Christopher Kohlhoff wrote:
Hi all,
As part of the asio library I have been using named placeholders for the common arguments to callback functions. For example:
asio::async_send_n(sock, buffer, length, boost::bind(mycallback, asio::arg::error, asio::arg::last_bytes_sent, asio::arg::total_bytes_sent));
rather than:
asio::async_send_n(sock, buffer, length, boost::bind(mycallback, _1, _2, _3));
I find having named placeholders saves time and helps prevent errors to do with the ordering of the parameters (e.g. in the case of something like the last_bytes_sent/total_bytes_sent example where both are size_t, and so incorrect order will not be detected by the compiler).
Unfortunately, from my reading of the TR1 bind doco at
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1455.htm
it simply defines the placeholders as:
namespace std { namespace placeholders { extern unspecified _1; extern unspecified _2; extern unspecified _3; // implementation defined number of additional placeholders } }
and it doesn't say anything about the type of the placeholders. Since the placeholders are CopyConstructible, it seems like I ought to be able to do something using a typedef (if there was one), e.g:
std::placeholders::_1_t my_placeholder(std::placeholders::_1);
Is there some TR1-compatible (and therefore portable) way of working around it that's eluding me (other than #define :).
We decided to not expose the types of the default _N placeholders in TR1, but you can add your own placeholder types by specializing std::tr1::is_placeholder.