c++ ifstream undeclared identifier

Question

I am in Visual Studio and am getting 'ifstream undeclared identifier' with this code (same for ofstream)

#include <iostream>
#include <iomanip>
#include <fstream>
void main()
{
    ifstream infile("file.txt");
    ofstream outfile("out.txt");
}

do I need to include something else?

[The correct return type for `main` is `int`.](http://stackoverflow.com/questions/4207134/what-is-the-proper-declaration-of-main/4207223#4207223) — James McNellis, May 09 '11 at 02:43

score 45 · Accepted Answer · answered May 09 '11 at 02:41

45

You need to scope it. Use using namespace std; or preface ifstream and ostream with std::

For example, std::ifstream

Currently, the compiler does not know where these structures are defined (since they are declared/defined within the std namespace). This is why you need to scope your structures/functions in this case.

answered May 09 '11 at 02:41

RageD

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5

Or add `using std::ifstream; using std::ofstream;` above where you use them. – Seth Carnegie May 09 '11 at 02:44
@rach if this is the answer, make sure to mark it so. Click the check mark underneath the up and down arrows. – Seth Carnegie May 09 '11 at 02:47

score 6 · Answer 2 · answered May 09 '11 at 02:42

6

You need to reference the standard namespace (std). Try this:

#include <iostream>
#include <iomanip>
#include <fstream>
void main()
{
    std::ifstream infile("file.txt");
    std::ofstream outfile("out.txt");
}

answered May 09 '11 at 02:42

jwismar

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score 1 · Answer 3 · edited May 12 '15 at 15:59

1

You can use

using namespace std;

instead of prefixing everyline with std::

edited May 12 '15 at 15:59

Bronumski

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answered May 12 '15 at 15:40

Ram Mokkapati

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1

Always pay attention to hint people to where exactly they can use "using namespace". It's alright inside your .cpp source, but could be horrible in the header. – Cadoiz Apr 15 '19 at 23:06

c++ ifstream undeclared identifier

3 Answers3