\'const int\' vs. \'int const\': What\'s the Difference in C and C ?

Function Parameters in C and C : 'const int' vs. 'int const'

Two similar function parameter declarations in C and C are 'const int' and 'int const'. While they may appear identical, there is a subtle distinction between the two.

Const Declarations in C

In C, the declaration of 'const int' implies that the variable passed as a parameter is constant, meaning its value cannot be modified within the function. However, the value that is passed as an argument to this parameter can be a constant or a variable.

Example:

<code class="c">int testfunc1 (const int a)
{
  // a is constant within the function, but the argument can be variable.
}</code>

Const Declarations in C

In C , 'int const' follows the same rules as in C, but 'const int' has an additional interpretation. It also implies that the variable is initialized with a constant value, which cannot be modified within the function.

Example:

<code class="cpp">int testfunc2(int const a)
{
  // Both a and the argument must be constants.
}</code>

Reading Trick for Declarations

To better understand the difference, a helpful trick is to read the declaration backwards:

• 'const int' reads as "a is an integer which is constant"
• 'int const' reads as "a is a constant integer"

Implications

Both declarations indicate that the passed value cannot be modified within the function. However, the initialization requirement in C (for 'const int') adds an additional constraint.

Example:

<code class="cpp">const char *s;      // s is a pointer to a char that is constant
char *const t = &c; // t is a constant pointer to a char</code>

In this case, the value of 's' can change (as it's a pointer), but the value of 't' cannot.

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