Update compiler-error-c2512.md

Expand explanations to address some common confusion.
Remove examples that now generate different errors.

Author: Colin Robertson

docs/error-messages/compiler-errors-2/compiler-error-c2512.md

@@ -18,74 +18,30 @@ manager: "ghogen"
 ms.workload: ["cplusplus"]
 ---
 # Compiler Error C2512
-'identifier' : no appropriate default constructor available  
-  
- No default constructor is available for the specified class, structure, or union. The compiler supplies a default constructor only if user-defined constructors are not provided.  
-  
- If you provide a constructor that takes a non-void parameter, and you want to allow your class to be created with no parameters—for example, as the elements of an array—you must also provide a default constructor. The default constructor can be a constructor with default values for all parameters.  
-  
- The following sample generates C2512 and shows how to fix it:  
-  
-```  
-// C2512.cpp  
-// C2512 expected  
-struct B {  
-   B (char *);  
-   // Uncomment the following line to fix.  
-   // B() {};  
-};  
-  
-int main() {  
-   B b;   
-}  
-```  
-  
- The following sample shows a more subtle C2512. To fix this error, rearrange the code to define the class before its constructor is referenced:  
-  
-```  
-// C2512b.cpp  
-// compile with: /c  
-struct S {  
-   struct X;  
-  
-   void f() {  
-      X *x = new X();   // C2512 X not defined yet  
-   }  
-  
-};  
-  
-struct S::X {};  
-  
-struct T {  
-   struct X;  
-   void f();  
-};  
-  
-struct T::X {};  
-  
-void T::f() {  
-   X *x = new X();  
-}  
-```  
-  
- C2512 can also be caused by a call to default-construct a class that contains a const or reference data member. These members must be initialized in a constructor initializer list, which prevents the compiler from generating a default constructor.  
-  
- The following sample generates C2512 and shows how to fix it:  
-  
-```  
-// C2512c.cpp  
-// Compile by using: cl /c /W3 C2512c.cpp  
-struct S {  
-   const int i_;  
-   int &r_;   
-};   
-  
-int SumS() {  
-   const int ci = 7;  
-   int ir = 42;  
-  
-   S s1; // C2512 - no default constructor available  
-   S s2{ci, ir};  // Fix - initialize const and reference members   
-   return s2.i_ + s2.r_;  
-}  
-```
+
+> '*identifier*' : no appropriate default constructor available  
+
+A *default constructor*, a constructor that requires no arguments, is not available for the specified class, structure, or union. The compiler supplies a default constructor only if no user-defined constructors are provided.
+
+If you provide a constructor that takes a non-void parameter, and you want to allow your class to be created with no parameters (for example, as the elements of an array), you must also provide a default constructor. The default constructor can be a constructor with default values for all parameters.
+
+## Example
+
+A common cause of error C2512 is when you define a class or struct constructor that takes arguments, and then you attempt to declare an instance of your class or struct without any arguments. For example, `struct B` below declares a constructor that requires a `char *` argument, but not one that takes no arguments. In `main`, an instance of B is declared, but no argument is supplied. The compiler generates C2512 because it can't find a default constructor for B.
+
+```cpp
+// C2512.cpp
+// Compile with: cl /W4 c2512.cpp
+// C2512 expected
+struct B {
+   B (char *) {}
+   // Uncomment the following line to fix.
+   // B() {}
+};
+
+int main() {
+   B b;   // C2512 - This requires a default constructor
+}
+```
+
+You can fix this issue by defining a default constructor for your struct or class, such as `B() {}`, or a constructor where all the arguments have default values, such as `B (char * = nullptr) {}`.