Initializing a struct with aggregate initialization and member initializers [duplicate]

Answer 1

Bjarne Stroustrup and Richard Smith raised an issue about aggregate initialization and member-initializers not working together.

The definition of aggregate is slightly changed in C++11 & C++14 standard.

From the C++11 standard draft n3337 section 8.5.1 says that:

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no brace-or-equal- initializers for non-static data members (9.2), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

But C++14 standard draft n3797 section 8.5.1 says that:

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).

So, when you use in class member initializer (i.e. equal initializer) for the data member id in C++11 it no longer remains aggregate & you can't write ABC abc{"hi", 0}; to initialize a struct ABC. Because it no longer remains aggregate type after that. But your code is valid in C++14. (See live demo here).

Answer 2

In c++ struct and classes are the same, except that structs have default public members and classes have private. If you want to use initial values I think you have to write a constructor or use something like this:

struct ABC
{
    std::string str;
    unsigned int id;
} ABC_default = {"init", 0 }; //initial values

int main()
{
    ABC abc = ABC_default;
    std::cout << abc.str << " " << abc.id << std::endl;
    return 0;
}