public member function
<istream> <iostream>
Get position in input sequence
Returns the position of the current character in the input stream.
Internally, the function accesses the input sequence by first constructing a sentry object (with noskipws set to true) without evaluating it. Then, if member fail returns true, the function returns -1.
Otherwise, returns rdbuf()->pubseekoff(0,cur,in). Finally, it destroys the sentry object before returning.
Notice that the function will work even if the eofbit flag is set before the call.
Calling this function does not alter the value returned by gcount.
Return Value
The current position in the stream.
If either the stream buffer associated to the stream does not support the operation, or if it fails, the function returns -1.
Member type pos_type is determined by the character traits: generally, it is an fpos type (such as streampos) that can be converted to/from integral types.
Errors are signaled by modifying the internal state flags:
| flag | error |
|---|
| eofbit | - |
| failbit | The construction of sentry failed (such as when the stream state was not good before the call). |
| badbit | Error on stream (such as when this function catches an exception thrown by an internal operation).
When set, the integrity of the stream may have been affected. |
Multiple flags may be set by a single operation.
If the operation sets an internal state flag that was registered with member exceptions, the function throws an exception of member type failure.
Example
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
|
// read a file into memory
#include <iostream> // std::cout
#include <fstream> // std::ifstream
int main () {
std::ifstream is ("test.txt", std::ifstream::binary);
if (is) {
// get length of file:
is.seekg (0, is.end);
int length = is.tellg();
is.seekg (0, is.beg);
// allocate memory:
char * buffer = new char [length];
// read data as a block:
is.read (buffer,length);
is.close();
// print content:
std::cout.write (buffer,length);
delete[] buffer;
}
return 0;
}
| |
In this example, tellg is used to get the position in the stream after it has been moved with seekg to the end of the stream, therefore determining the size of the file.
Data races
Modifies the stream object.
Concurrent access to the same stream object may cause data races.
Exception safety
Basic guarantee: if an exception is thrown, the object is in a valid state.
It throws an exception of member type failure if the resulting error state flag is not goodbit and member exceptions was set to throw for that state.
Any exception thrown by an internal operation is caught and handled by the function, setting badbit. If badbit was set on the last call to exceptions, the function rethrows the caught exception.