C ++内联初始化成员变量的正确方法 [英] C++ proper way to inline initialize member variables

问题描述

给出示例代码：

  struct S {
 char data [5]; 
 int a; 
}; 
  

在Microsoft Visual Studio中运行运行代码分析时，它会警告初始化所有变量。 / p>

现在，我知道您可以通过多种方式执行此操作，创建默认的构造函数，例如：

  S（）：
数据{0}，
a {0} {
} 
  

这使警告消失了。但是，如果您不想手动创建默认的构造函数，该怎么办。

类似这样的东西：

  struct S {
 char data [5]; 
 int a = 0; 
}; 
  

摆脱了 a 的警告，但不是 data ，尽管您可以通过以下方式添加 {} 来解决此问题： char data [5] {}; 这似乎使代码分析感到高兴。

这让我开始思考，您也可以初始化 a 像 int a {0};

所以我的问题是，

侧面说明：我注意到 std :: array 的 _Ty _Elems [_Size]; ，它从不初始化任何地方，也没有 {} ，我假设他们只是忽略此警告？还是他们在做我不注意修复警告的事情？

还想添加以下代码：
#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]; 
} 
  

在 gcc9.1 -std = c ++下17 -Wall 不产生警告，
但是 clang8.0 -std = c ++ 17下的相同代码17 -Wall 给我：

 警告：建议在子对象初始化周围使用括号[-Wmissing-braces] 
类型名称std :: aligned_storage< sizeof （T），alignof（T）> :: type data [N] = {0}; 
 ^ 
 {} 
  

我看到可以将其设置为 = {}; 可以解决此问题，只是想知道为什么一个编译器会在另一个不生成警告时发出警告？

解决方案

CPPCoreGuidelines 关于此状态：不要定义仅初始化数据成员的默认构造函数；而是使用类内成员初始化程序

所以您可以这样做：

  struct S {
 char data [5] = {0}; 
 int a = 0; 
}; 
  

关于您与 std相关的缺少警告的其他问题：数组，GCC的注释中指出：

通常禁止来自系统标题的警告，前提是它们通常并不表示真正的问题，只会使编译器的输出更难阅读。

我相信MSVC确实如此。好吧。

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?

解决方案

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.

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