我有一个模板单例类,有一个静态实例和一个静态getInstance()方法。但是,我收到来自g ++(MinGW)的实例的未定义参考错误;起初,我认为这是链接器顺序错误,但无论我把它放在哪个顺序中,我都可以得到相同的结果。模板类的静态成员没有实例化,除非明确专用?
但是,我发现如果我明确地专门化实例,它会编译(这是“可接受的解决方案)。所以,这里是代码:
Singleton.hpp:
#ifndef SINGLETON_HPP
#define SINGLETON_HPP
#include <iostream>
template<int X=100>
class Singleton {
protected:
static Singleton<X> *instance;
public:
static Singleton<X>& getInstance() {
if(!Singleton<X>::instance) {
Singleton<X>::instance = new Singleton<X>();
}
return *Singleton<X>::instance;
}
void foo() {
std::cout << "Test<" << X << ")::foo()" << std::endl;
}
};
#endif
Singleton.cpp:
#include "Singleton.hpp"
template<int X>
Singleton<X>* Singleton<X>::instance = NULL;
#ifdef SPECIALIZE
template<>
Singleton<100>* Singleton<100>::instance = NULL;
#endif
Main.cpp的:
#include "Singleton.hpp"
int main(int ac, char *av[]) {
Singleton<100> &sing = Singleton<100>::getInstance();
sing.foo();
}
那么这里的三种不同的方式编译和输出:
没有专业化,SingletonMain.o联第一:
C:\Test>g++ -c Singleton.cpp
C:\Test>g++ -c SingletonMain.cpp
C:\Test>g++ -o Singleton.exe SingletonMain.o Singleton.o
SingletonMain.o:SingletonMain.cpp:(.text$_ZN9SingletonILi100EE11getInstanceEv[Singleton<100>::getInstance()]+0x7): undefined reference to `Singleton<100>::instance'
SingletonMain.o:SingletonMain.cpp:(.text$_ZN9SingletonILi100EE11getInstanceEv[Singleton<100>::getInstance()]+0x1c): undefined reference to `Singleton<100>::instance'
SingletonMain.o:SingletonMain.cpp:(.text$_ZN9SingletonILi100EE11getInstanceEv[Singleton<100>::getInstance()]+0x21): undefined reference to `Singleton<100>::instance'
collect2: ld returned 1 exit status
没有专业化,Singleton.o联第一:
C:\Test>g++ -c Singleton.cpp
C:\Test>g++ -c SingletonMain.cpp
C:\Test>g++ -o Singleton.exe Singleton.o SingletonMain.o
SingletonMain.o:SingletonMain.cpp:(.text$_ZN9SingletonILi100EE11getInstanceEv[Singleton<100>::getInstance()]+0x7): undefined reference to `Singleton<100>::instance'
SingletonMain.o:SingletonMain.cpp:(.text$_ZN9SingletonILi100EE11getInstanceEv[Singleton<100>::getInstance()]+0x1c): undefined reference to `Singleton<100>::instance'
SingletonMain.o:SingletonMain.cpp:(.text$_ZN9SingletonILi100EE11getInstanceEv[Singleton<100>::getInstance()]+0x21): undefined reference to `Singleton<100>::instance'
collect2: ld returned 1 exit status
专业化:
C:\Test>g++ -DSPECIALIZE -c Singleton.cpp
C:\Test>g++ -DSPECIALIZE -c SingletonMain.cpp
C:\Test>g++ -DSPECIALIZE -o Singleton.exe Singleton.o SingletonMain.o
现在,似乎是有人在这里有类似的问题 - C++ template static member instantiation - 但“解决方案”是有明确的专业化每种类型,你可能会使用...这似乎总数最终打败模板的目的...所以必须有更好的方法,对吧?