inlined costructor

[mickey]

Hello, I read TC++ on page 393 and I don't understand if it says that in WithMember class it's better do a non-inlined class; and what does it refer with that //trivial? THanks

class Member {
   int i, j, k;
public:
   Member(int x = 0) : i(x), j(x), k(x) {}
  ~Member() { cout << "~Member" << endl; }
};
 
class WithMembers {
    Member q, r, s; // Have constructors
    int i;
public:
    WithMembers(int ii) : i(ii) {} // Trivial?
   ~WithMembers() {
          cout << "~WithMembers" << endl;
    }
};
 
int main() {
     WithMembers wm(1);
}

class Member {
   int i, j, k;
public:
   Member(int x = 0) : i(x), j(x), k(x) {}
  ~Member() { cout << "~Member" << endl; }
};

class WithMembers {
    Member q, r, s; // Have constructors
    int i;
public:
    WithMembers(int ii) : i(ii) {} // Trivial?
   ~WithMembers() {
          cout << "~WithMembers" << endl;
    }
};

int main() {
     WithMembers wm(1);
}

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[wysota]

Trivial means there is no logic in it - it's empty (apart from calling the base class constructor).

[jacek]

It says "Trivial?" because it only looks trivial. Although there is no code between {}, the constructor looks like this:

<invoke Member::Member() on q>
<invoke Member::Member() on r>
<invoke Member::Member() on s>
i = ii

<invoke Member::Member() on q>
<invoke Member::Member() on r>
<invoke Member::Member() on s>
i = ii

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In this case Member::Member() is rather simple, but in general case speed-up gained through inlining the constructor might be just a small fraction of the total time needed to construct the object, so you are just bloating your code.

[wysota]

Probably the word trivial refers to the code and not the logic behind it. And the question mark probably serves the same purpose as in the chess notation

[mickey]

It says "Trivial?" because it only looks trivial. Although there is no code between {}, the constructor looks like this:

<invoke Member::Member() on q>
<invoke Member::Member() on r>
<invoke Member::Member() on s>
i = ii

<invoke Member::Member() on q>
<invoke Member::Member() on r>
<invoke Member::Member() on s>
i = ii

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In this case Member::Member() is rather simple, but in general case speed-up gained through inlining the constructor might be just a small fraction of the total time needed to construct the object, so you are just bloating your code.

Yes, WithMember constructor is big because it calls 3 object costructors; WithMember is inlined, but (if I understand right) in the worst case the compiler won't do the constructor inlined; and with piece of code "big", there no reasons to do inline (we prefer don't soil the class declaration or the .h file); At the end, if I understand, that empty constructor ( { } ), it's trivial, in the sense that we are inlining without gain (but it's even true that in somewhere we have to write the constructor body; maybe, when we're writing the code, until the body constructor is empty, it's simple/faster(in the sense of "writing") put it within the class declaration and not in a cpp). Is it ok?

[jacek]

At the end, if I understand, that empty constructor ( { } ), it's trivial, in the sense that we are inlining without gain (but it's even true that in somewhere we have to write the constructor body; maybe, when we're writing the code, until the body constructor is empty, it's simple/faster(in the sense of "writing") put it within the class declaration and not in a cpp). Is it ok?

Not quite, whether the constructor is trivial or not depends not only on the code you wrote in its body, but also on the automatically generated code that calls the base class constructor and initializes member variables.