C++ Syntax : Constructor particularity

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Started by _Danneman_ December 29, 2002 09:46 AM

3 comments, last by _Danneman_ 21 years, 10 months ago

_Danneman_

Author

122

December 29, 2002 09:46 AM

Heh, feels like Im taking advantage of the expertise here

Anyway, this is about constructors. This one has an extra part to it that I dont understand: ----------------------------- SmyStruct::SmyStruct(int NumInputs):m_NumInputs(NumInputs){*code goes here*} ----------------------------- So, the unfamiliar part to constructors comes with the single semi-colon. What exactly am I doing with the ''...:m_NumInputs(NumInputs)'' part?

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CWizard

127

December 29, 2002 09:51 AM

That''s the constructor initializer list, used in the contructor of a class using inheritance. It is used to initialize the base object, in this case m_NumInputs, with proper parameters.

dalleboy

324

December 29, 2002 09:56 AM


MyStruct(int NumInputs)
: m_NumInputs(NumInputs)
{
}

is (almost) the same as:


MyStruct(int NumInputs)
{
   m_NumInputs = NumInputs;
}

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Fruny

1,658

December 29, 2002 04:39 PM

Not only is it used to initialise the (possible) base class, it is also used to initialise member variables by directly calling their constructors. If you assign to them in the body of the constructor, they will actually first be default-constructed THEN assigned to, which is not only inefficient (you essentially initialise the variable twice), but may be impossible if the member variable is a constant, a reference, or doesn''t have a default constructor.

Initializer lists are your friends.

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