c++中sizeof的理解

1. 例题

#include <iostream>

class A {};
class B { char m_data; };
class C {
    char m_data[];
};

class D {
    char *m_data;
    D(): m_data(new char[]) {}
    ~D() { delete []m_data; }
};

static void testSizeof() {
    std::cout << "bool sizeof " << sizeof(bool) << std::endl;
    std::cout << "char sizeof " << sizeof(char) << std::endl;
    std::cout << "int sizeof " << sizeof(int) << std::endl;
    char data[];
    char *pData = data;
    std::cout << "char data[10] sizeof " << sizeof(data) << std::endl;
    std::cout << "char* sizeof " << sizeof(pData) << std::endl;
    std::cout << "A sizeof " << sizeof(A) << std::endl;
    std::cout << "B sizeof " << sizeof(B) << std::endl;
    std::cout << "C sizeof " << sizeof(C) << std::endl;
    std::cout << "D sizeof " << sizeof(D) << std::endl;

    std::cout << "size_t sizeof " << sizeof(std::size_t) << std::endl;
};

static void unsignedAndSigned() {
    std::size_t a = ;
    int b = -;
    if (b < a)
        std::cout << "right\n";
    else
        std::cout << "bad\n";

}

int main()
{
    testSizeof();

    unsignedAndSigned();

    return ;
}