Nonstatic member as a default argument of a nonstatic member function [duplicate]

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Your code (simplified):

struct X
{
   int mem;
   void f(int param = mem); //ERROR
};

You want to use a non-static member data as default value for a parameter of a member function. The first question which comes to mind is this : which specific instance of the class the default value mem belongs to?

X x1 = {100};  //mem = 100
X x2 = {200};  //mem = 200

x1.f(); //param is 100 or 200? or something else?

Your answer might be 100 as f() is invoked on the object x1 which has mem = 100. If so, then it requires the implementation to implement f() as:

void f(X* this, int param = this->mem);

which in turn requires the first argument to be initialized first before initialization of other argument. But the C++ standard doesn’t specify any initialization order of the function arguments. Hence that isn’t allowed. Its for the same reason that C++ Standard doesn’t allow even this:

int f(int a, int b = a); //§8.3.6/9

In fact, §8.3.6/9 explicitly says,

Default arguments are evaluated each
time the function is called. The order
of evaluation of function arguments is
unspecified. Consequently, parameters
of a function shall not be used in
default argument expressions, even if
they are not evaluated.

And rest of the section is an interesting read.

An interesting topic related to “default” arguments (not related to this topic though):

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