wrong about fstream file(s.c_str(),std::ios_base::in | std::ios_base::app);

[DaVinci]

//error happen even when there existe a a.txt file
#include<fstream>
#include <string>
#include <iostream>
using std::string;
using std::fstream;
int main()
{
string s = "a.txt";
fstream file(s.c_str(),std::ios_base::in | std::ios_base::app);
if(!file)
{
std::cerr<<"error happen\n";
}
else
{
string readToStr;
file>>readToStr;
file<<"abc";
std::cout<<"successed"<<std::endl;
}
}

the output is alway "error happen" even when There exist a file named
a.txt.

what's the problem is ?
thanks very much.

 

[Sergiy Kanilo]

fstream file(s.c_str(),std::ios_base::in | std::ios_base::app);

Combination std::ios_base::in | std::ios_base::app is not valid
for opening the file and causes the failure.

Cheers,
Serge

 

[Fraser Ross]

"Sergiy Kanilo"

Combination std::ios_base::in | std::ios_base::app is not valid
for opening the file and causes the failure.

I think it is. out is implied by app.

What compiler and STL implementation is being used?

Fraser.


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[Alex Buell]

if(!file)

Try if (!file.is_open()) instead.

 

[BobR]

Fraser Ross wrote in message

"Sergiy Kanilo"

I think it is. out is implied by app.

What compiler and STL implementation is being used?
Fraser.

DejaVu all over again! <G>

using std::cout;
std::string s = "za.txt";
{ std:fstream Tfile(s.c_str()); Tfile<<s<<std::endl; }
{
std::fstream file(s.c_str(), std::ios_base::in | std::ios_base::app);
if(!file){ cout<<"1-error happen\n"; }
else{ // snipped content here
cout<<"1-successed"<<std::endl;
}
}
{
std::fstream file(s.c_str(), std::ios_base::in);
if(!file){ cout<<"2-error happen\n"; }
else{
std::string readToStr;
file>>readToStr; // snipped file write
cout<<"2-successed"<<std::endl;
}
}
{
std::fstream file(s.c_str(), std::ios_base:ut | std::ios_base::app);
if(!file){ cout<<"3-error happen\n"; }
else{
file<<"3-abc";
cout<<"3-successed"<<std::endl;
}
}
// --- output ---
// 1-error happen
// 2-successed // file contents: za.txt
// 3-successed // file contents: za.txt'\n' 3-abc

GCC g++ 3.3.1 MinGW

 

[DaVinci]

Combination std::ios_base::in | std::ios_base::app is not valid
for opening the file and causes the failure.

why?
fstream file(s.c_str(),std::ios_base::in | std::ios_base:ut) is ok.
the std::ios_base:ut and std::ios_base::app are both used to write
to a file.
The difference is just one will overwrite and the other will append.
I think there is no much difference between them,why one can the other
can't.

 

[DaVinci]

Try if (!file.is_open()) instead.

it doesn't work.
so that mean the file was not opened at all.

 

[Fraser Ross]

"Fraser Ross"

I think it is. out is implied by app.

My Rogue Wave library does not imply out when app is specified.

Fraser.