Is it possible to prevent omission of aggregate initialization members?

Question

I have a struct with many members of the same type, like this

struct VariablePointers {
   VariablePtr active;
   VariablePtr wasactive;
   VariablePtr filename;         


        

Answer 1

Here is a trick which triggers a linker error if a required initializer is missing:

struct init_required_t {
    template 
    operator T() const; // Left undefined
} static const init_required;

Usage:

struct Foo {
    int bar = init_required;
};

int main() {
    Foo f;
}

Outcome:

/tmp/ccxwN7Pn.o: In function `Foo::Foo()':
prog.cc:(.text._ZN3FooC2Ev[_ZN3FooC5Ev]+0x12): undefined reference to `init_required_t::operator int() const'
collect2: error: ld returned 1 exit status

Caveats:

• Prior to C++14, this prevents Foo from being an aggregate at all.
• This technically relies on undefined behaviour (ODR violation), but should work on any sane platform.