Why is Specifying Default Parameters in the Function Declaration Preferred in C ?

Understanding the Significance of Default Function Parameters

Question: In C , there are two ways to define a function with default parameters. Which method is preferred and why?

Response:

In C , default parameters can be specified either in the function declaration or definition. While both methods work syntactically, the preferred approach is to specify default parameters in the declaration itself.

This preference stems from potential compilation issues when the declaration and definition are separated across multiple files. Consider the following example:

lib.h (Header File)

<code class="cpp">int Add(int a, int b);</code>

lib.cpp (Source File)

<code class="cpp">int Add(int a, int b = 3) {
  // Function implementation
}</code>

test.cpp (Test File)

<code class="cpp">#include "lib.h"

int main() {
  Add(4);
}</code>

Compiling test.cpp will result in an error because the declaration in lib.h does not specify the default parameter value. This is because the compiler only sees the declaration when compiling test.cpp, and not the definition in lib.cpp.

Therefore, it's advisable to always define default parameters in the function declaration, as in the following:

lib.h

<code class="cpp">int Add(int a, int b = 3);</code>

By specifying the default parameter in the declaration, the compiler is aware of its presence even when the function definition is not available, preventing compilation errors.

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