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48 lines
1.4 KiB
Markdown
48 lines
1.4 KiB
Markdown
+++
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title = "The Rust dbg! macro"
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description = "A short article about a quite unknown but useful macro in Rust."
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date = 2021-07-25
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[taxonomies]
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categories = ["rust"]
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+++
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The `dbg!` macro is a useful macro to debug, and I think a not well known one, not to be confused with debug logs using format strings, this macro is useful when you are about to put `println` calls everywhere in your code to know if it reached a path, what value a variable has, etc.
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It uses the `Debug` trait implementation of the type of the given expression.
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Since Rust is an expression oriented language, you can use this macro nearly everywhere:
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```rust
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fn factorial(n: u32) -> u32 {
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if dbg!(n <= 1) {
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dbg!(1)
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} else {
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dbg!(n * factorial(n - 1))
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}
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}
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fn main() {
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dbg!(factorial(4));
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}
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```
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This outputs the following:
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```
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[src/main.rs:2] n <= 1 = false
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[src/main.rs:2] n <= 1 = false
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[src/main.rs:2] n <= 1 = false
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[src/main.rs:2] n <= 1 = true
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[src/main.rs:3] 1 = 1
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[src/main.rs:5] n * factorial(n - 1) = 2
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[src/main.rs:5] n * factorial(n - 1) = 6
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[src/main.rs:5] n * factorial(n - 1) = 24
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[src/main.rs:10] factorial(4) = 24
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```
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Using this macro without any argument will print the current file and line number.
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One thing to be aware is that this macro moves the input, in cases where this is a problem it's useful to borrow.
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Sources:
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- <https://doc.rust-lang.org/std/macro.dbg.html> |