how to initialize std::array with std::initializer_list in C++0x?

danath · 09-12-2009, 11:07 AM

Hi,

I am studying C++0x with gcc 4.4.0 these days.

Here I need to initialize a std::array object within a function which has a parameter of type std::initializer_list, as below:

Code:

template<typename T, int S>
void Foo(std::initializer_list<T> init_list)
{
    std::array<T, S> my_array = ****
}

What should I do in the place with '****' to make my_array initialized?

T is a type which needs explicit construction, which means I can not construct my_array as default, then copy every element from init_list to it.

The type & size of init_list can be known during compiling time, so I believe there should be some way to make use of them.

Can anyone give me some advice?

Thanks in advance.

anon · 09-12-2009, 12:11 PM

I haven't been able to figure that one out. initializer_list doesn't have anything but begin and end methods, but array doesn't have a constructor that can do anything with them.

Perhaps pass an array<T, S> instead of the initialization list:

Code:

template<typename T, unsigned S>
void Foo(std::array<T, S> my_array)
{
    ///...
}

Foo(std::array<int, 3u>{{1, 2, 3}});

danath · 09-12-2009, 08:48 PM

Thanks, anon.

Your suggestion should be workable, but when the compiler sees an ini list ({...}), it will store it into an object of std::initializer_list, so to change this behavior the client programmer needs to explicitly construct std::array at the place the calling happens -- this will obviously make the code ugly.

I may have to stick to what I did before, boost preprocessor...

BTW, I have another question, some articles mention the new way to init built-in array which is a member:

Code:

class C
{
        int n[5];
    public: 
        C(): n{0,1,2,3,4} {}//C++09 only
};

But seems g++4.4.0 does not support it. Where can I find more info about the new feature, the proposal?

Thanks

Elysia · 09-13-2009, 03:58 AM

Wikipedia has a huge amount of information. Perhaps it's a good starting point?

anon · 09-13-2009, 04:41 AM

Yet another thing you could do, perhaps, is not to use an array but just use the data in the initializer list (I think that's read-only only, though).

Quote:

BTW, I have another question, some articles mention the new way to init built-in array which is a member

This appears to work with GCC 4.4.1. It seems that currently all compiler versions implement their own subset of the standard and some things are changing (e.g 4.4.1 lets lvalues bind to rvalue references).

**CornedBee** · 09-14-2009, 08:20 AM

In C++0x, this should work:

Code:

template <size_t N, typename T>
void fn(std::initializer_list<T> init)
{
  std::array<T, N> ar(init); // or ar{init}
}

fn<3>({1, 2, 3});

If GCC doesn't support this, it simply means that either the initializer list support isn't there yet, or the implementation of std::array isn't.

Similarly for the array initialization example. It should work in C++0x, but GCC isn't a complete implementation of C++0x yet.

danath · 09-14-2009, 10:00 PM

Quote:

Originally Posted by anon

Yet another thing you could do, perhaps, is not to use an array but just use the data in the initializer list (I think that's read-only only, though).

This appears to work with GCC 4.4.1. It seems that currently all compiler versions implement their own subset of the standard and some things are changing (e.g 4.4.1 lets lvalues bind to rvalue references).

yes, I just built gcc 4.4.1 (quite smooth to build it with gcc 4.4.0, but toooo long time) and it worked to compile above syntax, good news to me.

danath · 09-14-2009, 10:02 PM

Quote:

Originally Posted by CornedBee

In C++0x, this should work:

Code:

template <size_t N, typename T>
void fn(std::initializer_list<T> init)
{
  std::array<T, N> ar(init); // or ar{init}
}

fn<3>({1, 2, 3});

If GCC doesn't support this, it simply means that either the initializer list support isn't there yet, or the implementation of std::array isn't.

Similarly for the array initialization example. It should work in C++0x, but GCC isn't a complete implementation of C++0x yet.

Thanks. Another good news.

Do you know which compiler is supporting this?

**CornedBee** · 09-15-2009, 03:14 AM

No. If GCC 4.4.1 doesn't, I suspect that none does so far. I know VC++ 2010 doesn't support initializer lists, and Comeau online doesn't provide the <initializer_list> header. CodeGear C++Builder 2010 doesn't support it according to its feature matrix, and I know Clang doesn't support it, so it doesn't look good.

09-12-2009, 11:07 AM	#1
danath Registered User Join Date: Sep 2009 Posts: 5	how to initialize std::array with std::initializer_list in C++0x? Hi, I am studying C++0x with gcc 4.4.0 these days. Here I need to initialize a std::array object within a function which has a parameter of type std::initializer_list, as below: Code: template<typename T, int S> void Foo(std::initializer_list<T> init_list) { std::array<T, S> my_array = ** } What should I do in the place with '**' to make my_array initialized? T is a type which needs explicit construction, which means I can not construct my_array as default, then copy every element from init_list to it. The type & size of init_list can be known during compiling time, so I believe there should be some way to make use of them. Can anyone give me some advice? Thanks in advance.
danath is offline

09-12-2009, 08:48 PM	#3
danath Registered User Join Date: Sep 2009 Posts: 5	Thanks, anon. Your suggestion should be workable, but when the compiler sees an ini list ({...}), it will store it into an object of std::initializer_list, so to change this behavior the client programmer needs to explicitly construct std::array at the place the calling happens -- this will obviously make the code ugly. I may have to stick to what I did before, boost preprocessor... BTW, I have another question, some articles mention the new way to init built-in array which is a member: Code: class C { int n[5]; public: C(): n{0,1,2,3,4} {}//C++09 only }; But seems g++4.4.0 does not support it. Where can I find more info about the new feature, the proposal? Thanks Last edited by danath; 09-13-2009 at 07:03 AM. Reason: typo
danath is offline

09-14-2009, 08:20 AM	#6
CornedBee Cat without Hat Join Date: Apr 2003 Posts: 8,439	In C++0x, this should work: Code: template <size_t N, typename T> void fn(std::initializer_list<T> init) { std::array<T, N> ar(init); // or ar{init} } fn<3>({1, 2, 3}); If GCC doesn't support this, it simply means that either the initializer list support isn't there yet, or the implementation of std::array isn't. Similarly for the array initialization example. It should work in C++0x, but GCC isn't a complete implementation of C++0x yet. __________________ All the buzzt! CornedBee "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code." - Flon's Law
CornedBee is offline

09-15-2009, 03:14 AM	#9
CornedBee Cat without Hat Join Date: Apr 2003 Posts: 8,439	No. If GCC 4.4.1 doesn't, I suspect that none does so far. I know VC++ 2010 doesn't support initializer lists, and Comeau online doesn't provide the <initializer_list> header. CodeGear C++Builder 2010 doesn't support it according to its feature matrix, and I know Clang doesn't support it, so it doesn't look good. __________________ All the buzzt! CornedBee "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code." - Flon's Law
CornedBee is offline

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
How to initialize a private static const array of a private child class in a class?	ryanli	C++ Programming	3	04-10-2009 02:20 AM
how to initialize char *	vignesh	C Programming	3	03-30-2009 01:17 AM
How do you use variable to initialize array size?	yougene	C Programming	11	09-04-2007 02:50 PM
initialize constant and initialization list	sawer	C++ Programming	5	07-09-2006 06:30 AM
using constructor to initialize data	Unregistered	C++ Programming	6	03-19-2002 08:55 PM