digitalmars.D.learn - Why isn't intended class constructor called?
- Zak (35/35) Jan 28 2019 I have defined a class that's meant to represent a data series,
I have defined a class that's meant to represent a data series,
which has an index and a set of values. Sometimes the user wants
to specify a particular index of custom type, other times they
don't care and we want to default to an array of contiguous "int"
starting from 0.
I have attempted to create a class where the index type is a
parameter, but defaults to int. I also tried to create two
constructors: one for if the index values are not specified (in
which the constructor makes the array of ints); and one where the
user passes in a literal of values that match the specified type.
However, it seems that only the first constructor is getting
called, even though I am passing in two parameters instead of
one. Why isn't the call matching the second constructor and
behaving as intended?
import std.stdio;
class MyClass(T, U = int) {
T[] values;
U[] index;
this(T[] values) {
this.values = values;
// Default index of contiguous ints
for (int i; i < values.length; i++) {
index ~= i;
}
}
this(T[] values, U[] index) {
this.values = values;
this.index = index;
}
}
void main() {
auto myc1 = new MyClass!(int)([1,2,-3]);
auto myc2 = new MyClass!(int, string)([1,2,-3], ["a", "b",
"c"]); // Error: cannot append type int to type string[]
}
Jan 28 2019
On Monday, 28 January 2019 at 18:34:44 UTC, Zak wrote:
I have defined a class that's meant to represent a data series,
which has an index and a set of values. Sometimes the user
wants to specify a particular index of custom type, other times
they don't care and we want to default to an array of
contiguous "int" starting from 0.
I have attempted to create a class where the index type is a
parameter, but defaults to int. I also tried to create two
constructors: one for if the index values are not specified
(in which the constructor makes the array of ints); and one
where the user passes in a literal of values that match the
specified type.
However, it seems that only the first constructor is getting
called, even though I am passing in two parameters instead of
one. Why isn't the call matching the second constructor and
behaving as intended?
import std.stdio;
class MyClass(T, U = int) {
T[] values;
U[] index;
this(T[] values) {
this.values = values;
// Default index of contiguous ints
for (int i; i < values.length; i++) {
index ~= i;
}
}
this(T[] values, U[] index) {
this.values = values;
this.index = index;
}
}
void main() {
auto myc1 = new MyClass!(int)([1,2,-3]);
auto myc2 = new MyClass!(int, string)([1,2,-3], ["a", "b",
"c"]); // Error: cannot append type int to type string[]
}
As the error states:
you are trying to append an int to a string array in the single
parameter constructor.
This would work:
´´´
import std.stdio;
class MyClass(T, U = int) {
T[] values;
U[] index;
this(T[] values) {
this.values = values;
// Default index of contiguous ints
static if(is(U == int))
{
for (int i; i < values.length; i++) {
index ~= i;
}
}
}
this(T[] values, U[] index) {
this.values = values;
this.index = index;
}
}
void main() {
auto myc1 = new MyClass!(int)([1,2,-3]);
auto myc2 = new MyClass!(int, string)([1,2,-3], ["a", "b",
"c"]); // Error: cannot append type int to type string[]
}
´´´
If design matters, I would even to expand the static if above the
constructor. So, the single parameter constructor would exist iff
is(U == int)
´´´
static if(is(U == int))
{
this(T[] values) {
this.values = values;
// Default index of contiguous ints
for (int i; i < values.length; i++) {
index ~= i;
}
}
}
´´´
Jan 28 2019
On Monday, 28 January 2019 at 18:50:18 UTC, Alex wrote:On Monday, 28 January 2019 at 18:34:44 UTC, Zak wrote:Thanks for the response, Alex! But it's not clear to me why the first constructor is called at all. Since I called with two parameters, shouldn't it invoke the second constructor?[...]As the error states: you are trying to append an int to a string array in the single parameter constructor. [...]
Jan 28 2019
On Monday, 28 January 2019 at 19:15:04 UTC, Zak wrote:On Monday, 28 January 2019 at 18:50:18 UTC, Alex wrote:I think I just realized the answer: this section of code is not called, it just fails compilation since it's not known that runtime doesn't do something like: auto myc = new MyClass!(int, string)([1,2,-3]); which "would" invoke this code block with type string.On Monday, 28 January 2019 at 18:34:44 UTC, Zak wrote:Thanks for the response, Alex! But it's not clear to me why the first constructor is called at all. Since I called with two parameters, shouldn't it invoke the second constructor?[...]As the error states: you are trying to append an int to a string array in the single parameter constructor. [...]
Jan 28 2019
Alex <sascha.orlov gmail.com> On Monday, 28 January 2019 at 19:24:21 UTC, Zak wrote:On Monday, 28 January 2019 at 19:15:04 UTC, Zak wrote:Yes. :)On Monday, 28 January 2019 at 18:50:18 UTC, Alex wrote:I think I just realized the answer: this section of code is not called, it just fails compilation since it's not known that runtime doesn't do something like: auto myc = new MyClass!(int, string)([1,2,-3]); which "would" invoke this code block with type string.On Monday, 28 January 2019 at 18:34:44 UTC, Zak wrote:Thanks for the response, Alex! But it's not clear to me why the first constructor is called at all. Since I called with two parameters, shouldn't it invoke the second constructor?[...]As the error states: you are trying to append an int to a string array in the single parameter constructor. [...]
Jan 28 2019