Why can const members be modified in a constructor?

Question

I\'m curious why const members can be modified in the constructor.

Is there any standard rule in initialization that overrides the \"const-ness\" of a member?

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Answer 1

When you do:

struct Bar {
    const int b = 5; // default member initialization
    ...
};

You are telling the compiler to do this with the default constructor:

...
Bar() : b(5) 
{}
...

Irrespective of if you've provided the default constructor or not. When you do provide default-constructor and the initial assignment, you override compiler's default assignment code (i.e. b(5)). Default initialization/assignment at the declaration is useful when you have multiple constructors, and you may or may not assign the const members in all constructors:

...
Bar() = default; // b=5
Bar(int x) : b(x) // b=x
Bar(double y) : /*other init, but not b*/  // b=5
...