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

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

Default arguments have to be known at compile-time. When you talk about something like a function invocation, then the function is known at compile-time, even if the return value isn't, so the compiler can generate that code, but when you default to a member variable, the compiler doesn't know where to find that instance at compile-time, meaning that it would effectively have to pass a parameter (this) to find mem. Notice that you can't do something like void func(int i, int f = g(i)); and the two are effectively the same restriction.

I also think that this restriction is silly. But then, C++ is full of silly restrictions.

Answer 2

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?