[英]C++ register pattern with static member works “sometimes”
我正在嘗試在運行時實現自動類注冊(我認為該技術采用“注冊模式”的名稱)。 在下面的示例中,我將int
存儲在靜態成員向量中,但目標是存儲可以在以后調用的函數指針。
我的理解是,由於下面代碼中的registeredClasses
成員被聲明為static
,因此在整個程序中應該只有一個實例。 但是,事實並非如此:
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,為簡潔起見省略)
#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,為簡潔起見省略)
#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";
}
我的問題是它只能“有時”工作。 初始輸出:
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
我刪除了class1.cpp和class2.cpp,在頭文件中聲明了構造函數,例如。 Class1() {}
。 注冊根本不起作用:
//no output here
registering classes works...
//...
然后我恢復了更改,突然得到了預期的輸出:
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
我一直在研究此問題,但未能了解原因。 我的猜測是,它與編譯順序(但是注冊如何工作?)或鏈接 / 存儲持續時間有關 。 有人可以闡明這一點嗎?
我認為您這里需要的技巧是在ClassRegister
而不是靜態數據成員中,請使用具有靜態局部函數。 有時稱為單例,盡管我不知道這是否真的是正確的術語。
代碼可能看起來像:
typedef std::vector<int> register_type;
register_type& getRegister() {
static register_type registeredClasses; //declaration
return registeredClasses;
}
在實現自動類注冊方案時,應注意靜態初始化順序,多線程。
刪除.cpp文件時,該文件不起作用的原因是,由於未編譯頭文件,因此沒有人知道有任何聲明。
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