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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