Re: g++ compile time calculations

[Daniel Lohmann <daniel dot lohmann at informatik dot uni-erlangen dot de>]

Wesley Smith schrieb:
> Hi,
> I'm trying to figure out how to do some compile time C++ template
> programming.  I have something working that calculates the length of a
> string, but I'm not sure how to evaluate what is actually being done
> at compile time and what is being done at run time.  Here's is my
> code:
>
> template <class T>
> class StringLength
> {
>     public:
>         static int eval(T* a)
>                 {    return ((*a) == '\0') ? 0 : 
> (StringLength<T>::eval(a+1) + 1);  }
> };
>
> template <class T>
> inline int stringlength(T* a) { return StringLength<T>::eval(a); }
>
>
>
> //in main somewhere
> cout << stringlength<>("This is a string") << endl;
>
>
> My GCC version is gcc version 4.0.1 (Apple Computer, Inc. build 5341).
> I'm on OSX 10.4.8 on a PPC.   Basically, I'm trying to find out what
> compiler output I can look at or somehow query things at runtime to
> find out what did and didn't compile down.


I suggest taking a close look at the generate assembler code, e.g. using 
objdump:

objdump -D --demangle --reloc objfile.o | less

However, without taking a close look at the generated code,  I am quite 
sure that there isn't much that is compiled down. Your template isn't 
actually a "template meta program" - it does not use recursive 
instantiation of templates, but just recursive function calls:

         static int eval(T* a)
                 {    return ((*a) == '\0') ? 0 :
(StringLength<T>::eval(a+1) + 1);  }

This is always the same StringLength::eval() method that is recursively 
invoked at *runtime*.

Daniel