Where should default parameters be defined in C : declaration or definition?

Default Parameter in Function Definition vs. Declaration

In C , functions can be declared with default parameters, allowing them to be called without providing values for all the arguments. However, the syntax for specifying the default parameter can vary.

Case 1:

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

Case 2:

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

Both syntaxes are valid. However, the standard and recommended approach is Case 1, where the default parameter is specified in the function declaration. This is because:

Consider the following scenario:

• A header file (lib.h) declares the function with either syntax.
• A separate implementation file (lib.cpp) defines the function with the default parameter.
• Another source file (test.cpp) includes the header file and calls the function without providing a value for the default parameter.

In Case 1:

The compiler will see the default parameter declaration in the header file during compilation of test.cpp. This ensures that the call to the function in test.cpp is valid, even though the default parameter was not explicitly provided.

In Case 2:

The compiler will not see the default parameter declaration in the header file during compilation of test.cpp, as it is only defined in the implementation file (lib.cpp). This will result in a compilation error in test.cpp, as the default parameter was not declared.

Therefore, specifying the default parameter in the function declaration (lib.h) is the standard way to ensure consistent behavior across different compilation units.

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