How to disable printf function?

Question

I have three files as below

Test.cpp

void helloworld()
{
    disable pf;
    pf.Disable();
    printf("No statement \n");
    }
int main()
{
    disable dis;
    helloworld();
    printf("Hello World");
    system("pause");
    return 0;
}

disable.cpp

    #include "StdAfx.h"
    #include "disable.h"
    disable::disable(void)
    {#define printf(fmt, ...) (0)}
    disable::~disable(void)
   {}
   void disable::Disable()
   {
    #define printf(fmt, ...) (0)
    }

disable.h

#pragma once
class disable
{
public:
    disable(void);
    ~disable(void);
    void Disable();
};

After executing, I am getting output as No Statement Hello World. But I would like to disable these two printf statements by calling Disable function and disable constructor.. Please help me why it is not working and how to solve this. Please help.

But things works fine if I do like

main()
{
#define printf(fmt, ...) (0)
printf("Hello World");
}

But why not if I am calling it from a function?

Answer 1

You can disable the printf ouput by:

close(STDOUT_FILENO);

or you can use also:

fclose(stdout);

This will disable all output to the stdout

Example:

#include<stdio.h>
#include<stdlib.h>

int main(){
    printf ("This message will be displayed\n");
    fclose(stdout);
    printf ("This message will not be displayed\n");
    // to reopen the stdout, this is another question
    return 0;
}

Note

If you are using sockets in your program, than you have to be careful here because the close of stout will cause the redirection of the output to the sockets

Answer 2

A macro doesnt obey scope rules, c++ syntax rules, or anything. It is a text replacement engine, only.

When you say #define printf(fmt, ...) (0) in disable.cpp, it is defined ONLY in disable.cpp. If you were to write that in disable.h, it would be defined in all files that include from disable.h.

The only way to control a macro is with a macro (#if and #ifdef and their ilk). So what you want to to can be achieved by the following.

#define DISABLE_PRINTF

#ifdef DISABLE_PRINTF
    #define printf(fmt, ...) (0)
#endif

But this will be a global disable and can only be undone by commenting out the first #define and recompiling the code. There is no way to do selective/scope based control of disabling using macros.

Edit: Instead of redefining printf itself, it is recommended to write a wrapper which is defined in terms of printf for this purpose.