可以通过“指针+偏移量”的方式访问私有变量。

    1. #include <iostream>
    2. using namespace std;
    4. class Test
    5. {
    6. private:
    7.     int a;
    8.     int b;
    10. public:
    11.     Test(int _a, int _b){a=_a; b=_b;}
    12.     void Print(){
    13.         cout << this->a << ", " << this->b << endl;
    14.     }
    15. };
    18. int main() {
    19.     Test a(3, 3);
    20.     a.Print();
    22.     int *p = (int*)&a;
    23.     *p = 10;
    24.     a.Print();
    26.     *(p+1) = 11;
    27.     a.Print();
    28.     return 0;
    29. }

    运行结果：
    image.png

    困惑：不同类型的成员变量的偏移量应该怎么计算

    1. #include <iostream>
    2. using namespace std;
    4. class TestA
    5. {
    6. private:
    7.     int a;
    8.     int b;
    10. public:
    11.     TestA(int _a, int _b){a=_a; b=_b;}
    12.     void Print() const{
    13.         cout << this->a << ", " << this->b << endl;
    14.     }
    15. };
    17. class TestB
    18. {
    19. private:
    20.     short a;
    21.     int b;
    22.     float c;
    23.     double d;
    24.     char e;
    26. public:
    27.     TestB(short _a, int _b, float _c, double _d, char _e){a=_a; b=_b; c=_c; d=_d; e=_e;}
    28.     void Print() const{
    29.         cout << this->a << ", " << this->b << ", " << this->c << ", " << this->d << ", " << this->e << endl;
    30.     }
    32.     void PrintAddress() const {
    33.         cout << &a << ", " << &b << ", " << &c << ", " << &d << ", " << &e << endl;
    34.     }
    35. };
    38. int main() {
    39.     TestA ta(3, 3);
    40.     ta.Print();
    41.     int* p = (int*)&ta;
    42.     *p = 4;
    43.     ta.Print();
    44.     *(p+1) = 5;
    45.     ta.Print();
    47.     cout << "------------------" << endl;
    48.     // 或者使用reinterpret_cast加字节偏移量
    49.     char ch = 'e';
    50.     TestB tb(13, 13, 13.0, 13.0, ch);
    51.     cout << &tb <<endl;
    52.     tb.PrintAddress();
    53.     tb.Print();
    54.     auto q1 = reinterpret_cast<short *>((char*)(&tb));
    55.     *q1 = 14;
    56.     tb.Print();
    57.     auto q2 = reinterpret_cast<int *>((char*)(&tb) + 4);
    58.     *q2 = 15;
    59.     tb.Print();
    60.     auto q3 = reinterpret_cast<float *>((char*)(&tb) + 2*4);
    61.     *q3 = 16;
    62.     tb.Print();
    63.     auto q4 = reinterpret_cast<double *>((char*)(&tb) + 4*4);
    64.     *q4 = 17;
    65.     tb.Print();
    66.     auto q5 = reinterpret_cast<char *>((char*)(&tb) + 6*4+1);
    67.     *q5 = 'f';
    68.     tb.Print();
    71.     cout << sizeof(short*) << sizeof(int*) << sizeof(float*) << sizeof(double*) << sizeof(char*) << endl;
    72.     cout << sizeof(short) << sizeof(int) << sizeof(float) << sizeof(double) << sizeof(char) << endl;
    73.     return 0;
    74. }

    运行结果：

    1. D:\jetbrains\cpp\untitled\cmake-build-debug\untitled.exe
    2. 3, 3
    3. 4, 3
    4. 4, 5
    5. ------------------
    6. 0x61fdc0
    7. 0x61fdc0, 0x61fdc4, 0x61fdc8, 0x61fdd0, e
    8. 13, 13, 13, 13, e
    9. 14, 13, 13, 13, e
    10. 14, 15, 13, 13, e
    11. 14, 15, 16, 13, e
    12. 14, 15, 16, 17, e
    13. 14, 15, 16, 17, e
    14. 88888
    15. 24481
    17. Process finished with exit code 0