Re: Compile time vs runtime?

From:

=?iso-8859-1?q?Erik_Wikstr=F6m?= <eriwik@student.chalmers.se>

Newsgroups:

comp.lang.c++

Date:

9 May 2007 07:19:05 -0700

Message-ID:

<1178720345.160346.238810@o5g2000hsb.googlegroups.com>

On 9 Maj, 13:57, desktop <f...@sss.com> wrote:

Erik Wikstr=F6m wrote:

On 9 Maj, 11:28, desktop <f...@sss.com> wrote:

I have read that using templates makes types know at compile time and
using inheritance the types are first decided at runtime. The use of
pointers and casts also indicates that the types will first be know at
runtime.

Don't know if this answers your question or not; the idea behind
templates it that you at compile-time knows all the types that can be
used, and then generate code for each of those types. That's why it's
called templates, since it's not the code you write that gets executed
since it's just a template used by the compiler to generate the code
that will be executed.

But is there some strict definitions that defines runtime code and
compile time code that can be used in general?

Compile-time is compile-time, meaning that all the magic takes place
before the application is compiled, which is about as typesafe as you
can get. With runtime the magic happens when needed and have not been
fully checked at compilation. This means that for compile-time
polymorphism you need to know most if not everything about the types
at compile-time, which is why you can't have containers that can be
used with any type as compiled code, but have to distribute the source
(see the standard containers).

Is not possible to have a heterogeneous container at compile-time? I
have read about a method that uses the heap and makes sure that each
element in the container is allocated:

http://gethelp.devx.com/techtips/cpp_pro/10min/10min0900.asp

In this article they store pointers to a base-class, so the elements
stored are all of the same type. However when you dereference them you
can utilize runtime polymorphism.

Another technique deals with simulating dynamic polymorphism (deriving
classes from a base class) with the "The Curiously Recurring Template
Pattern": CRTP:

http://en.wikipedia.org/wiki/Curiously_Recurring_Template_Pattern

Would either of these method not help me to generate a container with
different types at compile-time?

No, read this entry in the FAQ for a description about how to create
heterogeneous container using templates, notice that the first method
is the same as in the first article you posted.
http://www.parashift.com/c++-faq-lite/containers.html#faq-34.4

--
Erik Wikstr=F6m

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