Question

As an addendum to this question, what is going on here:

#include <string>
using namespace std;

struct A {
    string s;
};

int main() {
    A a = {0};
}

Obviously, you can’t set a std::string to zero. Can someone provide an explanation (backed with references to the C++ Standard, please) about what is actually supposed to happen here? And then explain for example):

int main() {
    A a = {42};
}

Are either of these well-defined?

Once again an embarrassing question for me – I always give my structs constructors, so the issue has never arisen before.

Answer 1

Your struct is an aggregate, so the ordinary rules for aggregate initialization work for it. The process is described in 8.5.1. Basically the whole 8.5.1 is dedicated to it, so I don’t see the reason to copy the whole thing here. The general idea is virtually the same it was in C, just adapted to C++: you take an initializer from the right, you take a member from the left and you initialize the member with that initializer. According to 8.5/12, this shall be a copy-initialization.

When you do

A a = { 0 };

you are basically copy-initializing a.s with 0, i.e. for a.s it is semantically equivalent to

string s = 0;

The above compiles because std::string is convertible from a const char * pointer. (And it is undefined behavior, since null pointer is not a valid argument in this case.)

Your 42 version will not compile for the very same reason the

string s = 42;

will not compile. 42 is not a null pointer constant, and std::string has no means for conversion from int type.

P.S. Just in case: note that the definition of aggregate in C++ is not recursive (as opposed to the definition of POD, for example). std::string is not an aggregate, but it doesn’t change anything for your A. A is still an aggregate.