Reading from text file or stdin - c++

Question

I have a program that basically reads a text file and counts the number of occurrences of each word on each line. Everything works properly when reading from a text file using an ifstream, however, if a file name is not entered on the command line, I need to read from stdin instead.
I use the following to open and read in the file currently:
map<string, map<int,int>,compare> tokens;
ifstream text;
string line;
int count = 1;
if (argc > 1){
try{
text.open(argv[1]);
}
catch (runtime_error& x){
cerr << x.what() << '\n';
}
// Read file one line at a time, replacing non-desired char's with spaces
while (getline(text, line)){
replace_if(line.begin(), line.end(), my_predicate, ' ');
istringstream iss(line);
// Parse line on white space, storing values into tokens map
while (iss >> line){
++tokens[line][count];
}
++count;
}
}
else{
while (cin) {
getline(cin, line);
replace_if(line.begin(), line.end(), my_predicate, ' ');
istringstream iss(line);
// Parse line on white space, storing values into tokens map
while (iss >> line){
++tokens[line][count];
}
++count;
}
Is there a way to assign cin to an ifstream and simply add an else statement if argc > 1 fails, using the same code afterwards instead of duplicating like this? I haven't been able to find a way to do this.

Answer 1

Make the reading part a function of its own. Pass either an ifstream or cin to it.
void readData(std::istream& in)
{
// Do the necessary work to read the data.
}
int main(int argc, char** argv)
{
if ( argc > 1 )
{
// The input file has been passed in the command line.
// Read the data from it.
std::ifstream ifile(argv[1]);
if ( ifile )
{
readData(ifile);
}
else
{
// Deal with error condition
}
}
else
{
// No input file has been passed in the command line.
// Read the data from stdin (std::cin).
readData(std::cin);
}
// Do the needful to process the data.
}

Answer 2

You cannot assign cin to an ifstream.
But you could reopen cin to some file.
Anyway, the better method would be modularising your code and just using a std::istream&.

Answer 3

I just put a easy way to let the program read file if given else use stdin, and close it properly.
#include <fstream>
#include <iostream>
#include <memory>
#include <string>
int main(int argc, char* argv[]) {
auto fp_deletor = [](std::istream* is_ptr) {
if (is_ptr && is_ptr != &std::cin) {
static_cast<std::ifstream*>(is_ptr)->close();
delete is_ptr;
std::cerr << "destroy fp.\n";
}
};
std::unique_ptr<std::istream, decltype(fp_deletor)> is_ptr{nullptr, fp_deletor};
if (argc > 2) {
std::cerr << "usage: " << argv[0] << "[input-file]";
return -1;
} else if (argc == 1) {
std::cerr << "using stdin as input.\n";
is_ptr.reset(&std::cin);
} else {
is_ptr.reset(new std::ifstream(argv[1]));
}
std::string line;
while (std::getline(*is_ptr, line)) {
// your logic....
}
// just return, unique_ptr manage the istream
return 0;
}