Pass reference to string as default parameter

[pdk5]

I have legacy code where the function takes five arguements. I want to put the last arguement as the default arguement (this is because i donot want to change the
legacy code everywhere. But the issue is the last arguement is of string type and for some reason it needs to be passed as reference. Not sure how i can pass a default arguement
string as a reference
bool FunctionName(
Atype * pCmd, const BType & stBInfo, const CType & stCType, const u8 nNum, XNameSpace::YNameSpace::ZRequestPtr req = XNameSpace::YNameSpace::IRequestPtr(),
string sClassName = "");

The issue is if i pass the last arguement sClassName by value , somehow the string is appearing as NULL. If i pass as reference, i am getting compile error..
Is there anyway i can pass as reference ?

In our codebase, the string is typedef as
typedef basic_string<char> string;

[laserlight]

You could consider overloading the function.

[2kaud]

For c++11, this works with MSVS 2015

typedef basic_string<char> bstring;

void byref(bstring& bst = bstring("default"))
{
	cout << bst << endl;
	bst = "qwerty";
}

int main()
{
	bstring st = "123";
	byref();
	byref(st);
	cout << st << endl;
	return 0;
}

However, the issue is why passing by value isn't working.

[pdk5]

@laseligt Thanks a lot for the help. I will try this.

[laserlight]

For c++11, this works with MSVS 2015

It is an extension to C++ that existed even before C++11, and it is not standard C++ even after C++11 (we have rvalue references by a slightly different syntax instead).

[pdk5]

@kaud: yes not sure why passing by value isnt working for me, may be something to do with the copy constructor ..i am not a c++ expert thought..havent dig deeper on this..

[laserlight]

The issue is if i pass the last arguement sClassName by value , somehow the string is appearing as NULL.

Is it appearing as an empty string (not NULL: that is a null pointer constant) inside the function or in the caller after you have called the function. For the latter, the obvious reason is that by passing by value, the function only changes a copy of the string, so the string in the caller remains unchanged. For the former... well, that should not be happening.

[pdk5]

@laserlight: I can see the string in the calling function being properly passed by the debugger. But it is appearing as "empty string" in the called function

[pdk5]

@laserlight: Overloading may be option, but creates lot of duplicate code

[laserlight]

Temporarily change the default argument to "default argument". Do you still see an empty string in the called function?

Overloading may be option, but creates lot of duplicate code

No, it doesn't. The overloaded function can forward to your modified function. But if you can pass by value, then you should not be passing by non-const lvalue reference. Perhaps you should be passing by const lvalue reference.

[pdk5]

Sorry for the delay in response, had some work yesterday.

@laserlight: tried the solution you mentioned, but getting compile error. ..sorry for my naiveness in c++

[pdk5]

I tried to compile the small program kaud has given earlier, with g++, but getting the error:
sing.cpp:9: error: default argument for 'bstring& bst' has type 'bstring'

[pdk5]

Sorry it was my mistake in the earlier code

Passing the parameter as default arguement (aftet little fiddling) worked..

Thanks a lot laserlight and kaud for the hints and help

[laserlight]

Passing the parameter as default arguement (aftet little fiddling) worked..

Thanks a lot laserlight and kaud for the hints and help

You're welcome. What solution did you settle on in the end?

[pdk5]

By passing as the default argument .

Overloading might have worked , but didnot try that yet (may be when i have sometime, i will explore these options )