Re: EVC++: compile errors disappear when preprocessor output is on

From:

"Alex Blekhman" <xfkt@oohay.moc>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 10 Jul 2006 14:18:58 +0300

Message-ID:

<#GbF#KBpGHA.516@TK2MSFTNGP05.phx.gbl>

<Mark.Stijnman@gmail.com> wrote:

[...]
class MyClass
{
public:
MyClass(const int i): m_i(i) {}

template <typename T>
MyClass(const T t): m_i(static_cast<int>(t)) {}

private:
int m_i;
};

and presto! I got heaploads of syntax errors again. Just
on a hunch, I
changed the line
MyClass(const T t)
to
MyClass(const T& t)
and it worked again, no more compile errors! I have no
idea why one
works and the other doesn't, but that's how it seems to
be.

Turned out that the previous, more elaborate template
class also had a
constructor of this form. Changing its argument to a
reference to T
also solved that one's compile errors when I used it as a
member
variable. So I now know what is causing those mysterious
compile
errors, I just still don't know why. I guess it's just
this compiler's
bad support for templates, or something. Anyone know why
const-reference-to-T works and const-T-by-value doesn't?
Is this a
known rule of thumb to get EVC++/VC6 to use templated
constructors
properly?

Without seeing actual compiler errors I can only guess. It
can be either weak compiler or erroneous type used for
instantiation of MyClass<> or both. Assuming that compiler
is OK, the problem with using const-T-by-value approach can
be that T hasn't appropriate copy constructor. Hence,
compiler fails to generate necessary code for constructor

    template <typename T>
    MyClass(const T t) { ... }

Why there is no available constructor for T is another
story. T can contain reference/const members and doesn't
define any copy constructor, or copy constructor can be
declared like this

    T(T& other); // <- notice absence of `const'

or million of other reasons.

HTH
Alex