C++ delegating constructor with initialization list. Which inits happen?
c++, class, constructor, definition, delegating-constructor
Solution
According to the C++ Standard
If a mem-initializer-id designates the constructor’s class, it shall be the only mem-initializer; the constructor is a delegating constructor, and the constructor selected by the mem-initializer is the target constructor. The principal constructor is the first constructor invoked in the construction of an object (that is, not a target constructor for that object’s construction). The target constructor is selected by overload resolution. Once the target constructor returns, the body of the delegating constructor is executed. If a constructor delegates to itself directly or indirectly, the program is ill-formed; no diagnostic is required.
So this constructor definition
MyClass::MyClass(int a, int b)
:
MyClass(a, b, NULL),
int1(a),
int2(b),
pOtherClass(NULL)
{
}
is invalid.
Must be
MyClass::MyClass(int a, int b)
:
MyClass(a, b, NULL)
{
}
Problem
I've looked at a very similar question, but I'm not quite sure I understand the answer. If I delegate a constructor, which initializations from initialization lists occur? Example: ``` MyClass::MyClass(int a, int b) : MyClass(a, b, NULL), int1(a), int2(b), pOtherClass(NULL) { } MyClass::MyClass(int a, int b, Other *p) : int1(a), int2(b), pOtherClass(p) { if (pOtherClass == NULL) { pOtherClass = &DefaultInstance; } } ``` Here I have to have full initializer lists for both classes due to compiler settings. But what I don't want is: - First constructor(`int, int`) calls the second constructor(`int, int, Other *`) - Second constructor assigns a default address to `pOtherClass` - First constructor's init list assigns `pOtherClass` to `NULL`. The question I linked at the top seems to indicate that this behavior wont occur, but then what is the point of the initializer list in the (`int, int`) constructor? Just to keep the compiler happy?