When to use class constructor member variable initializations?
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Hi,
There's a comma issue there ;-)
I usually go with all member initialisation in that list.
That also allows you to get a nice warning if you put them out of order.
Note that with recent C++ standards, you can also do member initialisation at declaration time. See here for what is allowed.
@SGaist said in When to use class constructor member variable initializations?:
Note that with recent C++ standards, you can also do member initialisation at declaration time. See here for what is allowed.
When I first (years ago, certainly before C++11) did some C++, I used to write:
class Foo { int z = 0; }but the guy who was a C++-er said to me: don't do it there, do it explicitly in the constructor. Are you now saying we can/should do initialization in the class declaration after all? Or have I misunderstood?
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@JonB said in When to use class constructor member variable initializations?:
would you be kind enough to spell out exactly what you are saying for me?
struct foo { std::string m_str; foo() } foo::foo() { m_str = "blub"; }In this case in the ctor first std::string() is called by the compiler, then in the ctor body the string is assigned. When you do
foo::foo() : m_string("blub") {}Then the std::string is assigned only once.
@Christian-Ehrlicher said in When to use class constructor member variable initializations?:
Then the std::string is assigned only once.
Yes, I know that. But that does not apply if it's
int m_iinstead? So what things do or do not get this auto-initialization? I have to look at what feature about the type of every member variable to see where I am? -
@JonB said in When to use class constructor member variable initializations?:
don't do it there, do it explicitly in the constructor
This did not work before c++11
So what things do or do not get this auto-initialization?
They're undefined (
You're correct for PODs)Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
Visit the Qt Academy at https://academy.qt.io/catalog -
@JonB said in When to use class constructor member variable initializations?:
don't do it there, do it explicitly in the constructor
This did not work before c++11
So what things do or do not get this auto-initialization?
They're undefined (
You're correct for PODs)@Christian-Ehrlicher said in When to use class constructor member variable initializations?:
This did not work before c++11
What didn't work < C++ 11?
class Foo { int z = 0; }didn't work??They're undefined (You're correct for PODs)
So are you saying I need to go look up whale pods for C++ in order to understand what is/is not initialized for class members?
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@JonB said in When to use class constructor member variable initializations?:
I need to go look up whale pods for C++
See http://www.cplusplus.com/reference/type_traits/is_pod/
What didn't work < C++ 11? class Foo { int z = 0; } didn't work??
Correct
Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
Visit the Qt Academy at https://academy.qt.io/catalog -
@JonB said in When to use class constructor member variable initializations?:
I need to go look up whale pods for C++
See http://www.cplusplus.com/reference/type_traits/is_pod/
What didn't work < C++ 11? class Foo { int z = 0; } didn't work??
Correct
@Christian-Ehrlicher said in When to use class constructor member variable initializations?:
Correct
Indeed! Gosh, I did not know that, I thought it did work < C++11. Clearly my memory is not to be relied on.
I ended up reading https://www.boost.org/doc/libs/1_65_1/libs/utility/value_init.htm. Which goes through all this initializer stuff, and made my head hurt.
I realize that, not for the first time, I am in a minority of one not liking any of this stuff. I was getting to quite like C++ over the past months, despite my misgivings of the past. Now I'm remembering just why I found it arcane [e.g. "C++ has at least four different initialization notations, some of which overlap."].. :(
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@Christian-Ehrlicher said in When to use class constructor member variable initializations?:
Correct
Indeed! Gosh, I did not know that, I thought it did work < C++11. Clearly my memory is not to be relied on.
I ended up reading https://www.boost.org/doc/libs/1_65_1/libs/utility/value_init.htm. Which goes through all this initializer stuff, and made my head hurt.
I realize that, not for the first time, I am in a minority of one not liking any of this stuff. I was getting to quite like C++ over the past months, despite my misgivings of the past. Now I'm remembering just why I found it arcane [e.g. "C++ has at least four different initialization notations, some of which overlap."].. :(
You know nothing Jon B. Actually, C++ was decently complicated to begin with, and is getting more complicated by the day. So the problem is trivial: when is the old stuff going to be cleaned up and the new stuff simplified? To which I suppose the answer's 'never'. How's that for a cheery perspective?
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@Christian-Ehrlicher said in When to use class constructor member variable initializations?:
Correct
Indeed! Gosh, I did not know that, I thought it did work < C++11. Clearly my memory is not to be relied on.
I ended up reading https://www.boost.org/doc/libs/1_65_1/libs/utility/value_init.htm. Which goes through all this initializer stuff, and made my head hurt.
I realize that, not for the first time, I am in a minority of one not liking any of this stuff. I was getting to quite like C++ over the past months, despite my misgivings of the past. Now I'm remembering just why I found it arcane [e.g. "C++ has at least four different initialization notations, some of which overlap."].. :(
@JonB said in When to use class constructor member variable initializations?:
@Christian-Ehrlicher said in When to use class constructor member variable initializations?:
Correct
Indeed! Gosh, I did not know that, I thought it did work < C++11. Clearly my memory is not to be relied on.
It's possible that the compiler you used implemented this behavior as a non-standard extension. There were, and still are lots of cases of non-standard code having perfectly reasonable, compiler defined behavior.
Asking a question about code? http://eel.is/iso-c++/testcase/
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@JonB said in When to use class constructor member variable initializations?:
@Christian-Ehrlicher said in When to use class constructor member variable initializations?:
Correct
Indeed! Gosh, I did not know that, I thought it did work < C++11. Clearly my memory is not to be relied on.
It's possible that the compiler you used implemented this behavior as a non-standard extension. There were, and still are lots of cases of non-standard code having perfectly reasonable, compiler defined behavior.
@jeremy_k
In those days I --- or, to be accurate, much of the time my colleague, whose code I would have looked at --- would have been using Visual Studio/MSVC, say VS2010 plus its compiler. It's possible that might have allowed these initializations before the C++11 standard. Equally possible is that I just misremember ;-) -
I'd suggest that a class with many instance vars (traits) often indicates a design error and less than optimal use of subclassing. IOW, trying to do too much in a single scope. This is often the result of incremental development, without a clear architecture proposal before coding....not always, but often.
It's not critical to initialize vars by the initializer list, but do initialize them in proper order. I often defer trait initialization to a private Init() method because it looks cleaner. At least in recent c++ versions you can do nested constructors.
