44 lines
1.6 KiB
Markdown
44 lines
1.6 KiB
Markdown
`String` Parameters in Rust Functions
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====================================
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{{#include ../links.md}}
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Avoid `String`
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--------------
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As must as possible, avoid using `String` parameters in functions.
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Each `String` argument is cloned during every single call to that function - and the copy
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immediately thrown away right after the call.
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Needless to say, it is _extremely_ inefficient to use `String` parameters.
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`&str` Maps to `ImmutableString`
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-------------------------------
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Rust functions accepting parameters of `String` should use `&str` instead because it maps directly to
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[`ImmutableString`][string] which is the type that Rhai uses to represent [strings] internally.
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The parameter type `String` involves always converting an [`ImmutableString`][string] into a `String`
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which mandates cloning it.
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Using `ImmutableString` or `&str` is much more efficient.
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A common mistake made by novice Rhai users is to register functions with `String` parameters.
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```rust
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fn get_len1(s: String) -> i64 { s.len() as i64 } // <- Very inefficient!!!
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fn get_len2(s: &str) -> i64 { s.len() as i64 } // <- This is better
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fn get_len3(s: ImmutableString) -> i64 { s.len() as i64 } // <- the above is equivalent to this
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engine
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.register_fn("len1", get_len1)
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.register_fn("len2", get_len2)
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.register_fn("len3", get_len3);
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let len = engine.eval::<i64>("x.len1()")?; // 'x' is cloned, very inefficient!
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let len = engine.eval::<i64>("x.len2()")?; // 'x' is shared
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let len = engine.eval::<i64>("x.len3()")?; // 'x' is shared
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```
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