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Let's play some more. At some point in our program we defined tt(int b). Then,
in a compound statement we need to construct an anonymous tt(Data) object,
initialized using tt(b), followed by displaying tt(b):
COMMENT(examples/ambiguity.cc)
verb( int b = 18;
{
Data(b);
cout << b;
})
About that tt(cout) statement the compiler tells us (I modified the
error message to reveal its meaning):
em(error: cannot bind `std::ostream & << Data const &')
Here we didn't insert tt(int b) but tt(Data b). Had we omitted the compound
statement, the compiler would have complained about a doubly defined tt(b)
entity, as tt(Data(b)) simply means tt(Data b), a tt(Data) object constructed
by default. The compiler may omit superfluous parentheses when parsing a
definition or declaration.
Of course, the question now becomes how a temporary object tt(Data),
initialized with tt(int b) can be defined. Remember that the compiler may
remove superfluous parentheses. So, what we need to do is to pass an tt(int)
to the anonymous tt(Data) object, without using the tt(int)'s name.
itemization(
it() We can use a cast: tt(Data(static_cast<int>(b)));
it() We can use a curly-brace initialization: tt(Data{ b }).
)
Values and types make big differences. Consider the following definitions:
verb( Data (*d4)(int); // 1
Data (*d5)(3); // 2)
Definition 1 should cause no problems: it's a pointer to a function,
expecting an tt(int), returning a tt(Data) object. Hence, tt(d4) is a pointer
variable.
Definition 2 is slightly more complex. Yes, it's a pointer. But it has
nothing to do with a function. So what's that argument list containing 3 doing
there? Well, it's not an argument list. It's an initialization that looks like
an argument list. Remember that variables can be initialized using the
assignment statement, by parentheses or by curly parentheses. So instead of
tt(`(3)') we could have written tt(`= 3') or tt(`{3}'). Let's pick the first
alternative, resulting in:
verb( Data (*d5) = 3;)
Now we get to `play compiler' again. Removing some superfluous
parentheses we get:
verb( Data *d5 = 3;)
It's a pointer to a tt(Data) object, initialized to 3. This is
semantically incorrect, but that's only clear after the syntactical
analysis. If I had initially written
verb( Data (*d5)(&d1); // 2)
the fun resulting from contrasting tt(int) and tt(3) would most likely
have been spoiled.
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