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  #1  
Old 25-10-2008
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Join Date: Oct 2008
Posts: 11
initialisation list in constructor

"Consider the following constructor that initializes member object x_ using an initialization list: Fred::Fred() : x_(whatever) { }. The most common benefit of doing this is improved performance. For example, if the expression whatever is the same type as member variable x_, the result of the whatever expression is constructed directly inside x_ ? the compiler does not make a separate copy of the object. Even if the types are not the same, the compiler is usually
able to do a better job with initialization lists than with assignments.

The other (inefficient) way to build constructors is via assignment, such as: Fred::Fred() { x_ = whatever; }. In this case the expression whatever causes a separate, temporary object to be created, and this temporary object is passed into the x_ object's assignment operator. Then that temporary object is destructed at the ;. That's inefficient.

As if that wasn't bad enough, there's another source of inefficiency when using assignment in a constructor: the member object will get fully constructed by its default constructor, and this might, for example, allocate some default amount of memory or open some default file. All this work could be for naught if the whatever expression and/ or assignment operator causes the object to close that file and/or release that memory (e.g., if the default constructor didn't allocate a large enough pool of memory or if it opened the wrong file). "

I believe that if you write Fred::Fred() {x_ = whatever;} then effectively what you're getting is Fred::Fred(): x_(whatever) {x_ = whatever;}. This is what I think the last paragraph is saying (the member object will get fully constructed by it's default constructor).

However, I don't see what the second paragraph is saying. If whatever is a primitive, then no 'temporary object' will be created. If whatever is an object, then a temporary object would have to be created anyway in the initialisation example anyway:

Fred::Fred(): x_(WhateverClass()) {}

When is the temporary copy being referred to created?

in a 'normal function':

if we write:

Foo foo = bar;

then whether a temporary Bar object is created depends on the
definition of the Foo constructor:

Foo(Bar &); // no copy made
Foo(Bar); // copy made
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  #2  
Old 25-10-2008
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Join Date: Aug 2008
Posts: 55
Re: initialisation list in constructor

If you assign x_ in the ctor body instead of the init list, Fred's ctor needs to allocate its members at the very least. This happens
before the ctor's body is processed.

in the same breath ...
int temp(0);

Whether a temporary is created depends on whether its creation can be optimized away or not. In your second example, Foo(Bar); you'ld get 2 copies if it wasn't optimized and you did use the init list (a temp is generated), 2 copies + op= if you didn't use the init list. In the first example you'ld get a copy unless you were setting a member reference (no temporary). Fortunately, that copy is usually optimized away.

For all intensive purposes, those paragraphs in faq-10.6 are relevent since they give an overview of the difference between using the init list and assignments. In reality the ctor, at the very least, will allocate / reserve memory for its members before its body is processed. Why not initialize those members using the init list then? Copies are very fast usually. The same can't be said of allocation + assignment.

Fred& Fred:perator= (const Fred& f)
{
if (this == &f) return *this; // Gracefully handle self
assignment

// Put the normal assignment duties here...

return *this;
}

There is no reason not to use the init list. For a programmer: its often a blessing. Take for example a simple way to zap the unitialized
pointer issue:

class P
{
int* p;
public:
P() : p(0) { }
...
};
int m = temp; // is a copy

note the difference now:
int temp(0);
int m;
m = temp; // is not a copy. assignment

The difference is that the first set of statements can be optimized away, the assignment usually cannot.
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  #3  
Old 25-10-2008
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Join Date: Sep 2008
Posts: 25
Re: initialisation list in constructor

The most common benefit of doing this is that it is cleaner; that you don't have to worry about uninitialized or incorrectly initialized variables in the body of your constructor. The general rule is to never declare a variable without initializing it; member variables are implicitlly declared before entering the constructor, so logically, you want to initialize them before entering the constructor.

Another frequent reason is that the type may not have a default constructor (a lot of my types don't), so you have to initialize it in the initializer list; otherwise, you get an error.
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