How to assign istringstream and ifstream to an istream variable?

Question

I want to have a variable of type istream which can hold either the contents of a file or a string. The idea is that if no file was specified, the variable of type istream would be assigned with a string.

std::ifstream file(this->_path)

and

std::istringstream iss(stringSomething);

to

std::istream is

I've tried just assigning them to the istream variable like I would with other objects that inherit from the same base class, but that didn't work.

How to assign istringstream and ifstream to an istream variable?

Answer 1

You can't assign to a std::istream but you can bind to a reference like this:

#include <string>
#include <sstream>
#include <fstream>
#include <iostream>

std::istringstream test_data(R"~(

some test data here
instead of in an external
file.

)~");

int main(int, char* argv[])
{
    // if we have a parameter use it
    std::string filename = argv[1] ? argv[1] : "";

    std::ifstream ifs;

    // try to open a file if we have a filename
    if(!filename.empty())
        ifs.open(filename);

    // This will ONLY fail if we tried to open a file
    // because the filename was not empty    
    if(!ifs)
    {
        std::cerr << "Error opening file: " << filename << '\n';
        return EXIT_FAILURE;
    }

    // if we have an open file bind to it else bind to test_data
    std::istream& is = ifs.is_open() ? static_cast<std::istream&>(ifs) : test_data;

    // use is here
    for(std::string word; is >> word;)
    {
        std::reverse(word.begin(), word.end());
        std::cout << word << '\n';
    }
}

Answer 2

Base class pointers can point to derived class data. std::istringstream and std::ifstream both derived from std::istream, so we can do:

//Note that std::unique_ptr is better that raw pointers
std::unique_ptr<std::istream> stream;

//stream holds a file stream
stream = std::make_unique<std::ifstream>(std::ifstream{ this->_path });

//stream holds a string
stream = std::make_unique<std::istringstream>(std::istringstream{});

Now you just have to extract the content using

std::string s;
(*stream) >> s;