Why Are 'const T*' and 'T const*' Both Valid Pointer Declarations in C ?

Why is "const T" and "T const" Both Valid?

The C language allows specifying const on both sides of the asterisk when declaring pointers. This flexibility arises from the C grammar's left-to-right parsing approach.

Left-to-Right Parsing

The C compiler parses input tokens from left to right. When parsing a pointer declaration, it encounters the token and assumes a pointer type is being defined. Any const qualifiers encountered before the apply to the type of data pointed to. However, if the const qualifier appears after the *, it applies to the pointer itself.

Semantic Equivalence

The semantic meaning of a pointer declaration remains the same regardless of the position of const. Whether the const is on the left or right of the type specifiers, the compiler interprets it the same way.

Usage

While "const T" and "T const" are equally valid, some programmers prefer to place const on the left to explicitly specify the constness of the data itself. However, the choice of one syntax over the other is a matter of personal preference.

Function Pointers

A similar situation arises when declaring function pointers. The syntax "void function1(void)" declares a function that returns void , while "void (* function2)(void)" declares a function pointer to a function that returns void.

Conclusion

The ability to specify const on either side of the asterisk in pointer declarations is a consequence of the left-to-right parsing mechanism of the C compiler. Semantically, the position of const does not affect the meaning of the declaration. Programmers are free to use whichever syntax they prefer for readability or consistency.

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