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digitalmars.D.learn - Template instance does not match template declaration

reply Marc <mailnawman gmail.com> writes:
Hello,
I'm trying to declare a templated member function that takes a 
value of size_t N. A simple example to reproduce what Im trying 
to do is the following:

```d
import std.stdio;
void getCol(N: size_t)() {
    return N;
}

void main() {
    // Call
    writeln(getCol!0());
}
```
I get the following error: `Error: template instance `getCol!0` 
does not match template declaration `getCol(N : ulong)()`
    writeln(getCol!0());
            ^
Error dmd failed with exit code 1.`

Can anyone please help me getting it to work?
Aug 15
next sibling parent reply 0xEAB <desisma heidel.beer> writes:
On Friday, 15 August 2025 at 12:02:19 UTC, Marc wrote:

 Can anyone please help me getting it to work?


It seems like you’ve accidentally used the wrong syntax for the 
template parameter.
To make the parameter a value parameter of type `size_t`, write:

```
size_t getCol(size_t N)() {
    return N;
}
```

Also, your return type was off. Can’t return data from a `void` 
function.
Aug 15
next sibling parent 0xEAB <desisma heidel.beer> writes:
On Friday, 15 August 2025 at 12:18:30 UTC, 0xEAB wrote:

 It seems like you’ve accidentally used the wrong syntax for the 
 template parameter.


With the syntax from your code snippet you’ve put a constraint on 
the types that can be substituted for `N` when instantiating the 
template.

```d
N getCol(N : size_t)(N n) {
    return n;
}

void main() {
     writeln(getCol(0));       // works
     writeln(getCol!uint(0));  // works
     writeln(getCol(0f));      // fails
     writeln(getCol!float(0)); // fails
}
```

This kind of constraint is commonly used with class-based 
polymorphism where it allows you to restrict the type to a 
certain class including subclasses.

```d
class Foo {}
class Bar {}
class Sub : Foo {}

auto doSth(T : Foo)(T value) {
    return value;
}

void main() {
     doSth(new Foo());
     doSth(new Sub());
     doSth(new Bar()); // fails
}
```
Aug 15
prev sibling parent Marc <mailnawman gmail.com> writes:
On Friday, 15 August 2025 at 12:18:30 UTC, 0xEAB wrote:

 On Friday, 15 August 2025 at 12:02:19 UTC, Marc wrote:

 Can anyone please help me getting it to work?


 It seems like you’ve accidentally used the wrong syntax for the 
 template parameter.
 To make the parameter a value parameter of type `size_t`, write:

 ```
 size_t getCol(size_t N)() {
    return N;
 }
 ```

 Also, your return type was off. Can’t return data from a `void` 
 function.


Thanks. It work now.
Aug 15
prev sibling parent reply Sergey <kornburn yandex.ru> writes:
On Friday, 15 August 2025 at 12:02:19 UTC, Marc wrote:

 Hello,
 I'm trying to declare a templated member function that takes a 
 value of size_t N. A simple example to reproduce what Im trying 
 to do is the following:

 ```d
 import std.stdio;
 void getCol(N: size_t)() {
    return N;
 }

 void main() {
    // Call
    writeln(getCol!0());
 }
 ```
 I get the following error: `Error: template instance `getCol!0` 
 does not match template declaration `getCol(N : ulong)()`
    writeln(getCol!0());
            ^
 Error dmd failed with exit code 1.`

 Can anyone please help me getting it to work?


D doesn't use ":" for types
```d
import std;

size_t getCol(size_t N)() {
    return N;
}

void main() {
    // Call
    writeln(getCol!0());
}
```
Aug 15
parent Marc <mailnawman gmail.com> writes:
On Friday, 15 August 2025 at 12:20:16 UTC, Sergey wrote:

 On Friday, 15 August 2025 at 12:02:19 UTC, Marc wrote:

 Hello,
 I'm trying to declare a templated member function that takes a 
 value of size_t N. A simple example to reproduce what Im 
 trying to do is the following:

 ```d
 import std.stdio;
 void getCol(N: size_t)() {
    return N;
 }

 void main() {
    // Call
    writeln(getCol!0());
 }
 ```
 I get the following error: `Error: template instance 
 `getCol!0` does not match template declaration `getCol(N : 
 ulong)()`
    writeln(getCol!0());
            ^
 Error dmd failed with exit code 1.`

 Can anyone please help me getting it to work?


 D doesn't use ":" for types
 ```d
 import std;

 size_t getCol(size_t N)() {
    return N;
 }

 void main() {
    // Call
    writeln(getCol!0());
 }
 ```


Thanks for your reply. It solved my problem.
Aug 15