Larry Evans wrote:
On 01/06/2005 02:29 PM, David Abrahams wrote:
* ... your ideas here ... :)
I've had some trouble understanding how placeholders and lambda expressions work. Specifically, I've wondered why, when a template is instantiated with _1, _2, ..., that the instantiation doesn't produce some sort of error messages.
Forming a lambda expression doesn't cause the component templates to be instantiated (on a conforming compiler).
For instance, if the template requires one of its arguments to satisfy some requirement, then how could _1 or _2 satisfy such a requirement since they couldn't know about it? Then I'm wondering about the complexity when the lambda is applied to an argument. Wouldn't each _1 have to be replaced by the actual argument, and wouldn't this require somehow parsing the template expression?
Yes, that's what Dave was talking about in his original message. In theory the implementation is not difficult; the hard part was thinking of it in the first place, and then getting it to work on a bunch of broken compilers. Jonathan