c++ - Will {0, 0} initialize array in the struct? -

in code, 100 items of c.b initialized zero?

struct { int b[100]; int d; }; c = {0, 0}; 

it seems work, memory have been empty in advance.

the line

a c = {0, 0};  

performs value initialization of aggregate a. according standard, braces can omitted aggregate initialization:

8.5.1 aggregates[dcl.init.aggr]/12

braces can elided in initializer-list follows. if initializer-list begins left brace, succeeding comma-separated list of initializer-clauses initializes members of subaggregate; erroneous there more initializer-clauses members. if, however, initializer-list sub- aggregate not begin left brace, enough initializer-clauses list taken initialize members of subaggregate; remaining initializer-clauses left initialize next member of aggregate of current subaggregate member.

[example:

 float y[4][3] = {    { 1, 3, 5 },    { 2, 4, 6 },    { 3, 5, 7 }, };  

is completely-braced initialization: 1, 3, , 5 initialize first row of array y[0], namely y[0][0], y[0][1], , y[0][2]. likewise next 2 lines initialize y[1] , y[2]. initializer ends , therefore y[3]'s elements initialized if explicitly initialized expression of form float(), is, initialized 0.0. in following example, braces in initializer-list elided; initializer-list has same effect completely-braced initializer-list of above example,

 float y[4][3] = {    1, 3, 5, 2, 4, 6, 3, 5, 7 };  

the initializer y begins left brace, 1 y[0] not, therefore 3 elements list used. likewise next 3 taken successively y[1] , y[2]. — end example ]

next

8.5.1 aggregates[dcl.init.aggr]/7

if there fewer initializer-clauses in list there members in aggregate, each member not explicitly initialized shall initialized brace-or-equal-initializer or, if there no brace-or-equal- initializer, empty initializer list.

in case, mean first 0 assigned b[0] , second 0 assigned b[1]. according 8.5.1/7, rest of elements value-initialized.

however, clarity in case, should use a c = {{0}, 0};, or, better

a c{}; // or c = {}; 

the thing worries me bit g++ warning (-wextra):

warning: missing initializer member 'main()::a::d' [-wmissing-field-initializers] c {0,0};

but according interpretation of standard above, should ok , d should have been initialized. tested placement new, , result expected

#include <iostream>  int main() {     struct { int b[100]; int d;};     memory{};     memory.d = 42;      std::cout << memory.d << std::endl;      // let's place @ location of memory     a* foo = new (&memory) a{0,0};      // line below outputs 0, d erased; not case if a* foo = new (&memory) a;      std::cout << memory.d << std::endl; // outputs 0 }