Boost::asio what is this kind of strange coding style?
I am about to debug something within my boost asio socket communication. And found this piece of code inside of the asio library (found in boost/asio/impl/write.hpp line 169 (boost 1.47) ):
switch (start)
{
case 1:
buffers_.prepare(this->check_for_completion(ec, total_transferred_));
for (;;)
{
stream_.async_write_some(buffers_,
BOOST_ASIO_MOVE_CAST(write_op)(*this));
return;
default:
total_transferred_ += bytes_transferred;
buffers_.consume(bytes_transferred);
buffers_.prepare(this->check_for_completion(ec, total_transferred_));
if ((!ec && bytes_transferred == 0)
|| buffers_.begin() == buffers_.end())
break;
}
handler_(ec, static_cast<const std::size_t&>(total_transferred_));
}
I have already a lot of years of C/C++ development experience, but never ever in my life I saw such kind of weird implementation. Look there, the default: label of the switch statement is within the for loop.
If I understand this right, then the switch statement is "misused" instead of a goto, right (for the cases where start != 1, goto default:)? Is it actually a valid C/C++ with respect to the standard? What will happen if I for example put
for(int i=0; i < 10; i++)
instead of the for loop in the original code. Will "i" be undefined in case when jump is performed to default: label? Sure I might use a debugger here, however this seems so suspicious for me, that I think this could produce different behavior for different compilers.
This is well-defined, valid code. A switch really is a glorified goto . For some clever use of this construct, take a look at Duff's device.
As for your for , that wouldn't be legal. A jump to a case label can't cross an initialization.
它似乎是达夫设备的一个版本。
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