Default argument/parameter to be another previous one

How to have a function default argument/parameter to be another previous argument value ?

let's make the below illustration only really work

Code:

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char* function(char* a, int b, char** c = &a){

  // if invoked only with two arg, c is assigned automatically with reference of a
  //...
}

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How to solve it? thanks much.

Instead of a default argument, add a two-parameter overload that forwards to the three-parameter version:

Code:

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char* function(char* a, int b, char** c) {
    // ...
}

char* function(char* a, int b) {
    return function(a, b, &a);
}

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