最烦人的解析 [英] Most vexing parse
问题描述
我从这里获得了代码.
class Timer {
public:
Timer();
};
class TimeKeeper {
public:
TimeKeeper(const Timer& t);
int get_time()
{
return 1;
}
};
int main() {
TimeKeeper time_keeper(Timer());
return time_keeper.get_time();
}
从它的外观来看,它应该由于以下行而出现编译错误:
From the looks of it, it should get compile error due to the line:
TimeKeeper time_keeper(Timer());
但只有在 return time_keeper.get_time();
存在时才会发生.
But it only happens if return time_keeper.get_time();
is present.
为什么这条线很重要,编译器会在 time_keeper(Timer() )
构造上发现歧义.
Why would this line even matter, the compiler would spot ambiguity on time_keeper(Timer() )
construction.
推荐答案
这是因为 TimeKeeper time_keeper(Timer());
被解释为函数声明而不是变量定义.这本身并不是错误,但是当您尝试访问 time_keeper 的 get_time()
成员(它是一个函数,而不是 TimeKeeper 实例)时,您的编译器会失败.
This is due to the fact that TimeKeeper time_keeper(Timer());
is interpreted as a function declaration and not as a variable definition. This, by itself, is not an error, but when you try to access the get_time()
member of time_keeper (which is a function, not a TimeKeeper instance), your compiler fails.
这是您的编译器查看代码的方式:
This is how your compiler view the code:
int main() {
// time_keeper gets interpreted as a function declaration with a function argument.
// This is definitely *not* what we expect, but from the compiler POV it's okay.
TimeKeeper time_keeper(Timer (*unnamed_fn_arg)());
// Compiler complains: time_keeper is function, how on earth do you expect me to call
// one of its members? It doesn't have member functions!
return time_keeper.get_time();
}
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