type_info不是RTTI的一部分吗？ [英] Is type_info not a part of RTTI?
问题描述
我曾问过一个问题 Do C ++ POD类型是否具有RTTI？，有人告诉我在评论：
I had asked a question Do C++ POD types have RTTI? and someone told me in the comments:
POD类型有type_info，但没有RTTI，这可能
，因为type_info不总是RTTI。
POD types do have type_info, but don't have RTTI, and that's possible because type_info isn't always RTTI.
似乎是正确的，因为我可以得到 type_info
and it seems right as i could get the type_info of a POD (non-polymorphic) type.
但是当我编译这个简单的程序：
But while I compile this simple program:
#include <iostream>
struct X
{
int a;
};
int main()
{
using namespace std;
std::cout << typeid(X) << std::endl;
return 0;
}
标志 -fno-rtti of GCC：
$ g++ -fno-rtti main.cpp && ./main
它不会编译：
main.cpp: In function ‘int main()’:
main.cpp:12:26: error: cannot use typeid with -fno-rtti
std::cout << typeid(X) << std::endl;
^
这意味着 type_info
推荐答案
是 / h2>
RTTI本身并不是真正的定义：C ++只说了 typeid 和 dynamic_cast do，而不是如何实现。
Abstract
RTTI per se is not something really formally defined: C++ only says what typeid and dynamic_cast do, not how they're implemented. However, it is convenient indeed to group such kind of operations under a common name which is RTTI.
请注意，实现不是必须严格遵守 在运行时获取此信息即
Notice an implementation is not required to strictly obtain this information at runtime i.e.
if（typeid（int）== typeid（double））
也可以在程序评估期间确定，很像 std :: is_same 。 int 是不可否认的非多态性（它没有'动态'类型）。 cppreference 甚至声称：
could also be determined during the program evaluation, much like std::is_same. int is undeniably non-polymorphic (it has no 'dynamic' type). cppreference even claims:
当应用于多态类型的表达式时，对typeid表达式的求值可能涉及运行时开销（虚拟表查找），否则typeid表达式在编译时解析。
When applied to an expression of polymorphic type, evaluation of a typeid expression may involve runtime overhead (a virtual table lookup), otherwise typeid expression is resolved at compile time.
但是要小心。
平均值type_info是RTTI的一部分，还是仅仅是GCC的行为？
Does that mean type_info is a part of RTTI, or is it just a behavior of GCC?
type_info 是一个类。你不能构造任何类型的对象 - 你只能通过 typeid 。
type_info is a class. You may not construct any object of that type - you only can through typeid.
-fno-rtti 禁用GCC下的RTTI：您不能使用 typeid 因此既不能 type_info 。
总而言之，原始报价完全正确：
To conclude, the original quote is totally right:
POD类型有
type_info，但没有RTTI，这是可能的，因为type_info不总是RTTI。
POD types do have
type_info, but don't have RTTI, and that's possible because type_info isn't always RTTI.
运行时信息可通过 typeid 获取。没有什么动态可以考虑（事实上， dynamic_cast 是没有意义的）。
The runtime information is available through typeid. There is just nothing dynamic to consider (indeed, dynamic_cast would make no sense).
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