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Akkara
Bodhisattva
Joined: 28 Mar 2006
Posts: 6702
Location: &akkara
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 Posted: Wed Mar 30, 2016 9:59 am Post subject: C++ template syntax -- ARGH! |
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I have a templated class, which contains a templated function. What's the syntax to define the function for a specialized version of the class?
I've tried various permutations of single and double templates, with parameters placed in various positions, and I can't seem to hit upon the correct syntax for some reason. This latest incarnation, compiled with g++-4.7.2, gives: | Quote: | error: function template partial specialization ‘doit<b>’ is not allowed
error: new declaration ‘void Something<32>::doit(int*, unsigned int, unsigned int)’
error: ambiguates old declaration ‘void Something<iA>::doit(int*, unsigned int, unsigned int) [with int iB = iB; int iA = 32]’ |
Ambiguates. That's a new one for me 
| Code: | #include <stdio.h>
template <int iA>
struct Something {
template <int iB>
void doit(int *, unsigned, unsigned);
};
template <>
template <int b>
void Something<32>::doit<b>(int *out, unsigned stride, unsigned n)
{
printf("In Something<%d>::doit<%d>\n", 32, b);
} |
--------------------
What if I try pre-instantiating the class? | Code: | template <int iA>
struct Something {
template <int iB>
void doit(int *, unsigned, unsigned);
};
template <>
struct Something<32>;
template <>
template <int b>
void Something<32>::doit<b>(int *out, unsigned stride, unsigned n)
{
printf("In Something<%d>::doit<%d>\n", 32, b);
} |
Nope.: | Quote: | | error: too many template-parameter-lists |
Get rid of the template <> in front of the definition? Still nope: | Quote: | | error: expected initializer before ‘<’ token |
I'm sure there's some rhyme and reason to all this, but I can't seem to figure it out right now.
Thanks!
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radio_flyer
Guru
Joined: 04 Nov 2004
Posts: 318
Location: Northern California
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| Posted: Wed Mar 30, 2016 5:43 pm Post subject: |
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| Function template partial specialization is not allowed by the standard..it has ambiguity issues with overloading. I believe MSVC allows it, but really it shouldn't. See http://www.gotw.ca/publications/mill17.htm. |
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SiberianSniper
Guru
Joined: 06 Apr 2006
Posts: 381
Location: Dayton, OH, USA
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| Posted: Wed Mar 30, 2016 5:49 pm Post subject: |
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| Code: | #include <stdio.h>
template <int iA>
struct Something {
template <int iB>
void doit(int *, unsigned, unsigned) {};
};
template <>
template <>
void Something<32>::doit<16>(int *out, unsigned stride, unsigned n)
{
printf("In Something<%d>::doit<%d>\n", 32, 16);
}
int main()
{
Something<32> s;
s.doit<16>(0, 0, 0);
}
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That compiles, though it doesn't give you the iA and iB inside the specialization.
Unfortunately, playing around with g++ and clang++ both tell me that you can't specialize for a only the class. I'm not sure if that's true or not, since I haven't looked at the C++ spec yet. |
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