Initialize a constant sized array in an initializer list

Question

I've got a situation which can be summarized in the following:

class Test { Test(); int MySet[10]; };

is it possible to initialize MySet in an initializer list?

Like this kind of initializer list:

Test::Test() : MySet({1, 2, 3, 4, 5, 6, 7, 8, 9, 10}) {}

Is there any way to initialize a constant-sized member array in a class's initalizer list?

For what it's worth, Set isn't just a pointer to an array of 10 integers, nor is it static here. Rather, the array name Set decays to a pointer to the first element of the array in certain situations. The difference can be seen clearly using sizeof - i.e. sizeof(Set) == 10 * sizeof(int) != sizeof(int*). — Stuart Golodetz
– Stuart Golodetz, Commented Aug 6, 2012 at 22:59
@StuartGolodetz Thanks for the clarification. When I said static, I meant static in the form that it is stored with the object/instance, and not just somewhere else in the heap. Of course, that's a gross misuse of the term static on my part; sorry. — Serge
– Serge, Commented Aug 6, 2012 at 23:00
No worries :) I was mainly just trying to clarify the distinction between arrays and pointers on the off-chance that there might have been a misunderstanding there. — Stuart Golodetz
– Stuart Golodetz, Commented Aug 6, 2012 at 23:53

obataku · Accepted Answer · 2012-08-06 23:41:36Z

While not available in C++03, C++11 introduces extended initializer lists. You can indeed do it if using a compiler compliant with the C++11 standard.

struct Test { Test() : set { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } { }; int set[10]; };

The above code compiles fine using g++ -std=c++0x -c test.cc.

As pointed out below me by a helpful user in the comments, this code does not compile using Microsoft's VC++ compiler, cl. Perhaps someone can tell me if the equivalent using std::array will?

#include <array> struct Test { Test() : set { { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } } { }; std::array<int, 10> set; };

This also compiles fine using g++ -std=c++0x -c test.cc.

Another good reason to love C++11 (or some parts of it)! =D Thank you so much!
Note that this code does not compile with Visual C++ version 11 and earlier (i.e., it does not compile with Microsoft Visual Studio 2012, and Visual C++ is the main compiler for the most common platform). So, if you want portable code, don't do this. Yet.
Thank you @Cheersandhth.-Alf for I'm not a C++ programmer and wouldn't have known that. :)
@Serge you should make sure you heed the above advice in case you want portable code. Look into using std::array<int, 10> as an alternative which hopefully should work in Visual C++ 11 as well.

Community · Accepted Answer · 2017-05-23 10:30:59Z

6

Unfortunately, in C++03, you cannot initialize arrays in initializer lists. You can in C++11 though if your compiler is newer :)

see: How do I initialize a member array with an initializer_list?

edited May 23, 2017 at 10:30

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answered Aug 6, 2012 at 23:00

John Humphreys

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1 Comment

Cheers and hth. - Alf Over a year ago

Even C++98 supported zero-initialization of arrays in initializer lists.

Cheers and hth. - Alf · Accepted Answer · 2012-08-06 23:07:33Z

"I understand that Set is just a pointer to the static array of 10 integers"

No, that's wrong: it's an array, not a pointer.

You can still initialize it in the constructor's initializer list.

For a compiler that doesn't support C++11 curly braces initialization (Visual C++ version 11 and earlier comes to mind) you'll have to jump through some hoops though, as shown below:

#include <iostream> #include <vector> using namespace std; #define CPP11 #if defined( _MSC_VER ) # if (_MSC_VER <= 1700) # undef CPP11 # endif #endif #ifdef CPP11 class Cpp11 { private: int set_[10]; public: Cpp11() : set_{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 } {} int foo() const { return set_[3]; } }; #endif class Cpp03 { private: struct IntArray10 { int values[10]; }; IntArray10 set_; static IntArray10 const& oneToTen() { static IntArray10 const values = { {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} }; return values; } public: Cpp03() : set_( oneToTen() ) {} int foo() const { return set_.values[3]; } }; int main() {}

Instead of using raw arrays, though, use std::vector and C+++11 std::array, both of which are supported even by Visual C++ 11.

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