quiz - cells

Q027#

#include <iostream>

struct A {
    virtual std::ostream &put(std::ostream &o) const {
        return o << 'A';
    }
};

struct B : A {
    virtual std::ostream &put(std::ostream &o) const override {
        return o << 'B';
    }
};

std::ostream &operator<<(std::ostream &o, const A &a) {
    return a.put(o);
}

int main() {
    B b;
    std::cout << b;
}

Answer：

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Q029#

#include <iostream>

struct A {
    A() {
        // std::cout << "A::A()" << std::endl;
        foo();
    }
    virtual ~A() {
        // std::cout << "A::~A()" << std::endl;
        foo();
    }
    virtual void foo() {
        // std::cout << "A::foo()" << std::endl;
        std::cout << "1";
    }
    void bar() {
        // std::cout << "A::bar()" << std::endl;
        foo();
    }
};

struct B : public A {
    virtual void foo() {
        // std::cout << "B::foo()" << std::endl;
        std::cout << "2";
    }
};

int main() {
    B b;
    b.bar();
}

Answer：

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121

Explain：

Even though A::foo() is a virtual function, it is not considered virtual during construction and destruction.

Before constructing B, A should be constructed, and during the construction of A, B is not yet constructed, so it should not be used, the foo() called in A::A() naturally calls A::foo().

When the object b is destroyed, the same problem arises because B's destructor is called first, followed by A's destructor, and after the object b is destroyed, B::foo() should not be used.

Q114#

#include <iostream>
#include <memory>
#include <vector>

class C {
public:
    void foo() {
        std::cout << "A";
    }
    void foo() const {
        std::cout << "B";
    }
};

struct S {
    std::vector<C> v;
    std::unique_ptr<C> u;
    C *const p1;
    C const *p2;

    S() : v(1)
        , u(new C())
        , p1(u.get()
        , p2(u.get()) {

    }
};

int main() {
    S s;
    s.v[0].foo();
    s.u->foo();
    s.p1->foo();
    s.p2->foo();

    const S &r = s;
    r.v[0].foo();
    r.u->foo();
    r.p1->foo();
    r.p2->foo();
}

Answer：

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AAABBAAB

Explain：

C *const p1 const modifies the p1 pointer, so when foo is called on the member variable p1 of object s, the non-const version of foo will be used. C const *p2 const modifies the *p (C object), so when foo is called on the member variable p2 of object s, the const version of foo will be used.

r is a constant reference to object s, and const changes the behavior of member v because std::vector overloads operator[] and the version called by a constant object returns a constant reference, so the returned value is also a constant reference, so calling foo will use the const version.

Q360#

// using C++17
#include <iostream>
#include <type_traits>

int main() {
    std::cout << std::is_const_v<const int *>;
    std::cout << std::is_const_v<int *const>;

    std::cout << std::is_const_v<const int[1]>;
    std::cout << std::is_const_v<const int **>;
    std::cout << std::is_const_v<int *const *>;

    std::cout << std::is_const_v<const int(*)[1]>;
    std::cout << std::is_const_v<const int *[1]>;
    std::cout << std::is_const_v<const int[1][1]>;
}

Answer：

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0110001

Explain：

const int * is a pointer to a constant integer, which means a constant pointer, although the content pointed to by the pointer is constant, the pointer itself is not constant, so the result is false.

int *const is a constant pointer, the pointer itself is constant, so the result is true.

const int[1] is an array containing constant integers, the elements of the array are constants, so the result is true.

const int ** is a pointer to a pointer to a constant pointer ( (const int *) *ptr ), the pointer itself is not constant, so the result is false.

int *const * is a pointer to a constant pointer ( (int *const) *ptr ), the pointer itself is not constant, so the result is false.

const int (*)[1] is a pointer to an array of constant integers, the pointer itself is not constant, so the result is false.

const int *[1] is an array of pointers to constant integers ( const int *ptr[1] ), the elements of the array are constant pointers, so the result is false.

const int[1][1] is a two-dimensional array, the elements of which are constants, so the result is true.