这个问题是在面试中问我的:
什么是静态构造函数?
它在C++中存在吗?如果存在,请用一个例子来解释它。
这个问题是在面试中问我的:
什么是静态构造函数?
它在C++中存在吗?如果存在,请用一个例子来解释它。
C++没有静态构造函数,但是您可以使用嵌套类的静态实例来模拟它们。
class has_static_constructor {
friend class constructor;
struct constructor {
constructor() { /* do some constructing here … */ }
};
static constructor cons;
};
// C++ needs to define static members externally.
has_static_constructor::constructor has_static_constructor::cons;
class constructor
应该是class has_static_constructor
的友元才能有用?否则它只是另一个静态成员。 - davkaunique_ptr
解决这个问题,它指向一个嵌套类,该类将所有“静态”成员作为实际上的非静态成员,并且在初始化为 0 并首次访问时重置为有效指针。 - Konrad RudolphC++中没有静态构造函数。在C#中(可能在Java中也是如此),您可以定义静态构造函数,它会被运行时自动调用以初始化静态成员。
如果您有进一步的问题和兴趣,可以阅读此主题:
由于在C++中技术上不存在静态构造函数,您必须决定是否值得做一些花招来强制执行(例如使用嵌套类的静态实例),或者只需稍微重新组织代码,在程序运行早期调用静态初始化器。
#include <iostream> // cout, endl
class Foo {
public:
static int s_count;
// Constructor definition
Foo (int l, int w, int h)
{
cout <<"Foo ctor called." << endl;
length = l;
width = w;
height = h;
// Increase every time object is created
s_count++;
}
int vol ()
{
return length * width * height;
}
static void initCount()
{
s_count = 0;
}
static int getCount()
{
return s_count;
}
private:
double length; // Length of a box
double width; // Width of a box
double height; // Height of a box
};
// Initialize static member of class Foo
int Foo::s_count; // Initializing here is non-deterministic
int main(void) {
Foo::initCount(); // Initializing here is deterministic
// Print total number of objects before creating object.
cout << "Inital Count: " << Foo::getCount() << endl;
Foo Foo1(3, 1, 1); // Declare box1
Foo Foo2(8, 6, 2); // Declare box2
// Print total number of objects after creating object.
cout << "Final Count: " << Foo::getCount() << endl;
return 0;
}
Output:
$ static_init_test
Inital Count: 0
Foo ctor called.
Foo ctor called.
Final Count: 2
我更喜欢这种方法;好的一面是,它使非确定性初始化变得可行。
但是还有一点需要注意 -- 如果你需要初始化静态常量变量,这种技术是不够的。对于静态常量变量,你必须将它们设置为类中的私有成员,并提供 getter 方法让外部读取。
注意:我更新了这段代码 -- 它通过以下方式编译和成功运行,没有出现警告:
g++ static_init_test.cpp -std=c++11 -o static_init_test
在C#和Java中存在静态构造函数。
它们用于初始化类的静态成员。
运行时会在类首次使用之前执行它们。
静态构造函数用于初始化类的静态数据。C++没有静态构造函数。但是可以通过使用友元类或嵌套类来模拟静态构造函数,如下所示。
class ClassStatic{
private:
static char *str;
public:
char* get_str() { return str; }
void set_str(char *s) { str = s; }
// A nested class, which used as static constructor
static class ClassInit{
public:
ClassInit(int size){
// Static constructor definition
str = new char[size];
str = "How are you?";
}
} initializer;
};
// Static variable creation
char* ClassStatic::str;
// Static constructor call
ClassStatic::ClassInit ClassStatic::initializer(20);
int main() {
ClassStatic a;
ClassStatic b;
std::cout << "String in a: " << a.get_str() << std::endl;
std::cout << "String in b: " << b.get_str() << std::endl;
a.set_str("I am fine");
std::cout << "String in a: " << a.get_str() << std::endl;
std::cout << "String in b: " << b.get_str() << std::endl;
std::cin.ignore();
}
输出:
String in a: How are you?
String in b: How are you?
String in a: I am fine
String in b: I am fine
class Cat
{
private:
Cat();
public:
static Cat getCat() {return Cat(); }
}
public class Bus {
// Static variable used by all Bus instances.
// Represents the time the first bus of the day starts its route.
protected static readonly DateTime globalStartTime;
// Property for the number of each bus.
protected int RouteNumber { get; set; }
// Static constructor to initialize the static variable.
// It is invoked before the first instance constructor is run.
static Bus()
{
globalStartTime = DateTime.Now;
// The following statement produces the first line of output,
// and the line occurs only once.
Console.WriteLine("Static constructor sets global start time to {0}",
globalStartTime.ToLongTimeString());
}
// Instance constructor.
public Bus(int routeNum)
{
RouteNumber = routeNum;
Console.WriteLine("Bus #{0} is created.", RouteNumber);
}
// Instance method.
public void Drive()
{
TimeSpan elapsedTime = DateTime.Now - globalStartTime;
// For demonstration purposes we treat milliseconds as minutes to simulate
// actual bus times. Do not do this in your actual bus schedule program!
Console.WriteLine("{0} is starting its route {1:N2} minutes after global start time {2}.",
this.RouteNumber,
elapsedTime.Milliseconds,
globalStartTime.ToShortTimeString());
} }
class TestBus {
static void Main()
{
// The creation of this instance activates the static constructor.
Bus bus1 = new Bus(71);
// Create a second bus.
Bus bus2 = new Bus(72);
// Send bus1 on its way.
bus1.Drive();
// Wait for bus2 to warm up.
System.Threading.Thread.Sleep(25);
// Send bus2 on its way.
bus2.Drive();
// Keep the console window open in debug mode.
System.Console.WriteLine("Press any key to exit.");
System.Console.ReadKey();
} } /* Sample output:
Static constructor sets global start time to 3:57:08 PM.
Bus #71 is created.
Bus #72 is created.
71 is starting its route 6.00 minutes after global start time 3:57 PM.
72 is starting its route 31.00 minutes after global start time 3:57 PM.
*/
以下是C++17中最简单的静态构造函数:
#include <iostream>
class h {
public:
static inline int i;
private:
struct constructor {
constructor() {
i=3;
}
};
static inline constructor cons;
};
int main() {
std::cout << h::i;
}