Add support for variadic Serial.print(...)

Allows combining multiple arguments into a single call to Print::print[ln], like Serial.print("Answer=", 66, HEX).
This feature requires C++11 or newer to work, but if it is not available the old calls can still be used (the change has been wrapped in `#if __cplusplus >= 201103L ... #endif`).
This change is backwards compatible: print(42, 16) prints "2A" as always, not "4216" (the latter can still be achieved with print(42, "", 16) ).  This also works with multiple arguments: any numeric argument followed by an int will be treated as a (number, format) pair.
The variadic function calls are inlined using an __attribute__ ((__always_inline__)), so they're equivalent to writing multiple sequential calls to print with one argument.

hardware/arduino/avr/cores/arduino/Print.h

@@ -77,8 +77,38 @@ class Print
 
     size_t println(void);
 
+#if __cplusplus >= 201103L
+    template<typename ...Ts> _always_inline size_t println(const Ts &...args) { size_t t = print(args...); return t + println(); }
+#else
     template<typename T> _always_inline size_t println(const T &arg)      { size_t t = print(arg);  return t + println(); }
     template<typename T> _always_inline size_t println(const T &n, int f) { size_t t = print(n, f); return t + println(); }
+#endif // __cplusplus >= 201103L
+
+
+    /** Variadic methods **/
+#if __cplusplus >= 201103L  // requires C++11
+    // Ensure there are at least two parameters to avoid infinite recursion.
+    // e.g. `StringSumHelper s; print(s)` may be treated as `print(s, ...)` 
+    // with `...` being the empty list, thus calling `print(s)` again.
+    // (This is because print(StringSumHelper) isn't explicitly defined.)
+    template<typename T, typename T2, typename ...Ts>
+    _always_inline size_t print(const T &arg, const T2 &arg2, const Ts &...args) {
+      size_t t = print(arg);
+      return t + print(arg2, args...);
+    }
+    // Some methods take an extra int parameter.  If so, use these templates.
+    // In a future, it would be nice to make the base/precision a special type.
+    template<typename ...Ts> _always_inline size_t print(  signed char  n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(  signed short n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(  signed int   n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(  signed long  n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(unsigned char  n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(unsigned short n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(unsigned int   n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(unsigned long  n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(    float      n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(    double     n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+#endif // __cplusplus >= 201103L
 };
 
 #undef _always_inline

hardware/arduino/sam/cores/arduino/Print.h

@@ -77,8 +77,38 @@ class Print
 
     size_t println(void);
 
+#if __cplusplus >= 201103L
+    template<typename ...Ts> _always_inline size_t println(const Ts &...args) { size_t t = print(args...); return t + println(); }
+#else
     template<typename T> _always_inline size_t println(const T &arg)      { size_t t = print(arg);  return t + println(); }
     template<typename T> _always_inline size_t println(const T &n, int f) { size_t t = print(n, f); return t + println(); }
+#endif // __cplusplus >= 201103L
+
+
+    /** Variadic methods **/
+#if __cplusplus >= 201103L  // requires C++11
+    // Ensure there are at least two parameters to avoid infinite recursion.
+    // e.g. `StringSumHelper s; print(s)` may be treated as `print(s, ...)` 
+    // with `...` being the empty list, thus calling `print(s)` again.
+    // (This is because print(StringSumHelper) isn't explicitly defined.)
+    template<typename T, typename T2, typename ...Ts>
+    _always_inline size_t print(const T &arg, const T2 &arg2, const Ts &...args) {
+      size_t t = print(arg);
+      return t + print(arg2, args...);
+    }
+    // Some methods take an extra int parameter.  If so, use these templates.
+    // In a future, it would be nice to make the base/precision a special type.
+    template<typename ...Ts> _always_inline size_t print(  signed char  n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(  signed short n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(  signed int   n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(  signed long  n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(unsigned char  n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(unsigned short n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(unsigned int   n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(unsigned long  n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(    float      n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+    template<typename ...Ts> _always_inline size_t print(    double     n, int f, const Ts &...args) { size_t t = print(n, f); return t + print(args...); }
+#endif // __cplusplus >= 201103L
 };
 
 #undef _always_inline