input and output questions about file

[Dic4000]

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#include<iostream>
#include<conio.h>
#include<fstream>


using namespace std;


int main()
{
fstream file("a.txt",fstream::in|fstream:ut|fstream::app);
string s1,s2;


if(!file) cerr<<"error"<<endl; //why the program have
//error when connecting
up "a.txt"


s1="abcd 1234\n";
file<<s1; //why don't write in a.txt?
file.flush();
file.seekg(0);
file>>s2;
cout<<"s2="<<s2<<endl;//s2 is empty


file.close();


getch();
return 0;}


the program can run without any error,but the result is:


error


s2=


the program don't create file "a.txt",How do i do?

 

[John Harrison]

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Ö»Ï붨ÒåÒ»¸öfstreamÀàÐÍÀ´Íê³ÉÊäÈëÊä³öµÄ¹¤×÷.
#include<iostream>
#include<conio.h>
#include<fstream>


using namespace std;


int main()
{
fstream file("a.txt",fstream::in|fstream:ut|fstream::app);
string s1,s2;


if(!file) cerr<<"error"<<endl; //why the program have
//error when connecting
up "a.txt"


s1="abcd 1234\n";
file<<s1; //why don't write in a.txt?
file.flush();
file.seekg(0);
file>>s2;
cout<<"s2="<<s2<<endl;//s2 is empty


file.close();


getch();
return 0;}


the program can run without any error,but the result is:


error


s2=


the program don't create file "a.txt",How do i do?

ofstream file("a.txt");

The file doesn't get created because you said you wanted to read from it
(that's what fstream::in does). Use ofstream for files you want to write
to, and ifstream from files you want to read from.

john

 

[Dic4000]

ofstream file("a.txt");

The file doesn't get created because you said you wanted to read from it
(that's what fstream::in does). Use ofstream for files you want to write
to, and ifstream from files you want to read from.

john- Òþ²Ø±»ÒýÓÃÎÄ×Ö -

- ÏÔʾÒýÓõÄÎÄ×Ö -

but I wanna use fstream type ,not ifstream or ofstream type,only use
it to complete input and output working.

 

[Victor Bazarov]

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Ö»Ï붨ÒåÒ»¸öfstreamÀàÐÍÀ´Íê³ÉÊäÈëÊä³öµÄ¹¤×÷.
#include<iostream>
#include<conio.h>
#include<fstream>


using namespace std;


int main()
{
fstream file("a.txt",fstream::in|fstream:ut|fstream::app);
string s1,s2;


if(!file) cerr<<"error"<<endl; //why the program have
//error when connecting
up "a.txt"


s1="abcd 1234\n";
file<<s1; //why don't write in a.txt?
file.flush();
file.seekg(0);
file>>s2;
cout<<"s2="<<s2<<endl;//s2 is empty


file.close();


getch();
return 0;}


the program can run without any error,but the result is:


error

Doesn't that contradict what you just wrote? How can the program
run without any error, yet display "error"?

s2=


the program don't create file "a.txt",How do i do?

I am not sure, but I think 'ios::in' is mutually exclusive with
'ios::app'. Try droppint 'ios::app'.

V

 

[Chris Theis]

Doesn't that contradict what you just wrote? How can the program
run without any error, yet display "error"?


I am not sure, but I think 'ios::in' is mutually exclusive with
'ios::app'. Try droppint 'ios::app'.

You're absolutely right on this one.

To the OP:

What you've been looking for is the stdio equivalent to "a+". However, the
standard mandates that only certain combinations of file open modes can be
supplied (see Table 92 of ISO:IEC 14882:2003(E)). Otherwise, your test of
the stream will fail as you've experienced. As Victor suggested already,
ios::app can only be used if neither ios::in nor ios::trunc is given.

By default there is a standard ctor for fstreams with the open mode set to
ios::in | ios:ut and this should do the trick. You actually won't need to
supply the flags yourself.

HTH
Chris

 

[BobR]

You're absolutely right on this one.

To the OP:

What you've been looking for is the stdio equivalent to "a+". However, the
standard mandates that only certain combinations of file open modes can be
supplied (see Table 92 of ISO:IEC 14882:2003(E)). Otherwise, your test of
the stream will fail as you've experienced. As Victor suggested already,
ios::app can only be used if neither ios::in nor ios::trunc is given.

By default there is a standard ctor for fstreams with the open mode set to
ios::in | ios:ut and this should do the trick. You actually won't need to
supply the flags yourself.

HTH
Chris

Add: For the 'app' part, you might try:

file.seekp( 0, std::ios::end ); // note: not 'seekg'.

file<<"output stuff";