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.