Re: How to avoid throwing an exceptions for a built-in function?

[Ali Çehreli via Digitalmars-d-learn]

On 05/10/2017 05:40 AM, k-five wrote:
> I have a line of code that uses "to" function in std.conv for a purpose
> like:
>
> int index = to!int( user_apply[ 4 ] ); // string to int
>
> When the user_apply[ 4 ] has value, there is no problem; but when it is
> empty: ""
> it throws an ConvException exception and I want to avoid this exception.
>
> currently I have to use a dummy catch:
> try{
>     index = to!int( user_apply[ 4 ] );
> } catch( ConvException conv_error ){
>     // nothing
> }
Are you really fine with 'index' being left with its previous value? If 
so, you can write a function like the following:

void setMaybe(To, From)(ref To var, From from) {
    import std.conv : to, ConvException;
    try {
        var = to!To(from);
    } catch( ConvException conv_error ) {
    }
}

unittest {
    int index = 41;
    index.setMaybe("42");
    assert(index == 42);
    index.setMaybe("forty three");
    assert(index == 42);
    index.setMaybe("");
    assert(index == 42);
}

void main() {
}

If you want the variable to be set to a default value (perhaps .init), 
then here is an idea:

void setMaybe(To, From)(ref To var, From from, To def = To.init) {
    import std.conv : to, ConvException;
    try {
        var = to!To(from);
    } catch( ConvException conv_error ) {
        var = def;
    }
}

unittest {
    int index = 41;
    index.setMaybe("42");
    assert(index == 42);

    index.setMaybe("forty three");
    assert(index == 0);

    index.setMaybe(7);
    index.setMaybe("", 8);
    assert(index == 8);
}

void main() {
}

Ali