当operator ==是朋友时,链接器错误 [英] Linker error when operator== is a friend
问题描述
以下代码是重现我的问题的最低代码.当我尝试编译它时,链接器找不到Config
的operator==
:
The following code is a minimum code to reproduce my problem. When I try to compile it, the linker can not find operator==
for Config
:
Undefined symbols for architecture x86_64:
"operator==(Config<2> const&, Config<2> const&)", referenced from:
_main in test2.o
operator==
是Config
的朋友.但是,当我不再将operator==
声明为朋友时,代码编译器将没有错误.
The operator==
is a friend of Config
. BUT when I do no longer declare operator==
as a friend, the code compilers with no error.
template <int DIM>
class Config{
// comment the following line out and it works
friend bool operator==(const Config<DIM>& a, const Config<DIM>& b);
public:
int val;
};
template <int DIM>
bool operator==(const Config<DIM>& a, const Config<DIM>& b){
return a.val == b.val;
}
int main() {
Config<2> a;
Config<2> b;
a == b;
return 0;
}
这是什么问题?
推荐答案
您错过了在friend
声明中声明模板的方法:
You have missed to declare the template in the friend
declaration:
template <int DIM>
class Config{
template <int DIM_> // <<<<
friend bool operator==(const Config<DIM_>& a, const Config<DIM_>& b);
public:
int val;
};
如果要使用friend
函数声明,则必须使用它的确切签名声明.如果这涉及模板参数,则必须与封闭的模板类或结构无关地指定这些参数.
If you want to have a friend
function declaration you must use it's exact signature declaration. If this involves template parameters these must be specified independently from the enclosing template class or struct.
这是一个精妙的答案,深入解释了friend
声明的几个方面.
Here's a brilliant answer explaining the several aspects of friend
declarations in depth.
这篇关于当operator ==是朋友时,链接器错误的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!