Cpp Usage

This page documents the C++ interface of cfstream, provided by cfs.hpp.

Use this page when you want to understand how the C++ version works in detail, including standard stream redirection, independent file streams, version query, closing behavior, and common usage rules.

Header

Use the C++ header:

#include "cfs.hpp"

The header provides all C++ features inside the cfs namespace.

cfs::cfs(...);
cfs::closecfs();

cfs::cppfs(...);
cfs::closecppfs();

cfs::cin;
cfs::cout;

cfs::getversion();

Requirements

cfs.hpp supports:

C++98 or later

The header includes the required standard headers internally:

#include <fstream>
#include <cstdio>
#include <iostream>

So in many simple programs, including cfs.hpp alone is enough.

Namespace

All C++ interfaces are placed in the cfs namespace.

Example:

#include "cfs.hpp"

int main() {
    cfs::cppfs("test.in", "test.out");

    int n;
    cfs::cin >> n;
    cfs::cout << n << '\n';

    cfs::closecppfs();
    return 0;
}

You may also import selected names with using, but this is optional.

#include "cfs.hpp"

using cfs::cppfs;
using cfs::closecppfs;
using cfs::cin;
using cfs::cout;

int main() {
    cppfs("test.in", "test.out");

    int n;
    cin >> n;
    cout << n << '\n';

    closecppfs();
    return 0;
}

Avoid using namespace cfs; in larger projects, because names such as cin and cout may become confusing when used together with std::cin and std::cout.

Two C++ Usage Modes

The C++ header provides two different file I/O modes:

Mode	Functions	Main Streams Used	Purpose
Standard stream redirection	`cfs::cfs`, `cfs::closecfs`	`std::cin`, `std::cout`, `scanf`, `printf`	Redirect standard input and output
Independent C++ file streams	`cfs::cppfs`, `cfs::closecppfs`	`cfs::cin`, `cfs::cout`	Use separate file streams without changing standard streams

These two modes are different and should not be confused.

Mode 1: Standard Stream Redirection

Function

void cfs::cfs(const char* input, const char* output);

This function redirects standard input and/or standard output.

Internally, it uses freopen.

Basic Example

#include <iostream>
#include "cfs.hpp"

int main() {
    cfs::cfs("test.in", "test.out");

    int n;
    std::cin >> n;
    std::cout << n << '\n';

    cfs::closecfs();
    return 0;
}

This program:

reads from test.in
writes to test.out

What Is Redirected

After calling:

cfs::cfs("test.in", "test.out");

the following streams/functions are affected:

std::cin
std::cout
scanf
printf

They will use the redirected standard input/output streams.

Redirect Only Input

#include <iostream>
#include "cfs.hpp"

int main() {
    cfs::cfs("test.in", NULL);

    int n;
    std::cin >> n;
    std::cout << n << '\n';

    cfs::closecfs();
    return 0;
}

Input is read from test.in, while output still goes to the console.

If you are using C++11 or later, you can also use nullptr:

cfs::cfs("test.in", nullptr);

Redirect Only Output

#include <iostream>
#include "cfs.hpp"

int main() {
    cfs::cfs(NULL, "test.out");

    int n;
    std::cin >> n;
    std::cout << n << '\n';

    cfs::closecfs();
    return 0;
}

Input still comes from the console, while output is written to test.out.

If you are using C++11 or later, you can also use nullptr:

cfs::cfs(nullptr, "test.out");

No Redirection

Both parameters can be NULL.

cfs::cfs(NULL, NULL);

In C++11 or later:

cfs::cfs(nullptr, nullptr);

This is allowed, but it has no practical effect.

Close File Redirection and Restore Console Streams

Function

void cfs::closecfs();

This function attempts to restore standard input and output back to the console.

Example:

#include <iostream>
#include "cfs.hpp"

int main() {
    cfs::cfs("test.in", "test.out");

    int n;
    std::cin >> n;
    std::cout << n << '\n';

    cfs::closecfs();

    std::cout << "Back to console" << '\n';
    return 0;
}

Platform behavior:

Platform	Console Device
Windows	`CON`
Linux / macOS / other Unix-like systems	`/dev/tty`

closecfs() uses these device names internally to restore the console streams.

Mode 2: Independent C++ File Streams

Function

void cfs::cppfs(const char* input, const char* output);

This function opens two independent C++ file streams:

cfs::cin
cfs::cout

They are separate from:

std::cin
std::cout

Basic Example

#include <iostream>
#include "cfs.hpp"

int main() {
    cfs::cppfs("test.in", "test.out");

    int n;
    cfs::cin >> n;
    cfs::cout << n << '\n';

    std::cout << "This goes to the console." << '\n';

    cfs::closecppfs();
    return 0;
}

This program:

reads n from test.in
writes n to test.out
writes the message to the console through std::cout

Streams

cfs.hpp defines:

static std::ifstream cfs::cin;
static std::ofstream cfs::cout;

Because they are file streams:

cfs::cin behaves like std::ifstream
cfs::cout behaves like std::ofstream

Example:

int x;
double y;
std::string s;

cfs::cin >> x >> y >> s;
cfs::cout << x << ' ' << y << ' ' << s << '\n';

Important Difference from Standard Redirection

cfs::cppfs() does not redirect standard streams.

This means:

std::cin
std::cout
scanf
printf

are not affected.

Only:

cfs::cin
cfs::cout

use the files opened by cfs::cppfs().

File Names Must Be Valid

For cfs::cppfs, both parameters should be valid file names.

Recommended:

cfs::cppfs("test.in", "test.out");

Do not pass NULL or nullptr to cfs::cppfs.

If you only need to redirect one side, use cfs::cfs() instead.

Reopening Protection

If cfs::cin or cfs::cout is already open, calling cfs::cppfs() again will not reopen the streams.

Example:

cfs::cppfs("a.in", "a.out");
cfs::cppfs("b.in", "b.out");

The second call will produce an error message:

Error-from-cfs Already open cppfs,please close first!

Correct usage:

cfs::cppfs("a.in", "a.out");
cfs::closecppfs();

cfs::cppfs("b.in", "b.out");
cfs::closecppfs();

Close Independent File Streams

Function

void cfs::closecppfs();

This function closes cfs::cin and cfs::cout if they are open.

Example:

#include "cfs.hpp"

int main() {
    cfs::cppfs("test.in", "test.out");

    int n;
    cfs::cin >> n;
    cfs::cout << n << '\n';

    cfs::closecppfs();
    return 0;
}

After calling cfs::closecppfs(), do not continue using cfs::cin or cfs::cout unless you call cfs::cppfs() again.

Version Query

Function

const char* cfs::getversion();

This function returns the current cfstream version as a const char*.

Example:

#include <iostream>
#include "cfs.hpp"

int main() {
    std::cout << cfs::getversion() << '\n';
    return 0;
}

For version 1.3.0, the output is:

1.3.0

Recommended Usage Style

For most C++ programs, especially competitive programming or local testing, the recommended style is:

#include "cfs.hpp"

int main() {
    cfs::cppfs("test.in", "test.out");

    int n;
    cfs::cin >> n;
    cfs::cout << n << '\n';

    cfs::closecppfs();
    return 0;
}

This keeps file I/O and console I/O separate.

Use cfs::cfs() only when you want behavior similar to freopen.

Using cfstream for Local OI Debugging

cfstream can be useful when practicing Informatics Olympiad (OI) problems locally.

For local debugging, you can redirect standard input and output to files, so that cin, cout, scanf, and printf all use the specified files.

This usage is intended only for convenient local debugging at home. It is not intended for submission on OI online judges or official contest judging machines.

Example:

#include <bits/stdc++.h>
#include "cfs.hpp"

using namespace std;

#define int long long
#define endl '\n'

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    cfs::cfs("test.in", "test.out");

    int n;
    cin >> n;
    cout << n << endl;

    int m;
    scanf("%lld", &m);
    printf("%lld\n", m);

    cfs::closecfs();
    return 0;
}

In this example:

cin reads from test.in
cout writes to test.out
scanf reads from test.in
printf writes to test.out

Because cfs::cfs() redirects the standard input and output streams, both C++-style I/O and C-style I/O use the redirected files.

For online judge submission, remove the cfstream header and all cfstream function calls unless the judge explicitly allows custom file I/O helpers.

Multiple Source Files

In cfs.hpp, cfs::cin and cfs::cout are declared with static.

This gives them internal linkage.

In practice, each translation unit that includes cfs.hpp gets its own private copy of:

cfs::cin
cfs::cout

This is intentional for cfstream.

It means file stream state is not shared globally across different .cpp files.

Common Mistakes

1. Calling `cppfs` twice without closing

Incorrect:

cfs::cppfs("a.in", "a.out");
cfs::cppfs("b.in", "b.out");

Correct:

cfs::cppfs("a.in", "a.out");
cfs::closecppfs();

cfs::cppfs("b.in", "b.out");
cfs::closecppfs();

2. Passing `NULL` or `nullptr` to `cppfs`

Incorrect:

cfs::cppfs(NULL, "test.out");

Use cfs::cfs() instead:

cfs::cfs(NULL, "test.out");

or in C++11 or later:

cfs::cfs(nullptr, "test.out");

3. Confusing `std::cin` with `cfs::cin`

cfs::cppfs() opens cfs::cin, not std::cin.

Incorrect:

cfs::cppfs("test.in", "test.out");

int n;
std::cin >> n;

Correct:

cfs::cppfs("test.in", "test.out");

int n;
cfs::cin >> n;

4. Forgetting to close file streams

Recommended:

cfs::cppfs("test.in", "test.out");

// use cfs::cin and cfs::cout

cfs::closecppfs();

Although file streams may be closed automatically when the program exits, explicitly closing them makes the program behavior clearer.

5. Expecting `closecppfs` to restore `std::cin` or `std::cout`

closecppfs() only closes cfs::cin and cfs::cout.

It does not affect standard streams.

For standard stream redirection, use:

cfs::closecfs();

Function Summary

Function / Object	Type	Description
`cfs::getversion()`	Function	Returns the current cfstream version
`cfs::cfs(input, output)`	Function	Redirects standard input/output
`cfs::closecfs()`	Function	Restores standard input/output to the console
`cfs::cppfs(input, output)`	Function	Opens independent C++ file streams
`cfs::closecppfs()`	Function	Closes independent C++ file streams
`cfs::cin`	`std::ifstream`	Input file stream used by `cppfs`
`cfs::cout`	`std::ofstream`	Output file stream used by `cppfs`

Cpp Usage

Cpp Usage

Header

Requirements

Namespace

Two C++ Usage Modes

Mode 1: Standard Stream Redirection

Function

Basic Example

What Is Redirected

Redirect Only Input

Redirect Only Output

No Redirection

Close File Redirection and Restore Console Streams

Function

Mode 2: Independent C++ File Streams

Function

Basic Example

Streams

Important Difference from Standard Redirection

File Names Must Be Valid

Reopening Protection

Close Independent File Streams

Function

Version Query

Function

Recommended Usage Style

Using cfstream for Local OI Debugging

Multiple Source Files

Common Mistakes

1. Calling cppfs twice without closing

2. Passing NULL or nullptr to cppfs

3. Confusing std::cin with cfs::cin

4. Forgetting to close file streams

5. Expecting closecppfs to restore std::cin or std::cout

Function Summary

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1. Calling `cppfs` twice without closing

2. Passing `NULL` or `nullptr` to `cppfs`

3. Confusing `std::cin` with `cfs::cin`

5. Expecting `closecppfs` to restore `std::cin` or `std::cout`