C++

Reference

<istream>

istream

public member function
<istream> <iostream>

std::istream::tellg

streampos tellg();
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.

Parameters

none

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.
streampos is an fpos type (it can be converted to/from integral types).

Errors are signaled by modifying the internal state flags:
flagerror
eofbit-
failbitThe construction of sentry failed (such as when the stream state was not good before the call).
badbitError 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

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// 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.

See also

istream::seekg
Set position in input sequence (public member function)
ostream::tellp
Get position in output sequence (public member function)