What the C ++ 14 standard says about cars as an argument type

Question

Let's look at the following code below:

#include <iostream>

class Test
{
private:
    int x;
public:
    Test(int _x) :
        x(_x)
    {
        std::cout << "Im being constructed" << std::endl;
    }

    int getx()
    {
        return this->x;
    }

    friend std::ostream& operator<<(std::ostream& os, Test& other)
    {
        os << other.getx();
        return os;
    }
};

auto func(auto x)
{
    std::cout << x << std::endl;
    return x.getx();
}

int main()
{
    auto y = func(20);

    return 0;
}

How does the compiler decide if (20) should be an int or Test object? The constructor for the test is not explicit, so what does the standard say about this?

Answer 1

So, although gcc allows auto as a parameter in functions, this is not part of C ++ 14, but part of concept-lite , which can become part of C ++ 1z according to the latest Herb Sutter post.

I believe gcc allows this as an extension, and if we compile your program with -pedanticgcc will warn:

 warning: ISO C++ forbids use of 'auto' in parameter declaration [-Wpedantic]
 auto func(auto x)
           ^

, , :

auto func(auto x)
{
    std::cout << x << std::endl;
    return x.getx();
}

:

template <typename T1>
auto func(T1 x)
{
    std::cout << x << std::endl;
    return x.getx();
}

, , x int. , gcc ( ):

error: request for member 'getx' in 'x', which is of non-class type 'int'
     return x.getx();
                   ^

5.1.1 [dcl.fct]:

- , . [: . -end note] [: ,

auto f(auto x, const Regular& y);

template<typename T1, Regular T2>
  auto f(T1 x, const T2&);

-end ]

Answer 2

auto , .

auto func(auto x) template<typename T> auto func(T x)

, .