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Add usage examples
This commit is contained in:
Peter Dimov
@ -19,6 +19,7 @@ Beman Dawes, Christopher Kohlhoff, Peter Dimov
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:leveloffset: +1
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include::system/introduction.adoc[]
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include::system/usage.adoc[]
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include::system/changes.adoc[]
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include::system/rationale.adoc[]
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include::system/reference.adoc[]
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612
doc/system/usage.adoc
Normal file
612
doc/system/usage.adoc
Normal file
@ -0,0 +1,612 @@
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////
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Copyright 2021 Peter Dimov
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Distributed under the Boost Software License, Version 1.0.
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https://www.boost.org/LICENSE_1_0.txt
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////
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[#usage]
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# Usage Examples
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:idprefix: usage_
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All of the following examples assume that these lines
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```
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#include <boost/system.hpp>
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namespace sys = boost::system;
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```
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are in effect.
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## Returning Errors from OS APIs under POSIX
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Let's suppose that we're implementing a portable `file` wrapper
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over the OS file APIs. Its general outline is shown below:
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```
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class file
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{
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private:
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int fd_;
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public:
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// ...
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std::size_t read( void * buffer, std::size_t size, sys::error_code& ec );
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std::size_t write( void const * buffer, std::size_t size, sys::error_code& ec );
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};
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```
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Since we're implementing the POSIX version of `file`, its
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data member is a POSIX file descriptor `int fd_;`, although other
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implementations will differ.
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Our `read` and `write` functions return the number of bytes transferred, and signal
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errors via the output parameter `ec`, of type `boost::system::error_code`.
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An implementation of `file::read` might look like this:
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```
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std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
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{
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ssize_t r = ::read( fd_, buffer, size );
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if( r < 0 )
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{
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ec.assign( errno, sys::system_category() );
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return 0;
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}
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ec = {}; // ec.clear(); under C++03
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return r;
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}
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```
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We first call the POSIX API `read`; if it returns an error, we store the `errno`
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value in `ec`, using the system category, and return 0 as bytes transferred.
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Otherwise, we clear `ec` to signal success, and return the result of `::read`.
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NOTE: Clearing `ec` on successful returns is an important step; do not omit it.
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Under POSIX, the system category corresponds to POSIX `errno` values, which is
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why we use it.
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In principle, since the generic category _also_ corresponds to `errno` values
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under all platforms, we could have used it here; however, by convention under
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POSIX, if the `errno` value comes from the OS (the "system"), we use the system
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category for it. That's because the system category values may be a
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platform-specific superset of the generic (platform-independent) values.
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The implementation of `file::write` is basically the same. We show it here for
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completeness:
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```
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std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
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{
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ssize_t r = ::write( fd_, buffer, size );
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if( r < 0 )
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{
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ec.assign( errno, sys::system_category() );
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return 0;
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}
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ec = {}; // ec.clear(); under C++03
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return r;
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}
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```
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## Returning Errors from OS APIs under Windows
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Under Windows, our `file` object will store a `HANDLE` instead of an `int`:
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```
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class file
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{
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private:
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HANDLE fh_;
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public:
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// as before
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};
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```
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and the implementation of `file::read` will look like this:
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```
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std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
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{
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DWORD r = 0;
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if( ::ReadFile( fh_, buffer, size, &r, 0 ) )
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{
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// success
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ec = {}; // ec.clear(); under C++03
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}
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else
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{
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// failure
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ec.assign( ::GetLastError(), sys::system_category() );
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}
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// In both cases, r is bytes transferred
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return r;
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}
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```
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Here, the system category corresponds to the values defined in the system
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header `<winerror.h>` and returned by `GetLastError()`. Since we use the
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Win32 API `ReadFile` to implement `file::read`, and it returns the error
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code via `GetLastError()`, we again store that value in `ec` as belonging
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to the system category.
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The implementation of `file::write` is, again, the same.
|
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```
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std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
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{
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DWORD r = 0;
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if( ::WriteFile( fh_, buffer, size, &r, 0 ) )
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{
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ec = {}; // ec.clear(); under C++03
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}
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else
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{
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ec.assign( ::GetLastError(), sys::system_category() );
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}
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return r;
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}
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```
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## Returning Specific Errors under POSIX
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Our implementation of `file::read` has a problem; it accepts `std::size_t`
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values for `size`, but the behavior of `::read` is unspecified when the
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requested value does not fit in `ssize_t`. To avoid reliance on unspecified
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behavior, let's add a check for this condition and return an error:
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```
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std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
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{
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if( size > SSIZE_MAX )
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{
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ec.assign( EINVAL, sys::generic_category() );
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return 0;
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}
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ssize_t r = ::read( fd_, buffer, size );
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if( r < 0 )
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{
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ec.assign( errno, sys::system_category() );
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return 0;
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}
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ec = {}; // ec.clear(); under C++03
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return r;
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}
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```
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In this case, since we're returning the fixed `errno` value `EINVAL`, which
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is part of the portable subset defined by the generic category, we mark the
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error value in `ec` as belonging to the generic category.
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|
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It's possible to use system as well, as `EINVAL` is also a system category
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value under POSIX; however, using the generic category for values belonging
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to the portable `errno` subset is slightly preferrable.
|
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|
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Our implementation of `file::write` needs to uindergo a similar treatment.
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There, however, we'll apply another change. When there's no space left on
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the disk, `::write` returns a number of bytes written that is lower than
|
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what we requested with `size`, but our function signals no error. We'll make
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it return `ENOSPC` in this case.
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|
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```
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std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
|
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{
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if( size > SSIZE_MAX )
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{
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ec.assign( EINVAL, sys::generic_category() );
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return 0;
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}
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ssize_t r = ::write( fd_, buffer, size );
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if( r < 0 )
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{
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ec.assign( errno, sys::system_category() );
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return 0;
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}
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if( r < size )
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{
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ec.assign( ENOSPC, sys::system_category() );
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}
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else
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{
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ec = {}; // ec.clear(); under C++03
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}
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return r;
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}
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```
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We've used the system category to make it appear that the `ENOSPC` value
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has come from the `::write` API, mostly to illustrate that this is also a
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possible approach. Using a generic value would have worked just as well.
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## Returning Specific Errors under Windows
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Not much to say; the situation under Windows is exactly the same. The only
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difference is that we _must_ use the generic category for returning `errno`
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values. The system category does not work; the integer values in the system
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category are entirely different from those in the generic category.
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```
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std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
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{
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DWORD r = 0;
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if( size > MAXDWORD )
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{
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ec.assign( EINVAL, sys::generic_category() );
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}
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else if( ::ReadFile( fh_, buffer, size, &r, 0 ) )
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{
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ec = {}; // ec.clear(); under C++03
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}
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else
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{
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ec.assign( ::GetLastError(), sys::system_category() );
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}
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return r;
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}
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std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
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{
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DWORD r = 0;
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if( size > MAXDWORD )
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{
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ec.assign( EINVAL, sys::generic_category() );
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}
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else if( ::WriteFile( fh_, buffer, size, &r, 0 ) )
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{
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if( r < size )
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{
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ec.assign( ENOSPC, sys::generic_category() );
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}
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else
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{
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ec = {}; // ec.clear(); under C++03
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}
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}
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else
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{
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ec.assign( ::GetLastError(), sys::system_category() );
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}
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return r;
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}
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```
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## Attaching a Source Location to Error Codes
|
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Unlike the standard `<system_error>`, Boost.System allows source locations
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(file:line:function) to be stored in `error_code`, so that functions handling
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the error can display or log the source code location where the error occurred.
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To take advantage of this functionality, our POSIX `file::read` function needs
|
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to be augmented as follows:
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```
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std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
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{
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if( size > SSIZE_MAX )
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{
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static constexpr boost::source_location loc = BOOST_CURRENT_LOCATION;
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ec.assign( EINVAL, sys::generic_category(), &loc );
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return 0;
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}
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ssize_t r = ::read( fd_, buffer, size );
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if( r < 0 )
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{
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static constexpr boost::source_location loc = BOOST_CURRENT_LOCATION;
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ec.assign( errno, sys::system_category(), &loc );
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return 0;
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}
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ec = {}; // ec.clear(); under C++03
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return r;
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}
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```
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That is, before every `ec.assign` statement, we need to declare a
|
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`static constexpr` variable holding the current source location, then pass
|
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a pointer to it to `assign`. Since `error_code` is small and there's no space
|
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in it for more than a pointer, we can't just store the `source_location` in it
|
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by value.
|
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`BOOST_CURRENT_LOCATION` is a macro expanding to the current source location
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(a combination of `++__FILE__++`, `++__LINE__++`, and `BOOST_CURRENT_FUNCTION`.)
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It's defined and documented in link:../../../assert/index.html[Boost.Assert].
|
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|
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Under {cpp}03, instead of `static constexpr`, one needs to use `static const`.
|
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Another option is `BOOST_STATIC_CONSTEXPR`, a
|
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link:../../../config/index.html[Boost.Config] macro that expands to either
|
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`static constexpr` or `static const`, as appropriate.
|
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|
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To avoid repeating this boilerplate each time we do `ec.assign`, we can define
|
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a macro:
|
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|
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```
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#define ASSIGN(ec, ...) { \
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BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION; \
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(ec).assign(__VA_ARGS__, &loc); }
|
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```
|
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|
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which we can now use to augment, for example, the POSIX implementation of `file::write`:
|
||||
|
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```
|
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std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
|
||||
{
|
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if( size > SSIZE_MAX )
|
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{
|
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ASSIGN( ec, EINVAL, sys::generic_category() );
|
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return 0;
|
||||
}
|
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|
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ssize_t r = ::write( fd_, buffer, size );
|
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|
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if( r < 0 )
|
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{
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ASSIGN( ec, errno, sys::system_category() );
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return 0;
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}
|
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|
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if( r < size )
|
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{
|
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ASSIGN( ec, ENOSPC, sys::generic_category() );
|
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}
|
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else
|
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{
|
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ec = {}; // ec.clear(); under C++03
|
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}
|
||||
|
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return r;
|
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}
|
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```
|
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## Obtaining Textual Representations of Error Codes for Logging and Display
|
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|
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Assuming that we have an `error_code` instance `ec`, returned to us by some
|
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function, we have a variety of means to obtain textual representations of the
|
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error code represented therein.
|
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|
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`ec.to_string()` gives us the result of streaming `ec` into a `std::ostream`,
|
||||
e.g. if `std::cout << ec << std::endl;` outputs `system:6`, this is what
|
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`ec.to_string()` will return. (`system:6` under Windows is `ERROR_INVALID_HANDLE`
|
||||
from `<winerror.h>`.)
|
||||
|
||||
To obtain a human-readable error message corresponding to this code, we can
|
||||
use `ec.message()`. For `ERROR_INVALID_HANDLE`, it would give us "The handle is
|
||||
invalid" - possibly localized.
|
||||
|
||||
If `ec` contains a source location, we can obtain its textual representation
|
||||
via `ec.location().to_string()`. This will give us something like
|
||||
|
||||
```text
|
||||
C:\Projects\testbed2019\testbed2019.cpp:98 in function 'unsigned __int64 __cdecl file::read(void *,unsigned __int64,class boost::system::error_code &)'
|
||||
```
|
||||
|
||||
if there is a location in `ec`, and
|
||||
|
||||
```text
|
||||
(unknown source location)
|
||||
```
|
||||
|
||||
if there isn't. (`ec.has_location()` is `true` when `ec` contains a location.)
|
||||
|
||||
Finally, `ec.what()` will give us a string that contains all of the above,
|
||||
something like
|
||||
|
||||
```text
|
||||
The handle is invalid [system:6 at C:\Projects\testbed2019\testbed2019.cpp:98 in function 'unsigned __int64 __cdecl file::read(void *,unsigned __int64,class boost::system::error_code &)']
|
||||
```
|
||||
|
||||
Most logging and diagnostic output that is not intended for the end user would
|
||||
probably end up using `what()`. (`ec.what()`, augmented with the prefix
|
||||
supplied at construction, is also what `boost::system::system_error::what()`
|
||||
would return.)
|
||||
|
||||
## Composing Functions Returning Error Codes
|
||||
|
||||
Let's suppose that we need to implement a file copy function, with the following
|
||||
interface:
|
||||
|
||||
```
|
||||
std::size_t file_copy( file& src, file& dest, sys::error_code& ec );
|
||||
```
|
||||
|
||||
`file_copy` uses `src.read` to read bytes from `src`, then writes these bytes
|
||||
to `dest` using `dest.write`. This continues until one of these operations signals
|
||||
an error, or until end of file is reached. It returns the number of bytes written,
|
||||
and uses `ec` to signal an error.
|
||||
|
||||
Here is one possible implementation:
|
||||
|
||||
```
|
||||
std::size_t file_copy( file& src, file& dest, sys::error_code& ec )
|
||||
{
|
||||
std::size_t r = 0;
|
||||
|
||||
for( ;; )
|
||||
{
|
||||
unsigned char buffer[ 1024 ];
|
||||
|
||||
std::size_t n = src.read( buffer, sizeof( buffer ), ec );
|
||||
|
||||
// read failed, leave the error in ec and return
|
||||
if( ec.failed() ) return r;
|
||||
|
||||
// end of file has been reached, exit loop
|
||||
if( n == 0 ) return r;
|
||||
|
||||
r += dest.write( buffer, n, ec );
|
||||
|
||||
// write failed, leave the error in ec and return
|
||||
if( ec.failed() ) return r;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
Note that there is no longer any difference between POSIX and Windows
|
||||
implementations; their differences are contained in `file::read` and
|
||||
`file::write`. `file_copy` is portable and works under any platform.
|
||||
|
||||
The general pattern in writing such higher-level functions is that
|
||||
they pass the output `error_code` parameter `ec` they received from
|
||||
the caller directly as the output parameter to the lower-level functions
|
||||
they are built upon. This way, when they detect a failure in an intermediate
|
||||
operation (by testing `ec.failed()`), they can immediately return to the
|
||||
caller, because the error code is already in its proper place.
|
||||
|
||||
Note that `file_copy` doesn't even need to clear `ec` on success, by
|
||||
using `ec = {};`. Since we've already tested `ec.failed()`, we know that
|
||||
`ec` contains a value that means success.
|
||||
|
||||
## Providing Dual (Throwing and Nonthrowing) Overloads
|
||||
|
||||
Functions that signal errors via an output `error_code& ec` parameter
|
||||
require that the caller check `ec` after calling them, and take appropriate
|
||||
action (such as return immediately, as above.) Forgetting to check `ec`
|
||||
results in logic errors.
|
||||
|
||||
While this is a preferred coding style for some, others prefer exceptions,
|
||||
which one cannot forget to check.
|
||||
|
||||
An approach that has been introduced by
|
||||
link:../../../filesystem/index.html[Boost.Filesystem] (which later turned
|
||||
into `std::filesystem`) is to provide both alternatives: a nonthrowing
|
||||
function taking `error_code& ec`, as `file_copy` above, and a throwing
|
||||
function that does not take an `error_code` output parameter, and throws
|
||||
exceptions on failure.
|
||||
|
||||
This is how this second throwing function is typically implemented:
|
||||
|
||||
```
|
||||
std::size_t file_copy( file& src, file& dest )
|
||||
{
|
||||
sys::error_code ec;
|
||||
std::size_t r = file_copy( src, dest, ec );
|
||||
|
||||
if( ec.failed() ) throw sys::system_error( ec, __func__ );
|
||||
|
||||
return r;
|
||||
}
|
||||
```
|
||||
|
||||
That is, we simply call the nonthrowing overload of `file_copy`, and if
|
||||
it signals failure in `ec`, throw a `system_error` exception.
|
||||
|
||||
We use our function name `++__func__++` (`file_copy`) as the prefix, although
|
||||
that's a matter of taste.
|
||||
|
||||
Note that typically under this style the overloads taking `error_code& ec`
|
||||
are decorated with `noexcept`, so that it's clear that they don't throw
|
||||
exceptions (although we haven't done so in the preceding examples in order
|
||||
to keep the code {cpp}03-friendly.)
|
||||
|
||||
## result<T> as an Alternative to Dual APIs
|
||||
|
||||
Instead of providing two functions for every operation, an alternative
|
||||
approach is to make the function return `sys::result<T>` instead of `T`.
|
||||
`result<T>` is a class holding either `T` or `error_code`, similar to
|
||||
link:../../../variant2/index.html[`variant<T, error_code>`].
|
||||
|
||||
Clients that prefer to check for errors and not rely on exceptions can
|
||||
test whether a `result<T> r` contains a value via `if( r )` or its more
|
||||
verbose equivalent `if( r.has_value() )`, then obtain the value via
|
||||
`*r` or `r.value()`. If `r` doesn't contain a value, the `error_code`
|
||||
it holds can be obtained with `r.error()`.
|
||||
|
||||
Those who prefer exceptions just call `r.value()` directly, without
|
||||
checking. In the no-value case, this will automatically throw a
|
||||
`system_error` corresponding to the `error_code` in `r`.
|
||||
|
||||
Assuming our base `file` API is unchanged, this variation of `file_copy`
|
||||
would look like this:
|
||||
|
||||
```
|
||||
sys::result<std::size_t> file_copy( file& src, file& dest )
|
||||
{
|
||||
std::size_t r = 0;
|
||||
sys::error_code ec;
|
||||
|
||||
for( ;; )
|
||||
{
|
||||
unsigned char buffer[ 1024 ];
|
||||
|
||||
std::size_t n = src.read( buffer, sizeof( buffer ), ec );
|
||||
|
||||
if( ec.failed() ) return ec;
|
||||
if( n == 0 ) return r;
|
||||
|
||||
r += dest.write( buffer, n, ec );
|
||||
|
||||
if( ec.failed() ) return ec;
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
The only difference here is that we return `ec` on error, instead of
|
||||
`r`.
|
||||
|
||||
Note, however, that we can no longer return both an error code and a
|
||||
number of transferred bytes; that is, we can no longer signal _partial
|
||||
success_. This is often not an issue at higher levels, but lower-level
|
||||
primitives such as `file::read` and `file::write` might be better off
|
||||
written using the old style.
|
||||
|
||||
Nevertheless, to demonstrate how `result` returning APIs are composed,
|
||||
we'll show how `file_copy` would look if `file::read` and `file::write`
|
||||
returned `result<size_t>`:
|
||||
|
||||
```
|
||||
class file
|
||||
{
|
||||
public:
|
||||
|
||||
// ...
|
||||
|
||||
sys::result<std::size_t> read( void * buffer, std::size_t size );
|
||||
sys::result<std::size_t> write( void const * buffer, std::size_t size );
|
||||
};
|
||||
|
||||
sys::result<std::size_t> file_copy( file& src, file& dest )
|
||||
{
|
||||
std::size_t m = 0;
|
||||
|
||||
for( ;; )
|
||||
{
|
||||
unsigned char buffer[ 1024 ];
|
||||
|
||||
auto r = src.read( buffer, sizeof( buffer ) );
|
||||
if( !r ) return r;
|
||||
|
||||
std::size_t n = *r;
|
||||
if( n == 0 ) return m;
|
||||
|
||||
auto r2 = dest.write( buffer, n );
|
||||
if( !r2 ) return r2;
|
||||
|
||||
std::size_t n2 = *r2;
|
||||
m += n2;
|
||||
}
|
||||
}
|
||||
```
|
||||
Reference in New Issue
Block a user