Const Placement in C Pointers: Left or Right, and Why Does It Matter?

Const Placement in Pointer Declarations: The Mystery Unraveled

In C , the const keyword can be used to modify either the data pointed to by a pointer or the pointer itself. This leads to the question: why are both "const T" and "T const" valid, and which one should be used?

Historical Origin

The reason for this seemingly arbitrary choice dates back to the early days of the C language. The original C grammar was defined to parse input from left to right, processing each token as it encountered it.

When parsing a declaration with an asterisk (*), the parser would complete processing of the declaration prior to encountering const. This meant that const could be applied either before or after the asterisk, without affecting the semantic meaning of the declaration.

Left-to-Right Parsing and Qualifier Placement

This left-to-right parsing mechanism also affects the placement of const in function pointer declarations. For example, "void function1(void)" declares a function that returns void , while "void (* function2)(void)" declares a function pointer to a function that returns void.

Which Syntax to Choose

Ultimately, the choice of which syntax to use is a matter of preference. However, the following guidelines may help:

• Const on the left of the pointer specifies that the data pointed to is constant.
• Const on the right of the pointer specifies that the pointer itself cannot be modified.

In general, it's recommended to place const on the left to clearly indicate the intended immutability of the data. This is particularly important when working with shared pointers or references to const objects to avoid unexpected surprises.

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