Question

struct X
{
   X():mem(42){}
   void f(int param = mem) //ERROR
   {
      //do something
   }
private: 
   int mem;
};

Can anyone give me just one reason as to why this is illegal in C++?! That is to say, I know that it is an error, I know what the error means, I just can’t understand why would this be illegal!

Answer 1

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.

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

• Default argument in the middle of parameter list?