问题
I want to write a simple program that depending on the options passed it the executable will print the output to the screen or to a file. The program is simple.
#include<iostream>
int main(int argc, char* argv[]){
... process options...
std::ostream& out = ... // maybe std::cout, maybe a *new* std::ofstream;
out << "content\n";
}
Is there a good idiom to make out refer alternatively to std::cout or a file stream at runtime?
I tried with pointers, but it is horrible. I couldn't avoid using pointers (Not to mention that more ugly code is needed to delete the pointer later).
#include<iostream>
#include<ofstream>
int main(int argc, char* argv[]){
std::string file = argc>1?argv[1]:"";
std::clog << "file: " << file << '\n';
// if there is no argument it will print to screen
std::ostream* out = (file=="")?&std::cout:(new std::ofstream(file)); // horrible code
*out << "content" << std::endl;
if(out != &std::cout) delete out;
}
I don't know, perhaps there is some feature of C++ streams that allows this. Perhaps I have to use some kind of type erasure. The problem, I think, is that std::cout is something that already exists (is global), but std::ofstream is something that has to be created.
I managed to use open and avoid pointers but it is still ugly:
int main(int argc, char* argv[]){
std::string file = argc>1?argv[1]:"";
std::clog << "file: " << file << '\n';
std::ofstream ofs;
if(file != "") ofs.open(file);
std::ostream& out = (file=="")?std::cout:ofs;
out << "content" << std::endl;
}
回答1:
My preference is to use streams with suitable stream buffers installed. Here is one way direct output to a file or to std::cout:
#include <iostream>
#include <fstream>
int main(int ac, char* av) {
std::ofstream ofs;
if (1 < ac) {
ofs.open(av[1]);
// handle errors opening the file here
}
std::ostream os(file? file.rdbuf(): std::cout.rdbuf());
// use os ...
}
回答2:
So much over-engineering.
#include <iostream>
#include <fstream>
int main(int argc, char* argv[]) {
std::ofstream ofs(argc > 1 ? argv[1] : "");
std::ostream& os = ofs.is_open() ? ofs : std::cout;
// use os ...
}
回答3:
A runtime binding of the desired stream will pretty much need to look like what you already have.
On the pointer issue, sure you can clean it up a bit... maybe something like this? This is assuming you only want to create the ofstream if the argument exists.
int main(int argc, char* argv[]){
std::string file = argc > 1 ? argv[1] : "";
std::clog << "file: " << file << '\n';
// if there is no argument it will print to screen
std::unique_ptr<std::ostream> fp;
if (file == "")
fp = std::make_unique<std::ofstream>(file);
std::ostream& out = (fp && fp->is_open()) ? std::cout : *fp; // not so horrible code
out << "content" << std::endl;
}
If the dynamic object is not required, the easiest may be something list this;
int main(int argc, char* argv[]){
std::string filename = (argc > 1) ? argv[1] : "";
std::ofstream file(filename);
// if there is no argument (file) it will print to screen
std::ostream& out = file.is_open() ? file : std::cout;
out << "content" << std::endl;
}
回答4:
I often use something like this for command-line tools:
int main(int, char* argv[])
{
std::string filename;
// args processing ... set filename from command line if present
if(argv[1])
filename = argv[1];
std::ofstream ofs;
// if a filename was given try to open
if(!filename.empty())
ofs.open(filename);
// bad ofs means tried to open but failed
if(!ofs)
{
std::cerr << "Error opeing file: " << filename << '\n';
return EXIT_FAILURE;
}
// Here either ofs is open or a filename was not provided (use std::cout)
std::ostream& os = ofs.is_open() ? ofs : std::cout;
// write to output
os << "Some stuff" << '\n';
return EXIT_SUCCESS;
}
回答5:
You could use a shared pointer to a stream for the polymorphic behavior:
#include <memory>
#include <fstream>
#include <sstream>
#include <iostream>
void nodelete(void*) {}
std::shared_ptr<std::ostream> out_screen_stream() { return std::shared_ptr<std::ostream>(&std::cout, nodelete); }
std::shared_ptr<std::ostream> out_file_stream() { return std::make_shared<std::ofstream>(); }
std::shared_ptr<std::ostream> out_string_stream() { return std::make_shared<std::ostringstream>(); }
int main ()
{
std::shared_ptr<std::ostream> out;
// case condition:
out = out_screen_stream();
out = out_file_stream();
out = out_string_stream();
*out << "content" << std::endl;
return 0;
}
Note: A std::shared_ptr allows managing different possible streams, where some streams should not get deleted (e.g.: std::cout).
Similar, but with std::unique_ptr:
#include <memory>
#include <fstream>
#include <sstream>
#include <iostream>
class Deleter
{
public:
Deleter(bool use_delete = true) : use_delete(use_delete) {}
template <typename T>
void operator () (const T* p) {
if(use_delete)
delete p;
}
bool nodelete() const { return ! use_delete; }
private:
bool use_delete;
};
using unique_ostream_ptr = std::unique_ptr<std::ostream, Deleter>;
unique_ostream_ptr out_screen_stream() { return unique_ostream_ptr(&std::cout, false); }
unique_ostream_ptr out_file_stream() { return unique_ostream_ptr{ new std::ofstream }; }
unique_ostream_ptr out_string_stream() { return unique_ostream_ptr{ new std::ostringstream }; }
int main ()
{
unique_ostream_ptr out;
// case condition:
out = out_screen_stream();
out = out_file_stream();
out = out_string_stream();
*out << "content" << std::endl;
return 0;
}
回答6:
Maybe a reference?
#include<iostream>
#include<ofstream>
int main(int argc, char* argv[])
{
auto &out = std::cout;
std::ofstream outFile;
std::string fileName = argc>1?argv[1]:"";
std::clog << "file: " << file << '\n';
// if there is no argument it will print to screen
if(!fileName.empty())
{
outFile.open(fileName);
out = outFile;
}
out<<"one, one, two";
return 0;
}
来源：https://stackoverflow.com/questions/38454951/is-there-a-good-idiom-to-deal-with-alternative-output-streams