为什么即使sizeof相同，指针也不能适合不同类型的变量?

为什么任何指针的大小为4或8个字节，但不能容纳任何其他变量?尝试为双指针分配int指针值时出现错误.

Why the sizeof any pointer is 4 or 8 bytes, but it cannot fit any different variable? I get error while trying to assign a double pointer an int pointer value.

int          *int_ptr{nullptr};
float        *float_ptr{nullptr};
double       *double_ptr{nullptr};
long double  *long_double_ptr{nullptr};
string       *string_ptr{nullptr};
vector<int>   *vector_ptr{nullptr};

cout << "sizeof int pointer is " << sizeof int_ptr;    //8 or 4
cout << "sizeof float pointer is " << sizeof float_ptr;    //8 or 4
cout << "sizeof double pointer is " << sizeof double_ptr;    //8 or 4
cout << "sizeof double double pointer is " << sizeof long_double_ptr;    //8 or 4
cout << "sizeof string pointer is " << sizeof string_ptr;    //8 or 4
cout << "sizeof vector int pointer is " << sizeof vector_ptr;  //8 or 4

double double_num{1020.7};
double_ptr = &int_ptr;      //cannot convert ‘int**’ to ‘double*’ in assignment