Section 16.3 of C++ Primer (5th edition) - Overloading and Templates -, teaches the function matching procedure in the presence of candidate function template(s) instantiati
EDIT: Thanks to Alf answer, and its elegant trick to conserve complete information about a type while using typeid
, I was able to write a program that addresses most of my questions (changing from std::string
to int
for output readability.)
The complete code can be edited and run rextester online IDE.
Let's define a few classes and methods:
template class Type{}; // Allowing to get full type information with typeid
template std::string typeStr()
{ return typeid(Type).name(); }
template void debug_rep(const T &a) /* 1 */
{
std::cout << "[1] T type is: " << typeStr()
<< "\t| arg type is: " << typeStr() << std::endl;
}
template void force_1(const T &a) /* 1 */
{
std::cout << "[forced 1] T type is: " << typeStr()
<< "\t| arg type is: " << typeStr() << std::endl;
}
template void debug_rep(T *a) /* 2 */
{
std::cout << "[2] T type is: " << typeStr()
<< "\t| arg type is: " << typeStr() << std::endl;
}
Running:
std::cout << "---First example---" << std::endl;
int i = 41;
debug_rep(&i);
force_1(&i);
Displays:
---First example---
[2] T type is: class Type | arg type is: class Type
[forced 1] T type is: class Type | arg type is: class Type
Q1: we can remark that, when we call force_1, instantiating a template corresponding to #1, the argument type is int * const &
, so the book is not correct, and the instantiated candidate #1 would be
debug_rep(int* const &)
Running:
std::cout << "---Second example---" << std::endl;
const int *ip = &i;
debug_rep(ip);
force_1(ip);
Displays:
---Second example---
[2] T type is: class Type | arg type is: class Type
[forced 1] T type is: class Type | arg type is: class Type
Q2.1: Calling force_1
, we remark that the argument type will be int const * const &
, so the book is missing const qualification on the reference. The instantiated candidate will actually be:
debug_rep(const int * const &)
Q2.2 The second candidate being debug_rep(const int *)
, it is an exact match for ip
(which is a pointer to constant integer). To check if the first candidate has a lower rank, let's write:
void debug_rep_plain_b(const int * const &) /* 1 */
{ std::cout << "[plain 1]" << std::endl;}
void debug_rep_plain_b(const int *) /* 2 */
{ std::cout << "[plain 2]" << std::endl; }
If we try to compile:
debug_rep_plain_b(ip)
There is a compilation error for ambiguous call: So the answer to Q2.2 is NO, it is still an exact match ! For the templated version, the compiler actually uses the rule regarding the most specialized template to resolve the ambiguity.
Even if there is a mistake in the deduced candidate, the book is correct regarding the fact that this example illustrates overload resolution using the most specialized template.
Running:
std::cout << "---Third example---" << std::endl;
const int ia[3] = {1, 2, 3};
debug_rep(ia);
force_1(ia);
Displays:
---Third example---
[2] T type is: class Type | arg type is: class Type
[forced 1] T type is: class Type | arg type is: class Type
Q3.1 The type deduced for T
by CL is array of const integer, so the book would be mistaken.
BUT the result is inconsistent with GCC or Clang, that would output:
---Third example---
[2] T type is:4TypeIKiE | arg type is: 4TypeIPKiE
[forced 1] T type is:4TypeIA3_iE | arg type is: 4TypeIRA3_KiE
The interesting part being:
[forced 1] T type is:4TypeIA3_iE
meaning that they deduce T
as an array of 3 non-const integers (because _i, not _Ki), which would agree with the book.
I will have to open another question for this one, I cannot understand the type deduction operated by GCC and Clang...