类模板实例化中的类型转换

Question

I have a template class item which stores objects of various types T . It also attaches attributes to those objects in instantiation/initialization.

One special thing I want to achieve is that whenever item sees a const char * , it deems and stores it as a std::string . This could be done, as follows.

But in type checking, I found an item instantiated from a const char * is still different in type from an item instantiated from a std::string . Please see the last line with comment false , which I want to make true .

#include <iostream>
#include <string>
#include <type_traits>

using namespace std;

template<typename T>
using bar = typename std::conditional<std::is_same<T, const char *>::value,
                                      string, T>::type;

template<typename T>
class item
{
    bar<T> thing;

    // other attributes ...

public:
    item(T t) : thing(t) {}

    // other constructors ...

    bar<T> what() const
    {
        return thing;
    }
};

int main()
{
    auto a = item("const char *");     // class template argument deduction (C++17)
    auto b = item(string("string"));   // class template argument deduction (C++17)

    cout << std::boolalpha;
    cout << (typeid(a.what()) == typeid(b.what())) << endl; // true
    cout << (typeid(a) == typeid(b)) << endl;               // false
}

My question is: is it possible to make any change to the template class item so that an item instantiated from a const char * becomes the same in type with an item instantiated from a std::string ?

In other words, can I make any change to the design of the template class item so that typeid(a) == typeid(b) evaluates to true ?

Thank you !

Note: This follows up a previous question on template function. But I think there's something intrinsically different that it deserves a stand-alone question.

Edit: My goal is to change the design of the template class item (eg item signatures), not the code in main , which is assumed to be supplied by users. I want to make life easier for the users of item , by not asking them to explicitly supply type T in instantiation. This is meant to be done by C++17 template class argument deduction or some equivalent workarounds.

Update: Thank you all! Special thanks to @xskxzr, whose one-liner exactly solves my question. With user-defined deduction guides for class template argument deduction, I don't even need the bar<T> technique in my previous code. I put updated code below for your comparison.

#include <iostream>
#include <string>

using namespace std;

template<typename T>
class item
{
    // UPDATE: no bar<T> needed any more
    T thing;

    // other attributes ...

public:
    item(T t) : thing(t) {}

    // other constructors ...

    // UPDATE: no bar<T> needed any more
    T what() const
    {
        return thing;
    }
};

item(const char *) -> item<std::string>;  // UPDATE: user-defined deduction guide !

int main()
{
    auto a = item("const char *");     // class template argument deduction (C++17)
    auto b = item(string("string"));   // class template argument deduction (C++17)

    cout << std::boolalpha;
    cout << (typeid(a.what()) == typeid(b.what())) << endl; // true
    cout << (typeid(a) == typeid(b)) << endl;               // UPDATE: now true !
}

Answer 1

You can add a user-defined deduction guide :

item(const char *) -> item<std::string>;

With this deduction guide, a will be deduced to be item<std::string> .

Answer 2

No, you can't directly make the typeid of two templated objects using different template arguements be the same.

But to achieve your end goal you can use a factory like pattern. It could look something like this:

template<typename T, typename R = T>
item<R> make_item(T&& t)
{
    return item<T>(std::forward<T>(t));
}

// Specialization for const char *
template<>
item<std::string> make_item(const char *&& str)
{
    return item<std::string>(str);
} 

The downside with this approach is that you'll need to construct all of your objects with this factory. And if you have a lot of exceptions you'll need to make a specialization for each exception.

Answer 3

This is more a guess than an answer, but I'd say no. Templates are expanded at compile time, so because you are creating an

item<const char*>

and an

item<std::string>

then the code that gets expanded looks something like

class item1
{
    bar<const char*> thing;

    // other attributes ...

public:
    item(const char* t) : thing(t) {}

    // other constructors ...

    bar<const char*> what() const
    {
        return thing;
    }
};


class item2
{
    bar<std::string> thing;

    // other attributes ...

public:
    item(std::string t) : thing(t) {}

    // other constructors ...

    bar<std::string> what() const
    {
        return thing;
    }
};

(More or less; they wouldn't actually be called item1 and item2)

How you chose to evaluate these two types is up to you, but to the compiler they are in fact two different types.

Answer 4

Ok, I'd never seen or used std::conditional before so I wasn't sure what that was doing, but after reading up on it and playing around with your code I did get it to "work" by using

bar<T>

as the template type. So instead of

auto a = item<const char*>("const char *");
auto b = item<string>(string("string"));

I did

auto a = item<bar<const char*>>("const char *");
auto b = item<bar<string>>(string("string"));

The thing is you need the template type to be the same in both cases, meaning the type needs to resolve to std::string before the template gets expanded. As long as you use your conditional, you can define any type.

auto c = item<bar<int>>(5);

Not sure that's a good solution (which is why I said "work"), but see my other answer about the class types actually being different.