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C: FILE freopen()

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Definition
std::FILE* freopen( const char* filename, const char* mode, std::FILE* stream );

File Access Mode String

From CPPReference

File access mode string Meaning Explanation Action if file already exists Action if file does not exist
"r" read Open a file for reading read from start return NULL and set error
"w" write Create a file for writing destroy contents create new
"a" append Append to a file write to end create new
"r+" read extended Open a file for read/write read from start return NULL and set error
"w+" write extended Create a file for read/write destroy contents create new
"a+" append extended Open a file for read/write write to end create new

Note

std::freopen is the only way to change the narrow/wide orientation of a stream once it has been established by an I/O operation or by std::fwide.

Example 
#include <cstdio>
#include <iostream>
using namespace std;

int main() {
    freopen("in.in", "r", stdin);
    freopen("out.out", "w", stdout);

    int a, b;
    cin >> a >> b;
    cout << a * a + b * b << endl;
    return 0;
}

Warning

If you have used the untie operation, you shouldn't use freopen because you would have already disabled <cstdio>.

Tip

If you are using an Online Judge, you can add this macro definition to your code:

ONLINE_JUDGE
#ifdef ONLINE_JUDGE
#else
#endif

Most OJ will use this macro definition, while your computer won't have one, thus you can add freopen in ONLINE_JUDGE so that you can use freopen as it'll be disabled on your OJ.

Example 
int main() {
    #ifdef ONLINE_JUDGE
    #else   // when run on local
        freopen("in.in", "r", "stdin");
        freopen("out.out", "w", "stdout");
    #endif
}

C++: fstream fiostream

headfile: <fiostream>

ifstream in("FILENAME", ios::in);

ofstream out("FILENAME", ios::out);

fstream foi("FILENAME", ios::in|ios::out);

function open()

void open(const char * filename,
    ios_base::openmode mode = ios_base::in | ios::base::out);

void open(const wchar_t *_Filename,
    ios_base::in | ios_base::out, int prot = ios_base::_Openprot);
Parameters 
    ios::app: //open the file as append  

    ios::ate: //Locate the file to the end of the file when it is opened. ios:app includes this property.  

    ios::binary: //Open the file as binary, the default is text. See the previous section for the difference between the two methods  

    ios::in: // the file is opened as input (file data is entered into memory)  

    ios::out: //files are opened as output (memory data is output to the file)  

    ios::nocreate: //doesn't create the file, so it fails to open if the file doesn't exist  

    ios::noreplace: // doesn't overwrite the file, so opening the file fails if the file exists  

    ios::trunc: // set file length to 0 if file exists

    0: Ordinary file, open access  
    1: Read-only file  
    2: Implicit files  
    4: system file

Tip

You can use "or" or "+" to connect the above attributes, such as 3 or 1 | 2 is to open the file with read-only and implicit attributes.

Status Indicator

is_open() //whether the file is normally open
bad() // read and write process whether the error
fail() // read and write process whether the error 
eof() // read the file to reach the end of the file, return true
good () // any of the above return true, this will return false

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