std::basic_ios<CharT,Traits>::tie

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< cpp‎ | io‎ | basic ios
 
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std::basic_ios
 
std::basic_ostream<CharT, Traits>* tie() const;
 (1)
std::basic_ostream<CharT, Traits>* tie( std::basic_ostream<CharT, Traits>* str );
 (2)

Manages the tied stream. A tied stream is an output stream which is synchronized with the sequence controlled by the stream buffer (rdbuf()), that is, flush() is called on the tied stream before any input/output operation on *this.

1) Returns the current tied stream. If there is no tied stream, a null pointer is returned.
2) Sets the current tied stream to str. Returns the tied stream before the operation. If there is no tied stream, a null pointer is returned. If str is not null and tie() is reachable by traversing the linked list of tied stream objects starting from str->tie(), the behavior is undefined.

Parameters

str - an output stream to set as the tied stream

Return value

The tied stream, or a null pointer if there was no tied stream.

Exceptions

May throw implementation-defined exceptions.

Notes

By default, the standard stream std::cout is tied to std::cin and std::cerr. Similarly, its wide counterpart std::wcout is tied to std::wcin and std::wcerr.

Example

Run this code
#include <fstream>
#include <iomanip>
#include <iostream>
#include <string>
 
int main()
{
    std::ofstream os("test.txt");
    std::ifstream is("test.txt");
    std::string value("0");
 
    os << "Hello";
    is >> value;
 
    std::cout << "Result before tie(): " << std::quoted(value) << "\n";
    is.clear();
    is.tie(&os);
 
    is >> value;
 
    std::cout << "Result after tie(): " << std::quoted(value) << "\n";
}

Output: 

Result before tie(): "0"
Result after tie(): "Hello"

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR Applied to Behavior as published Correct behavior
LWG 835 哋它亢++98 two streams could be tied to each other[1]
(either directly or through another intermediate stream object) 		the behavior is undefined in this case
  1. ↑ std::basic_ostream::flush() is an UnformattedOutputFunction, so it creates a sentry object while being called. When flush() is called on a stream object, the constructor of the sentry object will call flush() on its tied stream, and that flush() will construct another sentry object and its constructor will call flush() on the tied stream of that stream and so on. Therefore, if streams a and b are (directly or indirectly) tied to each other, calling a.flush() will eventually call b.flush(), which will eventually call a.flush(), and will result in an infinite loop.
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