Doing a std::endl before AllocConsole causes no display of std::cout

Question

I'm trying to redirect traces on an output console on Windows Visual 2012, Linker/Subsystem = Windows (/SUBSYSTEM:WINDOWS), using a classical RedirectIOToConsole function.

Performing std::endl before AllocConsole seems to causes problems to display traces.

Below is my test:

#include <windows.h>     
#include <stdio.h>
#include <iostream>  
void RedirectIOToConsole()
{
  FILE *conin, *conout; 
  AllocConsole();
  freopen_s(&conin, "conin$", "r", stdin);
  freopen_s(&conout, "conout$", "w", stdout);
  freopen_s(&conout, "conout$", "w", stderr);
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)
{
  // std::cout << "My Trace 1";              // Uncomment this line for Test 1
  // std::cout << "My Trace 1" << std::endl; // Uncomment this line for Test 2
  RedirectIOToConsole();
  printf( "redirected console\n");
  std::cout << "My Trace 2" << std::endl;
  ch = getchar();
  return 0;
}

• Run it as it is - output in Console window:

redirected console

My Trace 2 -> OK

• Uncomment line for test 1 - output in Console window:

redirected console

My Trace 2 -> OK

• Uncomment line for test 2 - output in Console window:

redirected console -> NOK

Answer 1

Before you call RedirectIOToConsole() there is nothing for cout to output onto. Since cout is buffered, the output is stored in a buffer until a flush is performed. endl contains a flush .

When you try to flush the output with no console available, the cout stream sets the badbit in the iostate .

When you then do the next step of cout << "My Trace 2" << endl; the output is not proceeding because before actually writing the data to the output file (stdout), cout checks the iostate and says "Oh, this is not good, we have badbit set" and bails out without trying to write.

You could, in theory, fix this by calling cout.clear(); , but I would suggest that it's a better plan to not call cout before you have allocated a console - it serves no purpose to output things before you have something to output to.