what are differences between language array and std::array initialization?

Question

std::array supports aggregate initialization, but what is the problem here? If code (1) is used, both vc10.0 and g++ 4.7.2 complain that too many initializers. But if I use code (2) instead, everything is OK.

#include <array>

struct elem_t {  char c;  unsigned n;};

struct my_struct_t
{
  int i;
  // std::array<elem_t, 2> a; // (1) cause error
  // elem_t a[2]; // (2) ok
};

int main()
{
  std::array<int, 3> ai[] = {{1,2,3},{4,5,6}}; // ok

  my_struct_t var[] =
  {
    { 0, { {'a',1U}, {'b',2U}} }, // in question?
  };
}

Kerrek SB · Accepted Answer · 2013-02-22 14:41:29Z

1

Try extra braces -- you need one extra pair for the array itself:

my_struct_t var[] = { { 0, { { { 'a', 1 } } } }
                    , { 1, { { { 'c', 3 } } } }
};

//                  ^-  mystruct[]
//                     ^-  mystruct
//                         ^-  array
//                           ^-  elem_t[2]
//                             ^-  elem_t

The braces can be collapsed at the top level, but this may either be a situation where collapse isn't permitted, or compiler support may just not be there yet.

edited Feb 22, 2013 at 14:41

answered Feb 22, 2013 at 14:24

Kerrek SB

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7 Comments

jgx Over a year ago

Thanks for your quick reply. But this code will not be compiled if I use (1) instead of (2), gcc complains that "错误：类型‘char’的标量初始化带花括号".

jgx Over a year ago

sorry, I mean that "this code will not be compiled if I use (2) instead of (1) ...".

jgx Over a year ago

gcc complains "error: initialization 'char' type scalar with brace".

jgx Over a year ago

Is there any way to make the line marked with "in question" correct under both in case (1) and (2) ?

Johannes Schaub - litb Over a year ago

sadly, these braces depend on implementation details of std::array . An implementation of std::array is not required to have a 2-depth aggregation.

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