c++ - C++11 class that can only be implicitly constructed? -

is there way make type has 0 size , can constructed implicitly?

the use case prevent public members of struct being initialized via brace syntax:

class barrier { ... };   struct foo {   int user_sets;   int* this_to;    barrier _bar;    int *must_be_zero_init_by_linker; };  foo foo = {1};               // ok foo bar = {1, nullptr};      // ok foo baz = {1, nullptr, {}};  // must error 

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edit: 1 other constraint: foo object must linker initialized can't define constructors or private members.

you define own constructor; prevents class being aggregate. example:

struct foo {   foo(int = 0, int * p = nullptr) constexpr   : user_sets(a), this_to(p), must_be(nullptr)   {}    int user_sets;   int* this_to;    int *must_be; };  foo foo = { 1 };                 // ok foo bar = { 1, nullptr };        // ok // foo baz = { 1, nullptr, {} }; // error 

in fact, recommend making constructor explicit - can't use copy-initialization, can still use list-initialization:

explicit foo(int = 0, int * p = nullptr) constexpr /* ... */  foo foo { 1 };                   // ok foo bar { 1, nullptr };          // ok // foo baz { 1, nullptr, {} };   // error