Const qualifier
A language feature that marks a value, binding, or access path as constant, restricting reassignment or modification according to the language’s rules.
Learn Rust in a Month of Lunches
- How it is used
- Uses const for compile-time, unchangeable values available throughout a Rust program.
- How deep it goes
- Defines declarations, typing, scope, and compile-time limits, while deferring deeper const-versus-static details.
- Examples
- Demonstrates numeric bounds, month counts, seasons, and calculator input validation with named constants.
- What you can do after
- Apply it by declaring typed compile-time values and reusing them across functions or validation logic.
Read how Learn Rust in a Month of Lunches explains const qualifier →
Modern C
A constant is a named C entity or value that does not change during program execution.
- How it is used
- Uses const qualification to mark objects and referenced character data as read-only during access.
- How deep it goes
- Explains read-only semantics, string-literal history, and indirect modification, while deferring pointer mechanics to later treatment.
- Examples
- Demonstrates const-qualified arrays of bird names, pronouns, and ordinal labels, plus null pointer declarations.
- What you can do after
- Evaluate/understand which parts of a declaration are read-only and distinguish const-qualified objects from compile-time constants.
Practices of the Python Pro
- How it is used
- Centralizes repeated delimiters and reusable computed data so changes happen in one place.
- How deep it goes
- Demonstrates constants through short Python refactorings but stops short of discussing immutability or language-level enforcement.
- Examples
- Uses CSV-to-TSV delimiters and alphabetized United States state capitals as practical examples.
- What you can do after
- Apply it via named Python values to reduce duplicated edits and repeated calculations.
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Kotlin in Action, 2E
The const modifier declares a compile-time constant property of a permitted primitive type or String type.
- How it is used
- Marks fixed top-level or object properties for compile-time use and direct Java field exposure.
- How deep it goes
- Defines placement, permitted types, initialization constraints, Java representation, and annotation usage without exploring compiler internals.
- Examples
- Uses UNIX_LINE_SEPARATOR for Java field exposure and TEST_TIMEOUT for a JUnit timeout annotation.
- What you can do after
- Apply const val to define Java-friendly constants and satisfy annotation arguments requiring compile-time values.
Secrets of the JavaScript Ninja, Second Edition
A 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.
- How it is used
- Uses const to prevent reassignment and name fixed values instead of unexplained literals.
- How deep it goes
- Explains initialization, reassignment errors, mutable object contents, and block scope while contrasting var and let.
- Examples
- Demonstrates reassignment failure with a string, object property addition, array mutation, and scoped loop variables.
- What you can do after
- Apply const to prevent binding reassignment and give fixed values meaningful names.
Read how Secrets of the JavaScript Ninja, Second Edition explains const qualifier →
Modern C, Third Edition
A const-qualified type is a C type whose referenced value is not modifiable through an expression having that type.
- How it is used
- Establishes read-only access and constrains pointer and string handling in C programs.
- How deep it goes
- Explains object and pointer qualification, alias-based mutation, string-literal history, and the boundary with constexpr.
- Examples
- Uses char const*const, string-literal arrays, extern double const, and constexpr declarations as concrete examples.
- What you can do after
- Evaluate/understand which values and pointer targets an expression may modify.
Rust in Action: Systems programming concepts and techniques
A const declaration defines a compile-time constant whose value cannot be changed.
- How it is used
- Distinguishes immutable constants from mutable global variables in global error-handling code.
- 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.
Read how Rust in Action: Systems programming concepts and techniques explains 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.
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