Question

#include <iostream>
using namespace std;

class Test {
    int a;
public:
    int getA() {
        return a;
    }

Test(): a(1){}
Test(int i): a(i){}

};



int main() {
    Test t1(100);
    cout << sizeof(t1) << " " << sizeof(1) << endl; // 4 4
    return 0;
}

It seems that classes in c++ have no overhead at all. t1 is of size 4 like an integer. If I add another int member to Test, it will increase its size to 8.

I would have expected something that is bigger than 4

Is it true that classes have no overhead?

Answer 1

It seems that classes in c++ have no overhead at all.

As long as a class doesn’t have virtual functions, then, yes. What kind of overhead do you expect? A virtual-less class is merely a collection of variables, with a set of functions associated with the type.

class Foo {
    int a;
    int bar() const { return a*a; }
};

could be trivially replaced by

struct Foo {
    int a;
}

int Foo_bar(Foo const *that) {
    return (that->a) * (that->a);
}

If you compiled each of those snippets, you’d see, that the assembly code looks almost identitcal.

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However if you add one single virtual function, the game changes dramatically.