# flexstr
[](https://crates.io/crates/flexstr)
[](https://docs.rs/flexstr)
A flexible, simple to use, immutable, clone-efficient `String` replacement for
Rust. It unifies literals, inlined, and heap allocated strings into a single
type.
## Overview
Rust is great, but it's `String` type is optimized as a mutable string
buffer, not for typical string use cases. Most string use cases don't
modify their contents, often need to copy strings around as if
they were cheap like integers, typically concatenate instead of modify, and
often end up being cloned with identical contents. Additionally, `String`
isn't able to wrap a string literal without additional allocation and copying.
Rust needs a new string type to unify usage of both literals and
allocated strings in these typical use cases. This crate creates a new string
type
that is optimized for those use cases, while retaining the usage simplicity of
`String`.
This type is not inherently "better" than `String`, but different. It
works best in 'typical' string use cases (immutability, concatenation, cheap
multi ownership) whereas `String` works better in "string buffer" use cases
(mutability, string building, single ownership).
## Installation
NOTE: The serde feature is optional and only included when specified.
```toml
[dependencies]
flexstr = { version = "0.6", features = ["serde"] }
```
## Examples
```rust
use flexstr::{flex_fmt, FlexStr, IntoFlexStr, ToCase, ToFlexStr};
fn main() {
// Use an `into` function to wrap a literal, no allocation or copying
let static_str = "This will not allocate or copy".into_flex_str();
assert!(static_str.is_static());
// Strings up to 22 bytes (on 64-bit) will be inlined automatically
// (demo only, use `into` for literals as above)
let inline_str = "inlined".to_flex_str();
assert!(inline_str.is_inlined());
// When a string is too long to be wrapped/inlined, it will heap allocate
// (demo only, use `into` for literals as above)
let rc_str = "This is too long to be inlined".to_flex_str();
assert!(rc_str.is_heap());
// You can efficiently create a new `FlexStr` (without creating a `String`)
// This is equivalent to the stdlib `format!` macro
let inline_str2 = flex_fmt!("in{}", "lined");
assert!(inline_str2.is_inlined());
assert_eq!(inline_str, inline_str2);
// We can upper/lowercase strings without converting to a `String`
// This doesn't heap allocate
let inline_str3: FlexStr = "INLINED".to_ascii_lower();
assert!(inline_str3.is_inlined());
assert_eq!(inline_str, inline_str3);
// Concatenation doesn't even copy if we can fit it in the inline string
let inline_str4 = inline_str3 + "!!!";
assert!(inline_str4.is_inlined());
assert_eq!(inline_str4, "inlined!!!");
// Clone is almost free, and never allocates
// (at most it is a ref count increment for heap allocated strings)
let static_str2 = static_str.clone();
assert!(static_str2.is_static());
// Regardless of storage type, these all operate seamlessly together
// and choose storage as required
let heap_str2 = static_str2 + &inline_str;
assert!(heap_str2.is_heap());
assert_eq!(heap_str2, "This will not allocate or copyinlined");
}
```
## How Does It Work?
Internally, `FlexStr` uses an enum with these variants:
* `Static` - A simple wrapper around a static string literal (`&'static str`)
* `Inlined` - An inlined string (no heap allocation for small strings)
* `Heap` - A heap allocated (reference counted) string
The type automatically chooses the best storage and allows you to use them
interchangeably as a single string type.
## Features
* Optimized for immutability and cheap cloning
* Allows for multiple ownership of the same string memory contents
* Serves as a universal string type (unifying literals and allocated strings)
* Doesn't allocate for literals and short strings (64-bit: up to 22 bytes)
* The same size as a `String` (64-bit: 24 bytes)
* Optional `serde` serialization support (feature = "serde")
* Compatible with embedded systems (doesn't use `std`)
* Efficient conditional ownership (borrows can take ownership without
allocation/copying)
* Both single threaded compatible (`FlexStr`) and multi-thread safe
(`AFlexStr`) options
* It is simple to use!
## Types
* `FlexStr` - regular usage
* `Heap` storage based on `Rc`
* `AFlexStr`- provides `Send` / `Sync` for multi-threaded use
* `Heap` storage based on `Arc`
## Usage
### Hello World
```rust
use flexstr::IntoFlexStr;
fn main() {
// From literal - no copying or allocation
let world = "world!".into_flex_str();
println!("Hello {world}");
}
```
### Creation Scenarios
```rust
use flexstr::{IntoAFlexStr, IntoFlexStr, ToFlexStr};
fn main() {
// From literal - no copying or allocation
// NOTE: `to_flex_str` will copy, so use `into_flex_str` for literals
let literal = "literal".into_flex_str();
// From borrowed string - Copied into inline string
let owned = "inlined".to_string();
let str_to_inlined = (&owned).to_flex_str();
// From borrowed String - copied into `str` wrapped in `Rc`
let owned = "A bit too long to be inlined!!!".to_string();
let str_to_wrapped = (&owned).to_flex_str();
// From String - copied into inline string (`String` storage released)
let inlined = "inlined".to_string().into_flex_str();
// From String - `str` wrapped in `Rc` (`String` storage released)
let counted = "A bit too long to be inlined!!!".to_string().into_flex_str();
// *** If you want a Send/Sync type you need `AFlexStr` instead ***
// From FlexStr wrapped literal - no copying or allocation
let literal = literal.into_a_flex_str();
// From FlexStr inlined string - no allocation
let inlined = inlined.into_a_flex_str();
// From FlexStr `Rc` wrapped `str` - copies into `str` wrapped in `Arc`
let counted = counted.into_a_flex_str();
}
```
### Passing FlexStr to Conditional Ownership Functions
This has always been a confusing situation in Rust, but it is easy with
`FlexStr` since multi ownership is cheap. By passing as `&FlexStr` instead
of `&str`, you retain the option for very fast multi ownership.
```rust
use flexstr::{IntoFlexStr, FlexStr};
struct MyStruct {
s: FlexStr
}
impl MyStruct {
fn to_own_or_not_to_own(s: &FlexStr) -> Self {
let s = if s == "own_me" {
// Since a wrapped literal, no copy or allocation
s.clone()
} else {
// Wrapped literal - no copy or allocation
"own_me".into()
};
Self { s }
}
}
fn main() {
// Wrapped literals - no copy or allocation
let s = "borrow me".into_flex_str();
let s2 = "own me".into_flex_str();
let struct1 = MyStruct::to_own_or_not_to_own(&s);
let struct2 = MyStruct::to_own_or_not_to_own(&s2);
assert_eq!(s2, struct1.s);
assert_eq!(s2, struct2.s);
}
```
## Performance Characteristics
* Clones are cheap and never allocate
* At minimum, they are just a copy of the enum and at max an additional
reference count increment
* Literals are just wrapped when used with `into()` and never copied
* Calling `into()` on a `String` will result in an inline string (if
short) otherwise copied into a `str` wrapped in `Rc`/`Arc`
(which will allocate, copy, and then release original `String` storage)
* `into_flex_str()` and `into_a_flex_str()` are equivalent to calling `into()`
on both literals and `String` (they are present primarily for `let`
bindings so there is no need to declare a type)
* `to_flex_str()` and `to_a_flex_str()` are meant for taking ownership of
borrowed strings and always copy into either an inline string (for short strings) or
an `Rc`/`Arc` wrapped `str` (which will allocate)
* `to_string` always copies into a new `String`
* Conversions back and forth between `AFlexStr` and `FlexStr` using `into()`
are cheap when using wrapped literals or inlined strings
* Inlined strings and wrapped literals just create a new enum wrapper
* Reference counted wrapped strings will always require an allocation
and copy for the new `Rc` or `Arc`
## Benchmarks
In general, creates are somewhat slower than `String`, but clones and
conversions from primitive types are vastly faster.
[Full benchmarks](BENCHMARKS.md)
## Negatives
There is no free lunch:
* Due to usage of `Rc` (or `Arc`), when on-boarding `String` it will need to
reallocate and copy
* Due to the enum wrapper, every string operation has the overhead of an extra
branching operation
* Since `FlexStr` is not `Send` or `Sync`, there is a need to consider
single-threaded (`FlexStr`) and multi-threaded (`AFlexStr`) use cases and
convert accordingly
## Status
This is currently beta quality and still needs testing. The API may very
possibly change but semantic versioning will be followed.
## License
This project is licensed optionally under either:
* Apache License, Version 2.0, (LICENSE-APACHE
or https://www.apache.org/licenses/LICENSE-2.0)
* MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)