Re: Template and default values of non-template pointers arguments
Clyde wrote:
what i'm trying to do is:
/////////////// Code Start
template <class TType, int* p = 0>
class Template
{
public:
Template<TType,p>() {};
};
/////////////// Code End
No, that's not what you're trying to do. That's *how* you're trying to
do what you think you need. You should consider explaining *what* you
think you need the null pointer for. There can be no compile-time check
for a null pointer...
That works on Visual Studio 2005, but doesn't work on mingw 5.1.4 and
comeau.
It must be an extension offered by VC++.
They both says:
mingw:
could not convert template argument `0' to `int*
comeau:
argument of type "int" is incompatible with template parameter of type
"int *"
template <class TType, int* p = 0>
^
so the problem is quite clear, it seems there's no an acceptable
conversion from int to int*
But if i try to do something like that:
/////////////// Code Start
#define zero (int*)0
template <class TType, int* p = zero>
class Template
{
public:
Template<TType,p>() {};
};
/////////////// Code End
Now, that works on comeau, but still doesn't work with mingw.
My questions:
1] Why is that illegal?
Here we just say "because the Standard says so". 14.3.2/5 prohibits '0'
(the integer literal) to be used as the non-type template *argument* for
a template *parameter* of type 'pointer to T'. The conversions do not
apply.
If you need the real rationale, you'll have to to ask in 'comp.std.c++'
when it's operational.
2] How can solve this problem?
Solve? Not sure what you mean. The requirement is that the pointer
non-type template argument to be the address of a real object, with
external linkage. A null pointer does not satisfy that requirement.
Now, if you do have a problem (that you're trying to solve by writing
your template with the wrong default argument), you haven't stated it
yet, so we can't really tell you how to solve it. Please explain what
you're trying to accomplish.
V
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