==, != operator overloading

练习12.20的 sbp != sb.end() 报错，原因是无法对两个StrBlobPtr进行默认的 != 操作，需要对该运算符进行重载。

Author: ZainChenDev

excersize/ch12.md

@@ -544,6 +544,8 @@ private:
 
 class StrBlobPtr
 {
+	friend bool operator==(const StrBlobPtr&,const StrBlobPtr&);
+    	friend bool operator!=(const StrBlobPtr&,const StrBlobPtr&);
 public:
 	StrBlobPtr() :curr(0) {}
 	StrBlobPtr(StrBlob &a, size_t sz = 0) :wptr(a.data), curr(sz) {}
@@ -580,6 +582,17 @@ StrBlobPtr StrBlob::end()
 {
 	return StrBlobPtr(*this, data->size());
 }
+
+bool operator==(const StrBlobPtr& lhs, const StrBlobPtr& rhs) {
+    if (!lhs.wptr.expired() && !rhs.wptr.expired()) {
+        return (lhs.wptr.lock() == rhs.wptr.lock()) && (lhs.curr == rhs.curr);
+    } else {
+        return false;
+    }
+}
+bool operator!=(const StrBlobPtr& lhs, const StrBlobPtr& rhs) {
+    return !(lhs==rhs);
+}
 ```
 
 ## 练习12.20