Question

I want to do something like this:

int displayAll(Message *m, string &lastIndex, int &NumPrinted = 0 );

It gives me error, cribbing about int to int&.

I tried this too:

int temp =0;

int displayAll(Message *m, string &lastIndex, int &NumPrinted = temp );

Still it gives following error:

error: ISO C++ forbids in-class initialization of non-const static member 'temp'

Even static int temp; gives error.

error: ISO C++ forbids in-class initialization of non-const static member ‘temp’

Answer 1

The problem with the first line of code you mention is that you are trying to pass a reference to a temporary variable

class Foo {

    int displayAll(Message *m, bool &moreElements, string &lastIndex, int &NumPrinted = 0 );

};

The second bit of code complains because you were trying to initialize a class member statically.

class Foo {

    int temp =0;

    int displayAll(Message *m, bool &moreElements, string &lastIndex, int &NumPrinted = temp );

};

(I am putting your code inside of a class declaration to be clear about what is happening).

An easy way out of your problem that does not introduce a static variable is explicit function overloading:

class Foo {

    inline int displayAll(Message *m, bool &moreElements, string &lastIndex) {
        int dummy = 0;
        return displayAll(m, moreElements, lastIndex, dummy);
    }
    int displayAll(Message *m, bool &moreElements, string &lastIndex, int &NumPrinted);

};

There’s a bit of boilerplate, but it achieves what you want.
Hope this helps.

EDIT: Some more clarification. The core of the problem stems from the fact that the function must take a reference to some memory that it can modify. If you pass it a temporary variable (temporary as in the C++ meaning of the term, not just the english language term ) (as in your first line of code), it’s illegal C++, since you usually copy a temporary to a value before you use it as an argument to a function:

void bar( int someNum = 0 ); // think of this as creating a temporary rvalue 0
                             // and then copying it into the function for use.

// temporary rvalues arise in expressions like
int v = 5 + 5; // the result of 5 + 5 is stored in a temporary rvalue, and then
               // copied into v (which is an lvalue in this case).

So we need something that is an “lvalue”, either some global variable somewhere or a temporary local variable ( in the english language sense ) as I gave in my answer. I was about to write a solution using a static variable, but there is a large flaw- since the static variable will be shared by all instances of your class, it will start out 0 and then be different every time you call the method ( since it would have been edted by the previous call). Even worse, in the case of multiple threads, you would be reading/writing to the same place of memory from several processors, so the value will be complete garbage, and you ill rape your processor cores’ caches as each write will invalidate the cache of every other core. It’s ugly, please don’t do it. 😛

By using my first solution you make the temporary variable very local, without much impact on anything else.