C ++ Struct无法进行POD测试

Question

I am writing code for a microcontroller, and need to be sure my struct is a POD. I thought I can use a "member initializer lists" to initialize members, but this does not pass the "is_pod" test. In example below, A is a POD, B and C are not. Adding "D() = default;" to D seems to make it a POD. But, by doing this, I can not longer have a "member initializer lists"? Is there a way for a structure to be a POD, and have a "member initializer lists"?

#include <iostream>
#include <string>

struct A {
    int var1;
};

struct B {
    int var1;
    //B() = default;
    B() : var1(100) {}
};

struct C {
    int bar [10];
    C() : bar{0} {}
};

struct D {
    int var1;
    D() = default;
    //D(int x) : var1(x) {}
};


int main()
{
    std::cout << std::boolalpha;
    std::cout << "\nIs A a POD = " << std::is_pod<A>::value;
    std::cout << "\nIs B a POD = " << std::is_pod<B>::value;
    std::cout << "\nIs C a POD = " << std::is_pod<C>::value;
    std::cout << "\nIs tD a POD = " << std::is_pod<D>::value;
}

=== Update 1 ===
Thanks for replies! So, seems like there is no way to initialize the member variables in the structure definition. The following works, but is not as elegant as having the initialization in the struct itself.

typedef struct A_ {
int var1;
} A;

A a = {
    .var1 = 100
};

Answer 1

POD must be a trivial type , this requires:

Has one or more default constructors , all of which are either trivial or deleted, and at least one of which is not deleted.

And a trivial constructor has a description of performing no action . So " having the initialization in the struct itself " is explicitly against it.

The hard requirements of your interest are:

• The constructor is not user-provided (ie, is implicitly-defined or defaulted on its first declaration)
• Every non-static member of class type has a trivial default constructor is that it has to be default, all all members constructors have

You are bound to use default constructor for var1 within the definition of the constructor. Ie var1(100) is "no bueno". ( Side note: you could use a member ctor but only in a default form eg var1() , which is the same as omitting it ).

Please note that there is a major difference in using aggregate initialization in declaration of the object, vs setting it as a default. It takes some work to set those variables, and a default creation of a POD "can't have none".