Initializing const struct member

[arnaudk]

How does one initialize a const struct member of an object? Since it's const, this has to be done in the constructor initialization list. Unfortunately I can't seem to get the syntax right...

 
class Example
{
public:
  Example() : st.a(1), st.b(5) // error
  {
  }
 
  const struct TheStruct 
  { 
    int a;
    int b;
  } st;
};
 

Obviously it looks like I'm trying to reference nonexistant methods of the struct. What is the correct syntax?

[Sick0Fant]

How does one initialize a const struct member of an object? Since it's const, this has to be done in the constructor initialization list. Unfortunately I can't seem to get the syntax right...

 
class Example
{
public:
  Example() : st.a(1), st.b(5) // error
  {
  }
 
  const struct TheStruct 
  { 
    int a;
    int b;
  } st;
};
 

Obviously it looks like I'm trying to reference nonexistant methods of the struct. What is the correct syntax?

Are you trying to declare a const instance of the struct in the class? If so, use "static const." That way, it will be initialized, then it cannot be changed.

something like

class MyClass
{
    public:
        MyClass(){};
    struct MyStruct
    {
        static const int a = 1;
        static const int b = 2;
    };
 
    MyStruct MyInstanceOfMyStruct;
};
 

[arnaudk]

Hi Sick0Fant, and thanks for your reply. If I initialized it as you proposed, then it would be the same accross all instances of Example objects. However, by const I merely mean to enforce that, once initialized, the struct should not change. It will, however, hold different values accross different instances of Example. Do you know how I may initialize such a struct in the initialization list of the Example constructor?

[weaknessforcats]

Here you go:

class Example
{
public:
  Example() : st(1,5)
  {
  }
 
  struct TheStruct 
  { 
    int a;
    int b;
   TheStruct(int first, int second);
  };
  const TheStruct st;
};
Example::TheStruct::TheStruct(int first, int second) : a(first), b(second)
{
 
}
int main()
{
   Example obj;
}
 

What you have to do is have a const data member. But that means the const values need to be set before the TheStruct constructor is called so st.a is too late.

What I did was create a TheStruct declaration followed by a const instance of TheStruct. That requires a constructor for the struct.

[arnaudk]

Great! Thanks agian.

[JosAH]

Here you go:

class Example
{
public:
  Example() : st(1,5)
  {
  }
 
  struct TheStruct 
  { 
    int a;
    int b;
   TheStruct(int first, int second);
  };
  const TheStruct st;
};
Example::TheStruct::TheStruct(int first, int second) : a(first), b(second)
{
 
}
int main()
{
   Example obj;
}
 

What you have to do is have a const data member. But that means the const values need to be set before the TheStruct constructor is called so st.a is too late.

What I did was create a TheStruct declaration followed by a const instance of TheStruct. That requires a constructor for the struct.

That is so incredibly filthy! No, seriously, very clever WFC; my compliments.

kind regards,

Jos