\'const int\' vs. \'int const\': What\'s the Difference in Function Parameters?

Ambiguity of 'const' Placement in Function Parameter Declaration

When declaring function parameters in C and C , you may encounter the choice between using 'const int' and 'int const'. While these two syntaxes appear similar, they carry subtle differences that can affect the behavior of your code.

Consider the following example:

int testfunc1 (const int a)
{
  return a;
}

int testfunc2 (int const a)
{
  return a;
}

Intuitively, it might seem that both functions have the same behavior, declaring a constant parameter 'a'. However, the placement of 'const' affects how the declaration is interpreted.

In the first declaration, 'const int a', the 'const' keyword modifies the type 'int,' indicating that the parameter 'a' is an integer that cannot be modified. This is commonly referred to as "constant data."

In the second declaration, 'int const a', the 'const' keyword modifies the parameter name 'a' itself, indicating that the parameter is a pointer to a constant integer. This is commonly referred to as "constant pointer."

The distinction becomes evident when examining the following code:

testfunc1(a); // Compiles without error, 'a' is a constant
testfunc2(&a); // Compiles without error, 'a' is a constant pointer

In summary, 'const int' declares a constant parameter, while 'int const' declares a constant pointer. Understanding this difference is crucial for ensuring the intended behavior of your functions and preventing potential errors.

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