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Placeholder arguments
Arguments in a function declaration can
be declared without identifiers. When these are used
with default arguments, it can look a bit funny. You can end up
with
void f(int x, int = 0, float = 1.1);
In C++ you don’t need identifiers
in the function definition, either:
void f(int x, int, float flt) { /* ... */ }
In the function body, x and
flt can be referenced, but not the middle argument, because it has no
name. Function calls must still provide a value for the placeholder, though:
f(1) or f(1,2,3.0). This syntax allows you to put the argument in
as a placeholder without using it. The idea is that you might want to change the
function definition to use the placeholder later, without changing all the code
where the function is called. Of course, you can accomplish the same thing by
using a named argument, but if you define the argument for the function body
without using it, most compilers will give you a warning message, assuming
you’ve made a logical error. By intentionally leaving the argument name
out, you suppress this warning.
More important, if you start out using a
function argument and later decide that you don’t need it, you can
effectively remove it without generating warnings, and yet not disturb any
client code that was calling the previous version of the
function.
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