How can a compiler generated default constructor be more efficient than a self-written one that does nothing but initialize members? [duplicate]

问题

This question already has answers here:

Why would the implicitly generated constructor (et al.) be more efficient than a user-defined (trivial) one? (4 answers)

Are user-defined default constructors less efficient? (2 answers)

Closed 7 months ago.

Triggered by this answer I was reading in the core guidelines:

C.45: Don’t define a default constructor that only initializes data members; use in-class member initializers instead

The reasoning given is

Reason

Using in-class member initializers lets the compiler generate the function for you. The compiler-generated function can be more efficient.

Note that this is specifically about a default constructor that does nothing but initialize the members and the guideline suggests that one should not write such a constructor.

The "bad" example is:

Example, bad

class X1 { // BAD: doesn't use member initializers
    string s;
    int i;
public:
    X1() :s{"default"}, i{1} { }
    // ...
};

What can the compiler generated constructor do more efficient than the user-provided one in that particular example (or in any other example)?

Naively I would expect the initializer list giving the same opportunities for optimization as in-class initializers.

来源：https://stackoverflow.com/questions/56272431/how-can-a-compiler-generated-default-constructor-be-more-efficient-than-a-self-w