Re: Function Parameters without Names?
On Saturday, 19 February 2022 at 23:37:01 UTC, Vijay Nayar wrote:
What is the main motivator to allow parameters with no names?
Do they get an automatic implied name like `_` or something?
The problem was about
[here](https://dlang.org/spec/class.html#constructors), it should
not be type inference:
https://dlang.org/spec/function.html#function-attribute-inference
Do you think so? But it works when you open inactive codes and
convert this(int) to this(T)! The program below throws an error
about ```_param_0``` when compiling because the inferred type is
not int.
```d
import std.conv;
class Foo
{
int i;
// v=== type of inferred
this(int)//(T i)
{
this.i = i;//.to!int;
}
string test()
{
return i.to!string;
}
}
void main()
{
long bar = 42;
auto foo = new Foo(bar);
assert(foo.test == "42"); /* Error:
constructor `source.Foo.this(int _param_0)` is not callable
using argument types `(long)`
cannot pass argument `bar` of type `long` to parameter `int
_param_0`
*/
}
```
The following doesn't work in a function outside the class:
```d
TYPE foo(TYPE) { return 42; }
```
The compiler gives the following error:
```undefined identifier TYPE```
SDB@79
Re: Function Parameters without Names?
On 2/19/22 15:37, Vijay Nayar wrote:
> The problem is that the parameter of the constructor actually has no
> name at all. Thus, the statement `this.val = val` simply sets the member
> variable to its own value
Wow! I've never come accross that one before. :)
> What is the main motivator to allow parameters with no names?
This is common in C++ as well. You have to have a parameter because an
interface requires it but you don't really have any use for that
parameter. If you name it, then there may be "unused variable" warning.
So, it is common to comment out the parameter name so that it is
instructive to people who look at the code:
void foo(int /* length */) {
// ...
}
> Do they get an automatic implied name like `_` or something?
The function is compiled to take the specified type of a parameter but
it is simply not used. (Names are only for source code so it doesn't
matter.)
Ali
Function Parameters without Names?
I encountered an odd bug in a project of mine which is
illustrated in the example below:
```d
class Thing {
int val;
this(int) {
this.val = val;
}
}
void main()
{
auto t = new Thing(3);
assert(t.val != 3);
}
```
The problem is that the parameter of the constructor actually has
no name at all. Thus, the statement `this.val = val` simply sets
the member variable to its own value, thus it stays the same as
`int.init`.
According to the
[specification](https://dlang.org/spec/function.html), this is
permissible via the following grammar rules: `FuncDeclaration` =>
`FuncDeclarator` => `FuncDeclaratorSuffix` => `Parameters` =>
`ParameterList` => `Parameter` => `Declarator` =>
`ParameterAttributes_opt Type`.
What is the main motivator to allow parameters with no names? Do
they get an automatic implied name like `_` or something?
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