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We mentioned earlier that an ordinary function overload of min( ) is preferable to using the template. If a function already exists
to match a function call, why generate another? In the absence of ordinary
functions, however, it is possible that overloaded function templates can lead
to ambiguities. To minimize the chances of this, an ordering is defined for
function templates, which chooses the most specialized template, if such
exists. A function template is considered more specialized than another if
every possible list of arguments that matches it also matches the other, but
not the other way around. Consider the following function template
declarations, taken from an example in the C++ Standard document:
template<class T> void f(T);
template<class T> void f(T*);
template<class T> void f(const T*);
The first template can be matched with any type. The second
template is more specialized than the first because only pointer types match
it. In other words, you can look upon the set of possible calls that match the
second template as a subset of the first. A similar relationship exists between
the second and third template declarations above: the third can only be called
for pointers to const, but the second accommodates any pointer type. The
following program illustrates these rules:
//: C05:PartialOrder.cpp
// Reveals ordering of function templates.
#include <iostream>
using namespace std;
template<class T> void f(T) {
cout << "T" << endl;
}
template<class T> void f(T*) {
cout << "T*" << endl;
}
template<class T> void f(const T*) {
cout << "const T*" << endl;
}
int main() {
f(0); // T
int i = 0;
f(&i); // T*
const int j = 0;
f(&j); // const T*
} ///:~
The call f(&i) certainly matches the first
template, but since the second is more specialized, it is called. The third
can t be called here since the pointer is not a pointer to const. The
call f(&j) matches all three templates (for example, T would
be const int in the second template), but again, the third template is
more specialized, so it is used instead.
If there is no most specialized template among a set of
overloaded function templates, an ambiguity remains, and the compiler will
report an error. That is why this feature is called a partial ordering it may
not be able to resolve all possibilities. Similar rules exist for class
templates (see the section Partial specialization below).
| Thinking in C++ Vol 2 - Practical Programming |
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