C++ proper way to inline initialize member variables

问题

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?

回答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.

回答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

来源：https://stackoverflow.com/questions/56271358/c-proper-way-to-inline-initialize-member-variables

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c++

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struct

initialization

member-variables