为什么要在 C++ 中初始化静态类变量？
在 C 和 C++ 中，所有静态变量都默认初始化为零。
这不是静态类数据成员的情况。这是为什么？
#include <iostream>
using namespace std;
int var;
class MyClass
{
public:
static int classVar;
};
int MyClass::classVar = 0; // Why I have to init it here?
int main(void)
{
cout << ::var << endl; // this is initalized to ZERO by default
static int var;
cout << var << endl; // and this also is initalized to Zero
cout << MyClass::classVar << endl;
return 0;
}
In C and C++ all static variables are initialized by default to ZERO.
This is not the case of static class data members. Why is that?
#include <iostream>
using namespace std;
int var;
class MyClass
{
public:
static int classVar;
};
int MyClass::classVar = 0; // Why I have to init it here?
int main(void)
{
cout << ::var << endl; // this is initalized to ZERO by default
static int var;
cout << var << endl; // and this also is initalized to Zero
cout << MyClass::classVar << endl;
return 0;
}
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在类范围内，
是一个定义，并且
是一个声明，即。承诺变量将在某处定义：您必须恰好定义一次您声明的变量。
理由是类声明可能会包含在多个源文件中。如果它的一部分是定义，它将发生乘法：这是错误的（例外是内联[成员]函数）。
注意，根据值初始化规则，您可以将 和
作为定义。
在命名空间范围内声明的变量也是定义（除非它们是
extern限定的）：是一个声明，也是一个定义：如果将其放入标头并将其包含在多个翻译单元中，则会出现错误（“多重定义的符号”，或类似的东西）。
[注意，在 C++ 中（而不是 C），如果上面的
var是const，它会自动变为static并且有如果将其放入多重包含的标头中，则不会违反单一定义规则。这有点偏离主题，但请随意询问详细信息]At class scope,
is a definition and
is a declaration, ie. a promise the variable will be defined somewhere: you must define exactly once the variables you declare.
The rationale is that the class declaration will likely be included in multiple source files. Would a part of it be a definition, it would take place multiply: this is erroneous (exceptions are inline [member] functions).
Note that according to value initialization rules, you can get along with
as a definition.
Variables declared at namespace scope are definitions too (unless they are
externqualified):is a declaration, and a definition: if you put this into a header and include it in multiple translation units, you have an error ("multiply defined symbol", or something along those lines).
[Note that in C++ (and not in C), if the
varabove isconst, it becomes automaticallystaticand there is no violation of the One Definition Rule should it be put into a multiply included header. This goes slightly off topic, but feel free to ask details]C++ FAQ 10.12 指出：
来自 C++ 常见问题解答 http://www.parashift.com/c++- faq-lite/ctors.html#faq-10.12
这是否回答了您的问题，或者您是否在参考 C++ 标准本身？
C++ FAQ 10.12 states that:
From C++ FAQ http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.12
Does that answer your question or were you after a reference to the C++ standard itself?
您必须初始化静态类数据变量，因为您必须告诉编译器它们的值是什么。类不需要有默认值的概念。
变量类型有一个逻辑“零值”，对于
int来说是0，对于double来说是0.0，对于string来说是“”等。相反，类不一定有默认值。例如，考虑类 Rectangle。它的零值是多少——正方形为零的矩形还是边长为单位的矩形？对于静态变量，编译器要求您自己定义静态变量必须具有什么值，因为并非每种数据类型都可以用默认值初始化。You have to initialize your static class data variables, because you have to tell the compiler what their value is. Classes need not have a notion of a default value.
Variables types have a logical "zero value", for
intit is 0, fordouble0.0, for astring"" etc. In contrast, classes do not necessarily have a default value. Consider, for exampleclass Rectangle. What is its zero value - a rectangle with zero square or a rectangle with unit side length? For static variables, a compiler asks you to define yourself, what value your static variable must have, because not every data type can be initialized by a default value.