C Function Parameters: What\'s the Difference Between `const int` and `int const`?

Constants as Function Parameters: 'const int' vs. 'int const'

In C , the seemingly similar parameter declarations of const int and int const have distinct implications for function behavior.

Consider the following functions:

<code class="c++">int testfunc1(const int a) {
  return a;
}

int testfunc2(int const a) {
  return a;
}</code>

To understand the difference, it's helpful to read the declarations right-to-left:

const int a = 1; // "a is an integer which is constant"
int const a = 1; // "a is a constant integer"

In both cases, a represents a constant value that cannot be modified within the function. However, the order of the keywords defines whether the constant defines the type or the variable:

• const int: The parameter is a constant of type int. The variable a can still be modified, but it must always contain an integer value.
• int const: The parameter is an integer that is constant. The variable a cannot be modified at all.

Therefore, these two functions are not interchangeable. In testfunc1, the value of a is protected from unexpected changes, while in testfunc2, both the value and the type are immutable.

This distinction becomes particularly important in more complex declarations like these:

<code class="c++">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"</code>

By reading the declarations backwards, we can determine that:

• The pointer in s is constant, but the character it points to can be modified.
• The character in c is non-constant, but the pointer in t cannot be reassigned to a different memory address.

This distinction in the order of keywords allows for fine-grained control over how data is handled within functions, ensuring both code clarity and predictable behavior.

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