why ostream cannot be constant

std::ostream is a standard output stream in C++ that is used to write data to the console or other output devices. It is commonly used with the << operator to send data to the console or a file.

The reason why std::ostream cannot be constant is because it represents an output stream, and by its nature, it is expected to change as data is written to it. If std::ostream were allowed to be constant, it would not be possible to write any data to it, which defeats the purpose of having an output stream.

When you declare a variable as constant using the const keyword, it means that the value of that variable cannot be changed after it is initialized. However, in the case of std::ostream, the value of the stream itself does not change, but rather the data that is written to it changes.

If you were to declare std::ostream as constant, it would prevent you from using the << operator to write data to the stream. For example, if you have the following code:

const std::ostream output(std::cout);
output << "Hello, world!";

The compiler would give you an error because you are trying to modify a constant object. In order to write data to the output stream, you need to have a non-constant std::ostream object.

In summary, std::ostream cannot be constant because it represents an output stream that is expected to change as data is written to it. Declaring it as constant would prevent you from using the << operator to write data to the stream.