Re: Recursive template instantiation error--help!

[Sam Varshavchik <mrsam at courier-mta dot com>]

Patrick Horgan writes:

> Building with:
>
> g++-4.6.real (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
>
> This program:
>
> template<unsigned int v>
> class testit{
> public:
>     static const unsigned int width =
>         ( v==0 ? 0 : testit<v >> 1>::width + 1);
> };
>
> int
> main()
> {
>     unsigned int foo=testit<3>::width;
> }
>
> Errors out with complaint of exceeding maximum template instantiation
> depth (error below) even though it seems it should compile like:
>
> testit<3>
>    3==0?  No, so width = testit <3 >> 1>::width + 1 ( which would be 1 + 1)
>    testit<1>
>        1==0? No, so width = testit<1 >> 1>::width+1 (which would be 0 + 1)
>        testit<0>
>        0==0 so width=0

The problem is that short-circuit evaluation gets carried out only /AFTER/  
the entire expression is _fully_ parsed. So:

testit<0>:

   0 == 0 ? 0 : testit<0>::width+1;

The compiler will then proceed and attempt to expand another instance of  
testit<0>, which then obviously results in an infinite loop.

The correct way to do this is to use specialization. Instead of the ternary  
expression, specialize your template:

template<unsigned int v> class testit {
public:
 static const unsigned int width=testit<v >> 1>::width+1;
};

template<> class testit<0> {
public:
 static const unsigned int width=0;
};

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