[英]C++ register pattern with static member works “sometimes”
我正在尝试在运行时实现自动类注册(我认为该技术采用“注册模式”的名称)。 在下面的示例中,我将int
存储在静态成员向量中,但目标是存储可以在以后调用的函数指针。
我的理解是,由于下面代码中的registeredClasses
成员被声明为static
,因此在整个程序中应该只有一个实例。 但是,事实并非如此:
classregister.h
#ifndef CLASSREGISTER_H
#define CLASSREGISTER_H
#include <vector>
#include <iostream>
class ClassRegister
{
public:
static std::vector<int> registeredClasses; //declaration
ClassRegister(int id) {
std::cerr << "registering " << id << " ";
registeredClasses.push_back(id);
std::cerr << "new size: " << registeredClasses.size() << "\n";
}
//code below just for testing
ClassRegister() {}
static int getSize() {
return registeredClasses.size();
}
};
#define REGISTER_CLASS(cls) \
static ClassRegister myClass_##cls(1);
#endif // CLASSREGISTER_H
classregister.cpp
#include "classregister.h"
std::vector<int> ClassRegister::registeredClasses; //definition
class1.h(class2.h遵循相同的模式,为简洁起见省略)
#ifndef CLASS1_H
#define CLASS1_H
#include "classregister.h"
class Class1
{
public:
Class1();
};
REGISTER_CLASS(Class1)
#endif // CLASS1_H
class1.cpp(class2.cpp遵循相同的模式,为简洁起见省略)
#include "class1.h"
Class1::Class1()
{
}
main.cpp中
#include "classregister.h"
#include <iostream>
int main(int argc, char *argv[])
{
std::cerr << "registering classes works...\n\n";
std::cerr << "but reading from main() does not...\n";
std::cerr << "checking size directly: " << ClassRegister::registeredClasses.size() << "\n";
std::cerr << "checking size through function: " << ClassRegister::getSize() << "\n";
std::cerr << "checking size on instance: " << ClassRegister().getSize() << "\n";
std::cerr << "\n";
std::cerr << "registration from main() uses a different register...\n";
ClassRegister(2);
ClassRegister(3);
ClassRegister(4);
std::cerr << "\n";
std::cerr << "...which is not the one used in the header files\n";
std::cerr << "checking size directly: " << ClassRegister::registeredClasses.size() << "\n";
std::cerr << "checking size on instance: " << ClassRegister().getSize() << "\n";
std::cerr << "checking size through function: " << ClassRegister::getSize() << "\n";
std::cerr << "\n";
}
我的问题是它只能“有时”工作。 初始输出:
registering 1 new size: 1
registering 1 new size: 2
registering classes works...
but reading from main() does not...
checking size directly: 0
checking size through function: 0
checking size on instance: 0
registration from main() uses a different register...
registering 2 new size: 1
registering 3 new size: 2
registering 4 new size: 3
...which is not the one used in the header files
checking size directly: 3
checking size on instance: 3
checking size through function: 3
我删除了class1.cpp和class2.cpp,在头文件中声明了构造函数,例如。 Class1() {}
。 注册根本不起作用:
//no output here
registering classes works...
//...
然后我恢复了更改,突然得到了预期的输出:
registering 1 new size: 1
registering 1 new size: 2
registering classes works...
but reading from main() does not... //it does work this time
checking size directly: 2
checking size through function: 2
checking size on instance: 2
registration from main() uses a different register...
registering 2 new size: 3
registering 3 new size: 4
registering 4 new size: 5
...which is not the one used in the header files
checking size directly: 5
checking size on instance: 5
checking size through function: 5
我一直在研究此问题,但未能了解原因。 我的猜测是,它与编译顺序(但是注册如何工作?)或链接 / 存储持续时间有关 。 有人可以阐明这一点吗?
我认为您这里需要的技巧是在ClassRegister
而不是静态数据成员中,请使用具有静态局部函数。 有时称为单例,尽管我不知道这是否真的是正确的术语。
代码可能看起来像:
typedef std::vector<int> register_type;
register_type& getRegister() {
static register_type registeredClasses; //declaration
return registeredClasses;
}
在实现自动类注册方案时,应注意静态初始化顺序,多线程。
删除.cpp文件时,该文件不起作用的原因是,由于未编译头文件,因此没有人知道有任何声明。
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