Break C ++ Const rules and cconst rules

Break C ++ Const rules and cconst rules

Starting with a C ++ cainiao Function

1 CString MyClass::GetStringValue() const2 {3 　　return m_strValue;    4 }

This value may not have been assigned a value. Well, I will first judge whether it is null. If it is null, it will be assigned a value.

CString MyClass::GetStringValue() const{ 　　if(m_strValue.IsEmpty()) 　　 　　SetStringValue();   　　return m_strValue;    }

As a result, the compilation fails because there is a rule that the const function cannot call non-const functions.

 

See the following compilation error:

Error C2662: "MyClass: SetStringValue": the "this" pointer cannot be converted from "const MyClass" to "MyClass &"

 

Hey, it turns out that when we define a member function of a class, a this pointer will be passed in by default, but if it is a const function, will it be passed to a const pointer? So I want the compiler to see this:

CString MyClass::GetStringValue(const MyClass* this){    if(this->m_strValue.IsEmpty())        this->SetStringValue();    return this->GetStringValue();}

After verification, we created a static function to simulate it:

CString MyClass::GetStringValueS(const MyClass* mcp){    if(mcp->m_strValue.IsEmpty())        mcp->SetStringValue();    return mcp->GetStringValue();}

Compile to get the same error:

Error C2662: "MyClass: SetStringValue": the "this" pointer cannot be converted from "const MyClass" to "MyClass &"

 

So I tried to break the const rule:

CString MyClass::GetStringValue()const{    MyClass * nonconstthis = (MyClass *)this;    if(m_strValue.IsEmpty())        nonconstthis->SetStringValue();    return m_strValue;}

The result is compiled successfully, and the execution is normal.

 

Therefore, there is a conversion const_cast <> specifically to do this:

CString MyClass::GetStringValue()const{    if(m_strValue.IsEmpty())        const_cast<MyClass *>(this)->SetStringValue();    return m_strValue;}

 

Finally, I think this is against the original intention of the const design. This is a kind of deception for callers (against the contract: I promise not to change you), so it is not recommended to use it like this.