Implicit type conversion depending on assignment

Mar 23

Alexander Zhirov

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struct MyVal { string value; // Here it would be possible to use an alias to this, but it can only be used 1 time } auto a = MyVal("100"); auto b = MyVal("11.2"); int MyInt = a; // Implicitly convert to target type float myFloat = b; // Implicitly convert to target type

March 23

Re: Implicit type conversion depending on assignment

Posted by user1234
in reply to Alexander Zhirov

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user1234

Posted in reply to Alexander Zhirov

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On Thursday, 23 March 2023 at 14:05:07 UTC, Alexander Zhirov wrote:

On Thursday, 23 March 2023 at 13:38:51 UTC, Alexander Zhirov wrote:

Is it possible to convert such records inside the structure to the assigned type?

struct MyVal
{
    string value;
    // Here it would be possible to use an alias to this, but it can only be used 1 time
}

auto a = MyVal("100");
auto b = MyVal("11.2");

int MyInt = a;        // Implicitly convert to target type
float myFloat = b;    // Implicitly convert to target type

Here is an example from the documentation, but for "several" types, for example, with a cast check and a return of the default value.

The best I can think of ATM

struct MyVal
{
    string value;
    auto opCast(T)()
    {
        import std.conv : to;
        return to!T(value);
    }
}

void main()
{
    auto a = MyVal("100");
    auto b = MyVal("11.2");

    int MyInt       = cast(int)a;
    float myFloat   = cast(float)b;
}

not exactly thing goal yet. The doc example you have put a link for is different, the struct with alias this a redefinition of the "alias this"'ed thing, that just cant work in what you ask in the first post.

struct MyVal { private string value; @property auto toString(T)() { return value.to!T; } alias toString this; } auto a = MyVal("100"); auto b = MyVal("11.2"); int myInt = a; float myFloat = b;

On 3/23/23 06:38, Alexander Zhirov wrote: > Is it possible to convert such records inside the structure to the > assigned type? D does not embrace implicit conversions. There is some support like 'alias this' as you mentioned but what you are looking for is not possible. Ali

struct MyVal { string value; T opBinary(string op, T)(T rhs) if (op == "+") { import std.conv : convto = to; static if (is(T==int)) return convto!T(value) + rhs; else static if (is(T==float)) return convto!T(value) + rhs; else static assert(0); } alias opBinary this; } void main() { auto a = MyVal("100"); auto b = MyVal("11.2");

On 3/23/23 07:36, Alexander Zhirov wrote: > @property auto toString(T)() The name is misleading because you want types other than string as well. > alias toString this; That should have been a compilation error because 'toString' does not have a known type (because it depends on a template parameter). How can the compiler accept that 'alias this'? > int myInt = a; > float myFloat = b; Since you need to spell out 'int' and 'float' in some form anyway, the following would be my choice, which I did use in my code before: struct MyVal { private string value; auto to(T)() { import std.conv : to; return value.to!T; } } void main() { auto a = MyVal("100"); auto b = MyVal("11.2"); auto myInt = a.to!int; auto myFloat = b.to!float; } Should it be an error to convert 'a' to float? If so, the following modules may be helpful: https://dlang.org/phobos/std_sumtype.html https://dlang.org/phobos/std_variant.html Ali

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