the : operator

Waldermort

Could somebody explain how this works and what it does. I know it can be used to declare a base class, but I have now come across it in another location and I can't work out what it does. class Error { public: Error(const int nErrCode, char* szErrMess); Error(const Error& e); private: int mnErrCode; char* mpszErrMess; }; Error::Error(const int nErrCode,char* szErrMess) : mnErrCode(nErrCode) { } Error::Error(const Error& e) : mnErrCode(e.mnErrCode) { }

BadKarma

Hi, I believe you mean the two following occurances

waldermort wrote:

Error::Error(const int nErrCode,char* szErrMess) : mnErrCode(nErrCode) ... Error::Error(const Error& e) : mnErrCode(e.mnErrCode)

When using the colon ':' after a constructor you open an initialisation list. This means that member like mnErrCode are initialised with the given error code even before you enter the scope of the constructor.

codito ergo sum

Waldermort

So it's basically the same as placing mnErrCode = nErrCode; inside the c'tor? I have to ask, what is the point of doing this?

Justin Tay

In this case, probably just convenience. But lets say you have a member that is a class object that doesn't have a default constructor.

BadKarma

supose the following situation

class A
{
public:
A(int i)
{
_i = i;
}

int _i;
};

// class a has no default constructor

class B
{
public:
B() {}

A _a;
};

// the Class B won't compile because there is no default constructor for _a
// solution the initialisation list

class C
{
public:
C() : _a(1) {}

A _a;
};

Now when maken an object of the type C the default constructor (of C) will initialize _a and calling the correct constructor of the member _a You might want to check the following http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.6[^]

codito ergo sum

Waldermort

Ahhh, I understand perfectly now. Thankyou for the explanation. Infact I now remember seeing and using this before, when using STL containers within a class, I remember first having to initialize them like this before being able to use them. -- modified at 20:20 Friday 25th August, 2006