Typeinfo how to get the name/id of the class in a polymorphic collection?

user_2612235 0
Viewed 134

In the following example I would expect not stdout : Base Foo Bar, but I get P4Base P4Base P4Base:

#include <iostream>
#include <typeinfo>
#include <vector>
#include <memory>

class Base {};
class Foo : public Base {};
class Bar : public Base {};

using Collection = std::vector<std::unique_ptr<Base> >;

int main() {
    Collection collection;
    collection.push_back(std::make_unique<Base>());
    collection.push_back(std::make_unique<Foo>());
    collection.push_back(std::make_unique<Bar>());
    for (auto &u:collection)
        std::cout << typeid(u.get()).name() << std::endl;
}

Is there a way to properly identify which kind of instance I have in my collection?

EDIT

A Working example after the advice of eerorika

struct Base {virtual ~Base() = default;};
struct Foo : public Base {};
struct Bar : public Base {};

using Collection = std::vector<std::unique_ptr<Base> >;

int main() {
    Collection collection;
    collection.push_back(std::make_unique<Base>());
    collection.push_back(std::make_unique<Foo>());
    collection.push_back(std::make_unique<Bar>());
    for (auto &u:collection)
        std::cout << typeid(*u).name() << std::endl;
}
typeid rtti polymorphism c++
4 Answers

Typeinfo how to get the name/id of the class in a polymorphic collection?

  • Firstly, the classes must be polymorphic in order for typeid to provide the dynamic type. Your classes are not polymorphic, so you would get the static type.
  • Secondly, you must use an lvalue to the pointed object instead of a pointer. Applying typeid to a pointer gives you the type info of the pointer type rather than type of the pointed object.
  • Thirdy, the behaviour of your program is undefined because derived objects are destroyed through pointer to base with non-virtual destructor.

To fix first and third point, provide a virtual destructor for the base. To fix second:

typeid(*u).name()

Lastly, your expectation of readable class names is misguided. std::type_info::name is not guaranteed to give you the name as you have written it for the class. The result is implementation defined, and in practice you typically get the mangled name of the type. There is no standard way to get a readable name, but there are implementation defined ways to demangle names.

user_20793030

The output you are seeing is caused by the fact that the u variable is auto deduced to always be a std::unique_ptr<Base> type.

One way of fixing this is to add virtual members to your classes that return the typeid of their respective this pointers. Then, each call to that function on the u pointer will call the actual derived class override:

class Base {
public:
    virtual const char* name() { return typeid(this).name(); }
};
class Foo : public Base {
public:
    const char* name() override { return typeid(this).name(); }
};
class Bar : public Base {
public:
    const char* name() override { return typeid(this).name(); }
};

using Collection = std::vector<std::unique_ptr<Base> >;

int main()
{
    Collection collection;
    collection.push_back(std::make_unique<Base>());
    collection.push_back(std::make_unique<Foo>());
    collection.push_back(std::make_unique<Bar>());
    for (auto& u : collection)
        std::cout << u->name() << std::endl;
    return 0;
}
user_108710730

There are several issues.

  1. u.get() has a type "pointer to Base". If you want to print the type of the object itself, try typeid(*u.get()).name().
  2. If you want polymorphic behaviour, use polymorphic classes. This requires defining at least one virtual function in Base. You need to define a virtual destructor anyway for correct behaviour of object deletion. Without one, the program has UB.
  3. name() returns an implementation-defined string, so you cannot expect Base Foo Bar. You might get something similar though if you fix the other problems. Live demo.
user_7758060

The expression typeid(var).name() for variable var should be unique among different types. However, for indexing and general distinguishing use you should use typeid(var).hash_code().

user_68968660
Related