Interleaving the standard streams with sync_with_stdio()

The sync_with_stdio() function allows you to interleave C standard streams with standard streams from either the Standard C++ Library or the USL I/O Stream Class Library. A call to sync_with_stdio() does the following:

cin, cout, cerr, and clog are initialized with stdiobuf objects associated with stdin, stdout, and stderr.
The flags unitbuf and stdio are set for cout, cerr, and clog.

This ensures that subsequent standard streams may be mixed on a per-character basis. However, a runtime performance penalty is incurred to ensure this synchronization. Figure 1 shows an example program and the output that it produces.

Figure 1. Interleaving I/O with sync_with_stdio()

//
// Example of interleaving USL I/O with sync_with_stdio()
//
// tsyncws.cxx
#include <stdio.h>
#include <fstream.h>

int main() {
    ios::sync_with_stdio();
    cout << "object: to show that sync_with_stdio() allows interleaving\n    "
            "    standard input and output on a per character basis\n" << endl;

    printf( "line 1 ");
    cout << "rest of line 1\n";
    cout << "line 2 ";
    printf( "rest of line 2\n\n");

    char string1[80] = "";
    char string2[80] = "";
    char string3[80] = "";
    char* rc = NULL;

    cout << "type the following 2 lines:\n"
            "hello world, here I am\n"
            "again\n" << endl;

    cin.get(string1[0]);
    string1[1] = getchar();
    cin.get(string1[2]);

    cout << "\nstring1[0] is \'" << string1[0] <<
"\'\n"
         <<   "string1[1] is \'" << string1[1] <<
"\'\n"
         <<   "string1[2] is \'" << string1[2] <<
"\'\n" << endl;

    cin  >> &string1[3];
    rc    = gets(string2);  // note: reads to end of line, so
    cin  >> string3;        // this line waits for more input

    cout << "\nstring1 is \"" << string1 << "\"\n"
         <<   "string2 is \"" << string2 << "\"\n"
         <<   "string3 is \"" << string3 << "\"\n" <<
flush;
}

// sample output (with user input shown underlined):
//
// object: to show that sync_with_stdio() allows interleaving
//         standard input and output on a per character basis
//
// line 1 rest of line 1
// line 2 rest of line 2
//
// type the following 2 lines:
// hello world, here I am
// again
//
// hello world, here I am
//
// string1[0] is 'h'
// string1[1] is 'e'
// string1[2] is 'l'
//
// again
//
// string1 is "hello"
// string2 is "world, here I am"
// string3 is "again"