Function Parameter List

A function’s parameter list can be empty but cannot be omitted. Typically we define a function with no parameters by writing an empty parameter list. For compatibility with C, we also can use the keyword void to indicate that there are no parameters:

void f1(){ /* ... */ }     // implicit void parameter list
void f2(void){ /* ... */ } // explicit void parameter list

A parameter list typically consists of a comma-separated list of parameters, each of which looks like a declaration with a single declarator. Even when the types of two parameters are the same, the type must be repeated:

int f3(int v1, v2) { /* ... */ }     // error
int f4(int v1, int v2) { /* ... */ } // ok

No two parameters can have the same name. Moreover, local variables at the outermost scope of the function may not use the same name as any parameter.

Parameter names are optional. However, there is no way to use an unnamed parameter. Therefore, parameters ordinarily have names. Occasionally a function has a parameter that is not used. Such parameters are often left unnamed, to indicate that they aren’t used. Leaving a parameter unnamed doesn’t change the number of arguments that a call must supply. A call must supply an argument for every parameter, even if that parameter isn’t used.