无法使用 ostream 打印但可以使用 cout 打印？

Question

Let this be the example code:

object o1(//parameters);
object o2(//parameters);
object o3(//parameters);
object *v[3];

using std::cout; //video output
ofstream of;     //save on file

//let's suppose

v[0]=&o1;
v[1]=&o2;
v[2]=&o3;
for (int i=0;i<3;i++) {
    v[i]->view(cout);
    v[i]->save(of);
}

view function is just a video print function of parameters of the class, while save function saves parameters of the class on a file. The question is, if i declare std::ostream; and ostream os; why can't I use v[i]->view(os) to have video output? If I use v[i]->view(os) it says:

'std::basic_ostream<_CharT, _Traits>::basic_ostream() [with _CharT = char; _Traits = std::char_traits<char>]' is protected

And while that's true for video output, for the saving function it is not, it works, just as intended in the code. Can someone explain? sorry if i made it long

Answer 1

The issues:

1) If the view function is defined as:

void view(std::ostream output, std::string text) // (1)
{
    output << text;
}

And used:

view(std::cout, "Hello, World!"); // (2)

Then an error message is given by the compiler:

In MSVC:

error C2280: 'std::basic_ostream<char,std::char_traits<char>>::basic_ostream(const std::basic_ostream<char,std::char_traits<char>> &)': attempting to reference a deleted function

GCC:

error: use of deleted function 'std::basic_ostream<_CharT, _Traits>::basic_ostream(const std::basic_ostream<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits<char>]'

Clang:

error: call to deleted constructor of 'std::ostream' (aka 'basic_ostream<char>')

---

2) For the declaration

std::ostream os;

The following error message is displayed:

MSVC:

error C2512: 'std::basic_ostream<char,std::char_traits<char>>': no appropriate default constructor available

GCC:

error: 'std::basic_ostream<_CharT, _Traits>::basic_ostream() [with _CharT = char; _Traits = std::char_traits<char>]' is protected within this context

Clang:

error: calling a protected constructor of class 'std::basic_ostream<char>'

---

The reason:

This is all according to the specification of std::basic_ostream

There is no definition for a default constructor - so a variable of the type std::ostream cannot be created, without specific constructor parameters.

And as C++ Reference says about the std::basic_ostream copy constructor :

The copy constructor is protected, and is deleted. Output streams are not copyable.

---

Explanation:

1) So the problem is that in (2) the parameter std::cout is been passed to a function which is defined in (1) to copy the std::ostream to the variable output .

But the definition of the class says that the copy constructor cannot be used, so the compiler gives an error message.

2) In the case of creating the variable os - it is not giving any constructor parameters, there is no default constructor, so the compiler gives an error message.

---

How to fix this?

1) In the declaration of the function change the definition to take a reference ( & ) of std::ostream as a parameter:

void view(std::ostream& output, std::string text) // (1)

This allows it to use the original object instead of making a copy (which the copy is not allowed).

---

2) If a variable is required, then also a reference should be used;

std::ostream& out = std::cout;