www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - variable template question

reply kdevel <kdevel vogtner.de> writes:
vartmpl.d
```
import std.stdio : writeln;
import decimal : decimal32;

template F(T) {
    immutable T c = 3;
}

void foo (T) ()
{
    immutable T t = 1;
}

void main ()
{
//   immutable decimal32 i = 1; // Error: none of the overloads 
of '__ctor' are
callable using a immutable object
//   foo!decimal32; //  Error: none of the overloads of '__ctor' 
are callable us
ing a immutable object, candidates are:
    alias c = F!decimal32.c;
    c.writeln;
    writeln (typeof (c).stringof);
}
```

$ dmd -g vartmpl.d decimal.git/libdecimal.a
$ ./vartmpl

3
immutable(Decimal!32)

Why does this compile while both of the commented lines give a 
compile error. decimal ist http://rumbu13.github.io/decimal/
Jan 14
parent ag0aep6g <anonymous example.com> writes:
On Sunday, 14 January 2018 at 16:23:18 UTC, kdevel wrote:
 Why does this compile while both of the commented lines give a 
 compile error.
The code boils down to this: ---- struct decimal32 { this(int x) {} } immutable decimal32 c = 3; /* works */ void main () { immutable decimal32 i = 1; /* error */ } ---- I think this is CTFE being unexpectedly smart. If you add the `pure` attribute to the constructor, then the `i` line works as well. That's because a strongly pure constructor is guaranteed to return a unique object, and a unique object can be converted implicitly to other mutability levels. The `pure` attribute is needed for `i`, because here the compiler only looks at the function attributes to determine purity. No `pure` attribute -> function is regarded as impure. But for `c`, the constructor goes through CTFE, and CTFE doesn't care all that much about the `pure` attribute. Instead, CTFE just tries to evaluate the function and aborts when it encounters an action that would be impure.
Jan 14