byvalue

By value is a parameter passing convention in programming languages in which a function receives a copy of the actual argument's value. The callee operates on that copy, and changes to the parameter do not affect the original variable in the caller.

How it behaves depends on the language and the type being passed. For primitive or value types, the language typically copies the entire value. For reference types, some languages copy the reference rather than cloning the object, so the function can mutate the shared object, while others copy the object itself if it is a value type.

Because by-value copies create a distinct copy, the callee has its own storage and cannot alter the caller’s data by assigning to the parameter. This makes reasoning about code easier and can aid predictability and thread safety, since the caller’s state is not directly mutated by the callee.

Trade-offs include performance costs for large data structures due to copying, and the potential loss of in-place updates. Modern languages mitigate these costs with techniques such as move semantics, copy-on-write, or by returning modified values rather than modifying inputs.

Examples:

- In C-like pseudocode, void f(int x) { x = x + 1; } int a = 5; f(a); // a remains 5

- In languages where objects are passed by value of the reference, such as some interpretations of Java, a function may receive a copy of the reference; the underlying object may still be mutated if it is mutable.

See also: pass by reference, copy semantics, shallow copy, deep copy, value type, reference type.