>(file) << "Hello World\n";
```

---
# Streaming Custom Types

--
Overload global `operator<<` for custom types:

```
template<typename Sink>
typename std::enable_if< 
*  is_streamable<Sink>::value, 
  Sink&
>::type 
operator<<( Sink& os,boost::uuids::uuid const& uuid) {
*  os << boost::uuids::to_string(uuid);
  return os;
}
```
---

# Formatting Numbers

```
template<T>
struct as_dec_impl {
  T m_f;
  ...
  // Decimal separator, precision, padding etc
      
  template<typename Sink>
  friend typename std::enable_if< 
*    is_streamable<Sink>::value, 
    Sink& 
  >::type 
  operator<<( Sink& os, as_dec_impl<T,Char> const& n) {
*        // Convert m_f to characters and stream those to os
        return os;
  }
};
```
--
_Formatters we have:_ 
- `as_dec(FloatingPointType f, CharType chDecimal)`
- `as_padded_lc_hex(IntegralType const& t)`

---
# Thank You!

// Stateless formatting, into arbitrary types, with encoding conversion:
      std::basic_string<char16_t> str;
      str << "Dec: " << as_dec(1000) << " Hex: " << as_hex(1000);

// We need that too (unfortunately):
      std::vector<char, some_custom_allocator> vecch;
      vecch << "Dec: " << as_dec(1000) << " Hex: " << as_hex(1000);

// And this of course:
      appendfile file(...);
      make_typed_stream<char16_t>(file) // write UTF16 to file
        << "Dec: " << as_dec(1000) << " Hex: " << as_hex(1000);

// Let's get crazy
      IStream* iStream = ...;
      make_typed_stream<char>(iStream)  // write UTF8 to iStream
        << "Dec: " << as_dec(1000) << " Hex: " << as_hex(1000);

---
class: middle
# Slides & Benchmark
## https://github.com/stheophil/iostreams_talk
## https://github.com/CppUGBerlin/Talks

---
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# _"iostreams are super slow!"_
--

## _... but are they?_
---
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<img alt="Column chart comparing iostream and file" src="/assets/en/career/talks/iostreams/benchmark_01.png" width="90%" />

**[Times in ms]**
???
- Small benchmark: Writing 1 Mb chunks or smaller chunk using formatter
- with ostream or with FILE*
- yes, ostream is slower than FILE* in this comparison, but as discussed,
you also get some features from ostreams that FILE* does not have
- BUT
---
class: middle
<img alt="Column chart comparing iostream, file and os" src="/assets/en/career/talks/iostreams/benchmark_02.png" width="90%" />

**[Times in ms]**
???
- What if we write 1M chunks directly using OS function?
- Performance diff between iostream and FILE* becomes almost irrelevant. 
- If you know what you're doing, you can do much faster than std functions. 
- What if we're writing smaller formatted chunks using OS functions?
---
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<img alt="Column chart comparing iostream, file and os formatted write" src="/assets/en/career/talks/iostreams/benchmark_03.png" width="90%" />

**[Times in ms]**

???
- Without userland buffer we can also do much worse than iostream/FILE

---
# Remarks for Implementors

1. The number of `std::FILE*` that can be returned is limited on POSIX and Windows systems. 
  - When using the Windows CRT, call `_setmaxstdio(8192)` to increase the limit to its maximum permitted value
  - On POSIX systems, call `setrlimit(RLIMIT_NOFILE, ...)` before you do any standard I/O call

2. According to the standard, when a `std::FILE*` is opened both for reading and writing, you must call `fseek` or `fflush` when you
switch from reading to writing or vice versa.

---
# Other File Implementations

- **Chromium:**
  https://chromium.googlesource.com/chromium/src.git/+/master/base/files/file.h

- **QT:**
  http://doc.qt.io/qt-5/qfile.html

- **Juce:**
  http://www.juce.com/doc/classFile

# Formatting
- **cppformat:** printf-style formatting but type-safe
  http://cppformat.github.io/latest/reference.html

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