C++ proper way to inline initialize member variables - c++

Question

Given the example code:
struct S {
char data[5];
int a;
};
When running the "Run code analysis" in Microsoft Visual Studio, It warns to initialize all variables.
Now I know you can do this a number of ways, create a default constructor such as:
S() :
data{0},
a{0} {
}
That makes the warning go away. But what if you don't want to manually create the default constructor.
something like:
struct S {
char data[5];
int a = 0;
};
gets rid of the warning for a but not data, though you can fix that by adding {} after like so: char data[5]{}; this seems to make the code analysis happy.
That got me thinking, you can also initialize a like int a{0};
So my question is, are these all valid, and which is preferred?
Side note: I noticed std::array has _Ty _Elems[_Size]; which it never initializes anywhere, nor does it have {} after it, I'm assuming they just ignore this warning? Or are they doing something I'm not noticing to "fix" the warning?
Also wanted to add that this code:
#include
#include
template<class T, std::size_t N>
struct static_vector {
typename std::aligned_storage<sizeof(T), alignof(T)>::type data[N] = {0};
T& operator[](std::size_t pos) {
return *std::launder(reinterpret_cast<T*>(&data[pos]));
}
};
int main(int argc, char**) {
static_vector<int, 10> s;
s[0] = argc;
return s[0];
}
under gcc9.1 -std=c++17 -Wall produces no warnings,
yet the same code under clang8.0 -std=c++17 -Wall gives me:
warning: suggest braces around initialization of subobject [-Wmissing-braces]
typename std::aligned_storage<sizeof(T), alignof(T)>::type data[N] = {0};
^
{}
I see that I can set it to = {}; which fixes it, just wondering why one compiler would produce a warning when the other doesn't? Which one is to spec?

Answer 1

The guideline from CPPCoreGuidelines on this states: Don’t define a default constructor that only initializes data members; use in-class member initializers instead
So you can just do:
struct S {
char data[5] = {0};
int a = 0;
};
As to your other question about the lack of warning related to std::array, GCC has a note which states:
Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read.
I believe this would be true of MSVC as well.

Answer 2

In C++ for each declarator, the initializer may be one of the following:
1. ( expression-list )
2. = expression
3. { initializer-list }
The description for these are as follows:
comma-separated list of arbitrary expressions and braced-init-lists in parentheses
the equals sign followed by an expression
braced-init-list: possibly empty, comma-separated list of expressions and other braced-init-lists
Well which type of initialization to prefer actually depends upon context. To initialize data members in a class I personally prefer in class initialization using braced initializer, as in that case we don't have to write a user defined default constructor, compiler generated one is always efficient.
Class members
Non-static data members can be initialized with member initializer
list or with a default member initializer.
In your case you can probably use:
struct S {
char data[5] = {0}; //initialize by zero
int a = 0;
};
or to give them different values also:
struct S {
char data[5] = {0,1,2,3,4};
int a = 0;
};
For more info see Initialization