\'const int\' vs. \'int const\' in Function Parameters: Does the Order of Qualifiers Matter?

'const int' vs. 'int const' in Function Parameters: Understanding the Distinction

In C and C , when declaring function parameters with type qualifiers such as 'const,' it's crucial to understand the difference between placing 'const' before or after the type. Consider the following two function declarations:

int testfunc1 (const int a);
int testfunc2 (int const a);

While these declarations appear similar, there is a subtle but significant distinction between them.

To clarify this difference, we can read the declarations backward:

• const int a: "a is an integer which is constant"
• int const a: "a is a constant integer"

Therefore, both declarations essentially mean the same thing. In either case, the value of 'a' cannot be modified within the function. The following code exemplifies this:

a = 2; // Can't do because a is constant

This line will result in an error because 'a' is declared as constant.

This "read backward" technique becomes particularly useful when dealing with more complex declarations, such as:

• const char *s: "s is a pointer to a char that is constant"
• char c;
• char *const t = &c: "t is a constant pointer to a char"

In this case, while 's' points to an immutable character, the pointer itself can be modified. On the other hand, 't' is a constant pointer, meaning its value cannot change. This is illustrated in the following code:

*s = 'A'; // Can't do because the char is constant
s++; // Can do because the pointer isn't constant
*t = 'A'; // Can do because the char isn't constant
t++; // Can't do because the pointer is constant

Understanding the difference between using 'const' before or after the type in function parameters is crucial to accurately convey the intended behavior of the function and avoid potential errors.

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