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Having determined the set of candidate functions and from that set the set of
viable functions the compiler must now determine the actual types of the
template type parameters.
It may use any of the three standard template parameter transformation
procedures (cf. section ref(TEMPFUNARGS)) when trying to match actual types to
template type parameters. In this process it concludes that no type can be
determined for the tt(T) in function 1's tt(T &t1) parameter as the argument
tt(3) is a constant tt(int) value. Thus function 1 is removed from the list of
viable functions. The compiler is now confronted with the following set of
potentially instantiated function templates and ordinary functions:
verb( void process(T1 [= int] const &t1, T2 [= int] const &t2); // 2
void process(T [= int] const &t, double d); // 3
void process(T [= int] const &t, int i); // 4
void process<int, int>(int i1, int i2); // 5
void process(int i1, int i2); // 6
void process(int i, double d); // 7
void process(double d, int i); // 8
void process(double d1, double d2); // 9)
The compiler associates a em(direct match count) value to each of the
viable functions. The direct match count counts the number of arguments that
can be matched to function parameters without an (automatic) type
conversion. E.g., for function 2 this count equals 2, for function 7 it is 1
and for function 9 it is 0. The functions are now (decrementally) sorted by
their direct match count values:
verb( match
count
void process(T1 [= int] const &t1, T2 [= int] const &t2); 2 // 2
void process(T [= int] const &t, int i); 2 // 4
void process<int, int>(int i1, int i2); 2 // 5
void process(int i1, int i2); 2 // 6
void process(T [= int] const &t, double d); 1 // 3
void process(int i, double d); 1 // 7
void process(double d, int i); 1 // 8
void process(double d1, double d2); 0 // 9)
If there is no draw for the top value the corresponding function is
selected and the function selection process is completed.
When multiple functions appear at the top the compiler verifies that no
ambiguity has been encountered. An ambiguity is encountered if the
em(sequences) of parameters for which type conversions were (not) required
differ. As an example consider functions 3 and 8. Using D for `direct match'
and C for `conversion' the arguments match function 3 as D,C and function 8 as
C,D. Assuming that 2, 4, 5 and 6 were not available, then the compiler would
have reported an ambiguity as the sequences of argument/parameter matching
procedures differ for functions 3 and 8. The same difference is encountered
comparing functions 7 and 8, but no such difference is encountered comparing
functions 3 and 7.
At this point there is a draw for the top value and the compiler
proceeds with the subset of associated functions (functions 2, 4, 5 and
6). With each of these functions an `ordinary parameter count' is associated
counting the number of non-template parameters of the functions. The functions
are decrementally sorted by this count, resulting in:
verb( ordin. param.
count
void process(int i1, int i2); 2 // 6
void process(T [= int] const &t, int i); 1 // 4
void process(T1 [= int] const &t1, T2 [= int] const &t2); 0 // 2
void process<int, int>(int i1, int i2); 0 // 5)
Now there is no draw for the top value. The corresponding function
(tt(process(int, int)), function 6) is selected and the function selection
process is completed. Function 6 is used in tt(main)'s function call
statement.
Had function 6 not been defined, function 4 would have been used. Assuming
that neither function 4 nor function 6 had been defined, the selection
process would continue with functions 2 and 5:
verb( ordin. param.
count
void process(T1 [= int] const &t1, T2 [= int] const &t2); 0 // 2
void process<int, int>(int i1, int i2); 0 // 5)
In this situation a draw is encountered once again and the selection
process continues. A `type of function' value is associated with each of the
functions having the highest ordinary parameter count and these functions are
decrementally sorted by their type of function values. Value 2 is associated
to ordinary functions, value 1 to template explicit specializations and value
0 to plain function templates.
If there is no draw for the top value the corresponding function is
selected and the function selection process is completed. If there is a draw
the compiler reports an ambiguity and cannot determine which function to
call. Assuming only functions 2 and 5 existed then this selection step
would have resulted in the following ordering:
verb( function
type
void process<int, int>(int i1, int i2); 1 // 5
void process(T1 [= int] const &t1, T2 [= int] const &t2); 0 // 2)
Function 5, the template explicit specialization, would have been
selected.
figure(functiontemplates/selection)
(The function template selection mechanism)
(selection)
Here is a summary of the function template selection mechanism (cf. figure
fig(selection)):
itemization(
it() The set of candidate functions is constructed: identical names;
it() The set of viable functions is constructed: correct number of
parameters and available type conversions;
it() Template type determination, dropping templates whose type parameters
cannot be determined;
it() Decrementally sort the functions by their direct match count
values. If there is no draw for the top value the associated function
is selected, completing the selection process.
it() Inspect the functions associated with the top value for ambiguities
in automatic type conversion sequences. If different sequences are
encountered report an ambiguity and terminate the selection process.
it() Decrementally sort the functions associated with the top value by
their ordinary parameter count values. If there is no draw for the
top value the associated function is selected, completing the
selection process.
it() Decrementally sort the functions associated with the top value by
their function type values using 2 for ordinary functions, 1 for
template explicit specializations and 0 for plain function templates.
If there is no draw for the top value the associated function is
selected, completing the selection process.
it() Report an ambiguity and terminate the selection process.
)
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