Update to use reference, not pointer. Improve documentation.

Author: phillipjohnston

ArduinoPrintf.cpp

@@ -1,19 +1,17 @@
 #include "ArduinoPrintf.h"
 #include "Arduino.h"
 
-static Print* print_instance = &Serial;
+static Print& print_instance = Serial;
 
-void printf_init(Print* PrintClass)
+void printf_init(Print& PrintClass)
 {
-	if(PrintClass)
-	{
-		print_instance = PrintClass;
-	}
+	print_instance = PrintClass;
 }
 
+
 // If you use the default printf() implementation, this function will route the output
 // to the Serial class
 extern "C" __attribute__((weak)) void _putchar(char character)
 {
-	print_instance->print(character);
+	print_instance.print(character);
 }

ArduinoPrintf.h

@@ -4,8 +4,12 @@
 #include "Print.h"
 #include "printf.h"
 
-// In Setup(), you must initialize printf with a serial class.
-// You must also configure the Serial interface in setup() and call begin().
-void printf_init(Print* StreamClass = nullptr);
+// In Setup(), you must initialize printf with a Print class if you don't want
+// to use the default Serial object. If you want the default behavior, calling this
+// function is not necessary.
+//
+// The caller is responsible for configure the Serial interface in setup() and calling
+// Serial.begin().
+void printf_init(Print& StreamClass);
 
 #endif //ARDUINO_PRINTF_H_

README.md

@@ -4,12 +4,18 @@ This library adds support for the `printf()` function to Arduino projects. This
 
 ## Using the Library
 
+To use this library in your Arduino project, you need to include the header:
+
+```
+#include ArduinoPrintf.h
+```
+
 By default, the library can be used without any special initialization. The `Serial` object is the default output target. You must still initialize the `Serial` object in `setup()`, the library will not do this for you.
 
 You can specify any class derived from the `Print` base class for use with `printf()`. To change the output class, use the `printf_init` function in `setup()`:
 
 ```
-printf_init(&Serial1);
+printf_init(Serial1);
 Serial1.begin(115200);
 ```
 
@@ -36,6 +42,10 @@ void _putchar(char character)
 
 And your prototype will be used instead of the library's version.
 
+## Advanced Use
+
+You can include `printf.h` directly and supply your own implementation of `_putchar`. This approach is useful if you want to use the library in a test suite (skipping Arduino SDK headers).
+
 ## Disabling Specific Formats
 
 If memory footprint is critical, floating point, exponential and 'long long' support and can be turned off via the `PRINTF_DISABLE_SUPPORT_FLOAT`, `PRINTF_DISABLE_SUPPORT_EXPONENTIAL` and `PRINTF_DISABLE_SUPPORT_LONG_LONG` compiler switches. You must define these symbols in the build system.

examples/specify_print_class/specify_print_class.ino

@@ -3,7 +3,7 @@
 void setup() {
   // Specify the print class to use with printf().
   // Any class derived from Print will work.
-  printf_init(&Serial1);
+  printf_init(Serial1);
 
   // But the important detail: you are responsible for initializing the interface!
   Serial1.begin(115200);