Inout

Inout is a programming concept describing a parameter passing mechanism in which a function can read the initial value of a variable passed to it and also modify that variable, with the modifications visible to the caller after the function returns. It is often described as pass-by-reference with two-way data flow: the parameter acts as both input and output.

In languages that support inout parameters, the parameter is typically passed by reference rather than by value. This allows functions to update the caller's variable without returning a separate result. The term is commonly used in shader languages and in some general-purpose languages.

In GLSL, the OpenGL Shading Language, a function parameter can be declared inout to indicate that the parameter is passed by reference to both read and write the caller’s variable. For example: void swap(inout int a, inout int b) { int t = a; a = b; b = t; } Then calling swap(x, y) will exchange x and y.

In Swift, inout parameters enable a similar effect for functions, but require the caller to pass the variable with an explicit reference marker, usually &. For example: func increment(_ value: inout Int) { value += 1 } var x = 3; increment(&x); // x becomes 4. Swift enforces rules to prevent aliasing, such as disallowing multiple inout arguments referencing the same value in a single call.

Limitations and notes: inout parameters introduce side effects, can complicate reasoning about code, and may be restricted by language rules (for example around escaping closures or aliasing). They are contrasted with pass-by-value and with separate return values. See also: pass-by-reference, reference parameters, in, out, copy-on-write.

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