why I need to use std::decay in the following case?

user_9603886 0
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I am a newbie in C++ and made a test program to learn more about decltype, std::decay, std::is_same_v(traits) and also typeid.

I have following simple class, in which I wanted to get the template parameter type Type in the Base class's constructor using decltype.

like decltype(content of std::vector<Type>::iterator::begin). Some how, it did not work.

#include <iostream>
#include <vector>
#include <type_traits>
#include <initializer_list>
#include <typeindex>

template<typename Type> class Base
{
    std::vector<Type> vec;
public :
    Base(std::initializer_list<decltype(*vec.begin())> liVec): vec(liVec) {}
    // here                    ^^^^^^^^^^^^^^^^^^^^^^^^ . isn't enough?? 
};

int main()
{
    //Base<int> obj{ 1, 2, 3, 4, 5 };     // does not works: error !

    // to see the type, I wrote the following code
    std::vector<int> vec;
    std::cout << typeid(decltype(*vec.begin())).name() << std::endl; // prints i means int type(I know not reliable though)
    // and
    std::cout << std::boolalpha << std::is_same_v<int, decltype(*vec.begin())> << std::endl; // prints false 

    return 0;
}

The error on the line Base<int> obj{ 1, 2, 3, 4, 5 }; was(in GCC 6.1, C++14)

include\c++\initializer_list||In instantiation of 'class std::initializer_list<int&>':|
include\c++\initializer_list|54|error: forming pointer to reference type 'int&'|
include\c++\initializer_list|55|error: forming pointer to reference type 'int&'|
main.cpp|17|error: no matching function for call to 'Base<int>::Base(<brace-enclosed initializer list>)'|
candidate: 'Base<Type>::Base(std::initializer_list<decltype (*((Base<Type>*)(void)0)->Base<Type>::vec.begin())>) [with Type = int; decltype (*((Base<Type>*)(void)0)->Base<Type>::vec.begin()) = int&]'|
main.cpp|11|note:   candidate expects 1 argument, 5 provided|
main.cpp|7|note: candidate: 'Base<int>::Base(const Base<int>&)'|
main.cpp|7|note:   candidate expects 1 argument, 5 provided|
main.cpp|7|note: candidate: 'Base<int>::Base(Base<int>&&)'|
main.cpp|7|note:   candidate expects 1 argument, 5 provided|

As I saw the compiler told me something about int&, I simply tried it with following.(i.e, std::decay_t) and it worked. 

template<typename Type> class Base
{
    std::vector<Type> vec;
public :
    Base(std::initializer_list<std::decay_t<decltype(*vec.begin())>> liVec): vec(liVec) {}
                               ^^^^^^^^^^^^^^^^^^^^^^^^ why I need this here?
};

int main()
{
    Base<int> obj{ 1, 2, 3, 4, 5 };     // works now

    std::vector<int> vec;
    std::cout << typeid(decltype(*vec.begin())).name() << std::endl; // prints i means int type(not reliable though)
    // and
    std::cout << std::boolalpha << std::is_same_v<int, std::decay_t<decltype(*vec.begin())>> << std::endl; // true now: WHY?

    return 0;
}

But I don't know the meaning of the error and why it worked. Could someone explain me what exactly happend?

c++14 templates c++
1 Answers

The first line of your error messages says (among other things):

instantiation of 'class std::initializer_list<int&>'

So it is trying to create an initializer_list with a reference type which is not allowed.

Looking at your template code you have:

 std::initializer_list<decltype(*vec.begin())>

Now vec.begin() yields an iterator and dereferencing an iterator yields a reference so you can do things like:

*iter = whatever;

So you need to remove the reference part of the type. std::decay_t does that.

user_38077290
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