Const qualifier

As explained in Rust in Action: Systems programming concepts and techniques

A const declaration defines a compile-time constant whose value cannot be changed.

How Rust in Action: Systems programming concepts and techniques covers it

How deep it goes

Defines const as an unchanging compile-time value and contrasts it with mutable global state, without developing its declaration details.

Examples

Contrasts an unchanging const value with the mutable ERROR global used to propagate file-reading errors.

What you can do after

Evaluate whether a global value should remain constant or be mutable in Rust error-handling code.

How it connects

Connects const with global variables, static mut, unsafe code, and error handling.

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Also explained in

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  Learn Rust in a Month of Lunches
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  Modern CA constant is a named C entity or value that does not change during program execution.
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  Practices of the Python Pro
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  Kotlin in Action, 2EThe const modifier declares a compile-time constant property of a permitted primitive type or String type.
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  Secrets of the JavaScript Ninja, Second EditionA JavaScript binding declared with const that must be initialized at declaration and cannot be reassigned, although an object or array it references can still be modified.
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  Modern C, Third EditionA const-qualified type is a C type whose referenced value is not modifiable through an expression having that type.

Compare how 7 books explain const qualifier →

Related concepts

• factoryused by · A factory is a function, method, class, or component that creates and returns objects or other values while encapsulating construction details or selection.
• constantuses · A constant is a named program value or binding that is fixed or intended not to change after initialization.
• let keywordcontrasts with · A let construct is a language feature that introduces a name binding associating an identifier with a value or expression.
• read-only objectkind of · A read-only object is an object that a program is not permitted to modify through its applicable access or type qualification.

This explainer was generated by Manning's knowledge graph from the text of Rust in Action: Systems programming concepts and techniques. The section links point to the parts of the book it draws on.