Default argument in the middle of parameter list?

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That code would work if in the very first declaration of the function, the last parameter has default value, something like this:

//declaration
void error(char const *msg, bool showKind, bool exit = false);

And then in the same scope you can provide default values for other arguments (from right side), in the later declaration, as:

void error(char const *msg, bool showKind = true, bool exit); //okay

//void error(char const *msg = 0 , bool showKind, bool exit); // error

which can called as:

error("some error messsage");
error("some error messsage", false);
error("some error messsage", false, true);

Online Demo : http://ideone.com/aFpUn

Note if you provide default value for the first parameter (from left), without providing default value for the second, it wouldn’t compile (as expected) : http://ideone.com/5hj46

§8.3.6/4 says,

For non-template functions, default
arguments can be added in later
declarations of a function in the same
scope.

Example from the Standard itself:

void f(int, int);
void f(int, int = 7);

The second declaration adds default value!

Also see §8.3.6/6.

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