refactor: prefer passing by reference instead of copy (synfig#3637)

Author: rodolforg

synfig-core/src/synfig/color/color.cpp

@@ -53,7 +53,7 @@ using namespace synfig;
 /* === M E T H O D S ======================================================= */
 
 ColorReal
-Color::hex2real(String s)
+Color::hex2real(const String& s)
 {
 	std::istringstream i(s);
 	int n;
@@ -76,7 +76,7 @@ Color::real2hex(ColorReal c)
 }
 
 void
-Color::set_hex(String& str)
+Color::set_hex(const String& str)
 {
 	value_type r, g, b;
 	String hex;

synfig-core/src/synfig/color/color.h

@@ -111,7 +111,7 @@ class Color
 	const value_type& get_alpha()const { return get_a(); }
 
 	//! Converts a 2 character hex string \a s (00-ff) into a ColorReal (0.0-1.0)
-	static ColorReal hex2real(String s);
+	static ColorReal hex2real(const String& s);
 
 	//! Converts a ColorReal \a c (0.0-1.0) into a 2 character hex string (00-ff)
 	static const String real2hex(ColorReal c);
@@ -120,7 +120,7 @@ class Color
 	inline const String get_hex()const;
 
 	//! Sets the color's R, G, and B from a 3 or 6 character hex string
-	void set_hex(String& hex);
+	void set_hex(const String& hex);
 
 	//! Sets the RED component to \a x
 	Color& set_r(const value_type& x) { r_ = x; return *this; }