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I am writing an Arduino program that uses Bluetooth on Serial1 to print text to a Bluetooth terninal on an Android phone and also normal Serial to print text to the serial monitor on a laptop. I would like to wrap the Serial.print() and Serial.println() functions so that they work with either or both Serial and Serial1. For example the code below works fine depending on the values of the global variables. But this only works for single chars, but print() and println() can take a very wide variety of datatypes. If I also define overloading functions for int and String types it works fine, but that is a very verbose and maybe fragile solution, it also ignores the optional inputs to the underlying functions. What is the proper way to do this ?

void print(char x) 
 if (g_use_Serial)
 Serial.print(x);
 if (g_use_Serial1)
 Serial1.print(x);


void println(char x) 
 if (g_use_Serial)
 Serial.println(x);
 if (g_use_Serial1)
 Serial1.println(x);

I have implemented the solution suggested and it works nearly perfectly. There seems to be a problem with the new line characters on the bluetooth (Serial1) connection.

The code is:

class DualPrint : public Print

public:
 DualPrint() : use_Serial(false), use_Serial1(false) 
 virtual size_t write(uint8_t c) 
 if (use_Serial) Serial.write(c);
 if (use_Serial1) Serial1.write(c);
 return 1;
 
 bool use_Serial, use_Serial1;
 out;

void setup() 
 Serial.begin(9600);
 Serial1.begin(9600);
 delay(1000); 


void loop() 

 out.use_Serial = true;
 out.print("Printed to USB only.n");
 out.use_Serial1 = true;
 out.print("Printed to both USB and BT.n");
 out.use_Serial = false;
 out.print("Printed to BT only.n");
 out.use_Serial1 = false;

 delay(3000);

The output on the USB Serial monitor is as expected:

Printed to USB only.
Printed to both USB and BT.
Printed to USB only.
Printed to both USB and BT.

The output on the Bluetooth terminal is not as expected, the new lines are not in the right position:

Printed to both USB and BT.Pri
nted to BT only.
Printed to both USB and BT.Pr
inted to BT only.
Printed to both USB and BT.Pr
inted to BT only.
Printed to both USB and BT.Pr
inted to BT only.

Any suggestions on how to fix this. Are there other function members that need to be re-implemented to make this work properly?

Many thanks for high quality answers so far.

You can create a class derived from Print that forwards its output to
either or both Serial and Serial1. The only method you need to
implement for this to work is write(uint8_t):

class DualPrint : public Print

public:
 DualPrint() : use_Serial(false), use_Serial1(false) 
 virtual size_t write(uint8_t c) 
 if (use_Serial) Serial.write(c);
 if (use_Serial1) Serial1.write(c);
 return 1;
 
 bool use_Serial, use_Serial1;
 out;

You would use it like this:

out.use_Serial = true;
out.println("Printed to Serial only");
out.use_Serial1 = true;
out.println("Printed to both Serial and Serial1");
out.use_Serial = false;
out.println("Printed to Serial1 only");

Note that with this approach, unlike yours, printing number will format
them as text only once, and the underlying Serial and Serial1 will
only handle the resulting characters.

Edit: To answer the question in OP's comment, the construct

class ClassName

 ...definition...
 classInstace;

is a shotcut for

class ClassName

 ...definition...
;
ClassName classInstace;

A very similar construct exists in plain C:

struct struct_name 
 ...the struct fields...
 struct_instance;