std :: remove_reference有什么意义呢？

Question

Let me dive in C++14 generic lambdas with:

#include <iostream>

// g++ -std=c++14 

template<typename T>
T incr(T v)
{
    return v + 1;
}


int main()
{
    float f = 2.0;
    int i = 3;

    auto selfincr = [] (auto & value)
        {
            value = incr<std::remove_reference<decltype(value)>>(value);    // A
            value = incr<decltype(value)>(value);                           // B
        };


    selfincr(f);
    selfincr(i);

    std::cout << "f " << f << ", i " << i << std::endl;
    return 0;
}

Since line // B causes a

invalid initialization of non-const reference of type 'T&' from an rvalue of type 'T'

My immediate guessing has been the removal of the reference, so I added line // A . But this yield a

no matching function for call to 'incr(T&)'

So how could I remove that reference ?

Answer 1

So how could I remove that reference ?

incr<std::remove_reference<decltype(value)>>(value) , you're specifying std::remove_reference<T> as the template parameter, but not the type referred by T (ie decltype(value) ). What you want should be

value = incr<typename std::remove_reference<decltype(value)>::type>(value);    // A
//           ~~~~~~~~                                       ~~~~~~        

And since C++14 you could make it simpler:

value = incr<std::remove_reference_t<decltype(value)>>(value);    // A
//                                ~~ 

LIVE

Answer 2

This:

value = incr<std::remove_reference<decltype(value)>>(value);    // A

doesn't work due to what songyuanyao explained.

---

This:

value = incr<decltype(value)>(value);                           // B

doesn't work becuase decltype(value) is a reference type and you're trying to instantiate:

float& incr(float& v) { return v + 1; }
int& incr(int& v) { return v + 1; }

You can't bind those expressions to non-const lvalue references, hence the compile error.

---

The simplest solution is to just let template deduction do its thing:

value = incr(value); // C

That will call incr<int> and incr<float> as desired.