# Ruby from C++ (new to both) question

**URL:** <https://www.ruby-forum.com/t/ruby-from-c-new-to-both-question/172606>\
**Category:** Ruby\
**Created:** [July 24, 2009, 12:27am UTC](https://www.ruby-forum.com/t/ruby-from-c-new-to-both-question/172606 "2009-07-24T00:27:44Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Steven\_LeBeau](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/f07891/32.png) [@Steven\_LeBeau](https://www.ruby-forum.com/u/Steven_LeBeau)\
**Post date:** [July 24, 2009, 12:27am UTC](https://www.ruby-forum.com/t/ruby-from-c-new-to-both-question/172606/1 "2009-07-24T00:27:44Z")

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Hi,

I’m currently taking a beginning C++ class and learning Ruby on my own.

One thing I’m curious about (but don’t have the vocabulary to articulate  
it  
to a search engine, apparently) is if there’s an equivalent to declaring  
functions and then defining them later in the file so you don’t have to  
worry about where the function is in relation to another function that  
calls  
it (or is called by it). In Ruby, I guess I’m talking about methods  
rather  
than functions, but the same principle: is there a way for method x to  
call  
method y (which appears later on in the program) by having declaration  
statements at the top of your program? (I think that time my question  
may  
have made more sense).

This wasn’t addressed on the Ruby webpage on the “Ruby From C and C++”  
page  
as being a “difference” or a “similarity”.

-Steven

P.S.–I’m new. Hi!

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<div class="post-metadata">

**Author:** ![Steven\_LeBeau](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/f07891/32.png) [@Steven\_LeBeau](https://www.ruby-forum.com/u/Steven_LeBeau)\
**Post date:** [July 24, 2009, 5:05am UTC](https://www.ruby-forum.com/t/ruby-from-c-new-to-both-question/172606/2 "2009-07-24T05:05:24Z")

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Steven,  
C/++ and Ruby are fundamentally different in a few ways, and you have  
stumbled onto one of the effects of those differences.

With C/++, the reason you name those functions before calling them is  
for  
something which basically sanity-checks your code before compiling it to  
a  
binary. Regardless, when you run a program written with C/++, it has  
parsed  
the entire file and the function you wrote at the end of the file are  
available from the beginning of the execution. This is a gross  
oversimplification, I know, but I hope that it’s sufficiently clear.

In Ruby, each line is executed one after the other. So, the short answer  
is  
that there is not a way to do what you are saying.  
Technically, you could def method; end and then redefine the method  
later,  
however, a method does not need to be defined for you to define a method  
which calls it: that is, there is no sanity check to confirm that  
“variable”  
knows how to “do\_something” in the following code.

def method  
variable.do\_something  
end

So this allows you to include all of your libraries at the top of your  
file  
and then actually call the methods later. Provided that the method  
exists at  
the time that the line is actually executed, you won’t run into  
problems.

I hope this makes sense. Welcome to Ruby! I think you’ll like it quite a  
lot.

-chris

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<div class="post-metadata">

**Author:** ![Steven\_LeBeau](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/f07891/32.png) [@Steven\_LeBeau](https://www.ruby-forum.com/u/Steven_LeBeau)\
**Post date:** [July 24, 2009, 5:20am UTC](https://www.ruby-forum.com/t/ruby-from-c-new-to-both-question/172606/3 "2009-07-24T05:20:00Z")

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Steven LeBeau wrote:

> method y (which appears later on in the program) by having declaration  
> statements at the top of your program? (I think that time my question may  
> have made more sense).
> 
> This wasn’t addressed on the Ruby webpage on the “Ruby From C and C++” page  
> as being a “difference” or a “similarity”.
> 
> -Steven
> 
> P.S.–I’m new. Hi!

Hi, welcome to ruby!

In ruby, there is no need to declare a function before the compiler sees  
it, only before your program calls it. If your functions are all in  
classes or modules, you rarely need to think about how they are ordered  
in files.

The case where order matters is when you have a function called directly  
from a script file, as you would if you were writing a script to be  
called from the command line (rather than a library). In that case,  
there is a trick that you can use to call a function above where it is  
defined, if you prefer. You can use the BEGIN construct to designate a  
block to be evaluated after the file is parsed, but before anything else  
in the file is executed:

####################  
main

BEGIN {  
def main  
puts “Hello, world.”  
end  
}  
####################

There is also an END construct, which you can use to get the same  
effect:

####################  
END {  
main  
}

def main  
puts “Hello, world.”  
end  
####################

Without any special trickery, you would have to put the main call below  
the definition, which arguably obscures the intent of the script.  
(There’s no need to call the method “main”–that’s just an example.)

HTH.