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Class Object Initialization Question ...

tmason (112)
Hello,

I have a quick question that arouse out of curiosity. Suppose you have a class like so:

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class SimpleClass {

public:

    SimpleClass();
	~SimpleClass();

protected:

	float 	protectedFloat;
	int		protectedInteger;

private:

}


My question is why is it better to do this:

 
 
SimpleClass::SimpleClass () : protectedFloat(0.0f), protectedInteger(0){}


As opposed to:

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SimpleClass::SimpleClass () {

     protectedFloat = 0.0f;
     protectedInteger = 0;

}


Is the former faster?

Thank you for your time.
LB (13399)
You should prefer the first. It doesn't make much of a different for primitive types but for more complex types it makes a big difference.
Gamer2015 (810)
When you have more complex types, and you use the second method, the Objects are created with the default constructor and then you change it's members which takes more time because you basically initialize the Object twice.

please correct me if I'm wrong
MikeyBoy (5631)
When you have more complex types, and you use the second method, the Objects are created with the default constructor and then you change it's members which takes more time because you basically initialize the Object twice.

please correct me if I'm wrong

That's correct - although it's more precise to say you're initialising it once and then assigning new values to it.
closed account (D80DSL3A)
Some examples of when the 1st method must be used:

1) Data members lacking any no-arg constructor.

2) Data members which are a reference type. These must be assigned on construction.

EDIT: Example
Class A lacks a no-arg ctor.
Class B has an A and a type double reference as members.
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#include<iostream>

class A
{
private:
    int x;
public:
    // class A has only a 1 arg ctor.
    A( int X ) { x = X; }// 2nd method OK. type int is basic
};

class B
{
private:
    double& rd;// reference member
    A a;// member with no no-arg ctor
public:
    B( int X, double & rDbl ): rd(rDbl), a(X) {}// must use initializer list for these 2 members
//    B( int X, double & rDbl ) { rd = rDbl; a.x = X; }

    double& get_ref(){ return rd; }// added to show use of refernce member in main()
};


int main()
{
    double dblVar = 3.14;
    B b( 5, dblVar );

    b.get_ref() = 7.77;// assign new value through b's reference to dblVar
    std::cout << "dblVar now = " << dblVar << '\n';

    return 0;
}

The example compiles and runs as is.
A 2nd B class ctor is commented out. Change to this from the 1st ctor for errors.

I get the following:

D:\my_programs\codeBlocksProjects\forumProbs\main.cpp||In constructor 'B::B(int, double&)':|
D:\my_programs\codeBlocksProjects\forumProbs\main.cpp|28|error: uninitialized reference member in 'double&' [-fpermissive]|
D:\my_programs\codeBlocksProjects\forumProbs\main.cpp|24|note: 'double& B::rd' should be initialized|
D:\my_programs\codeBlocksProjects\forumProbs\main.cpp|28|error: no matching function for call to 'A::A()'|
D:\my_programs\codeBlocksProjects\forumProbs\main.cpp|28|note: candidates are:|
D:\my_programs\codeBlocksProjects\forumProbs\main.cpp|18|note: A::A(int)|
D:\my_programs\codeBlocksProjects\forumProbs\main.cpp|15|error: 'int A::x' is private|
D:\my_programs\codeBlocksProjects\forumProbs\main.cpp|28|error: within this context|

That last error provides a 3rd reason, which I hadn't thought of.
Assigning a.x = X; in the body of the ctor is illegal because A::x is private.
Last edited on
LB (13399)
#2 is a subset of #1
closed account (D80DSL3A)
I guess they are, since both are examples of objects/entities which must be initialized on construction/declaration.
tmason (112)
Thank you for the examples! Good to know this stuff :)
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