独習C++ 1章

• When I declare any variable, the value are not decided.

how to declare and initilize at the same time?

int a(0);
int b = 1;
int c{2};
int d = {3};

• const

const int a = 3;
int const a = 3;

• fall through

switch(a)
{
    case 1: FALL THROUGH:
    case 2: FALL THROUGH:
    case 3: 
        std:: cout <<~
}

In the above code, If a is 1, 2, or 3, we hit the same code.(std::cout~~)

1.5.4 const and pointer

1. What the pointer points to is const.(*a = 5ができない)
   There is const to the left of *.

const int* a = 10;
int const* a = 15;

1. The pointer itself is immutable.(a = &bができない)
   There is const to the right of *.

int *const a = 30;

これはダメ we cannot change what the pointer points to.
*ptr = 2でerror

    #include <iostream>
     int main()
     {
        int a = 42;
        const int* ptr = &a;
        *ptr = 2;
        std::cout << *ptr << std::endl;
     }

これはいい。we change the pointer itself.

    #include <iostream>
     int main()
     {
        int a = 42;
        int b = 10;
        const int* ptr = &a;
        ptr = &b;
        std::cout << *ptr << std::endl;
     }

we cannot assign const variable to not const pointer.

#include <iostream>

 int main()
 {
    const int a = 42;
    int b = 10;
    int* ptr = &a;
    ptr = &b;
    std::cout << *ptr << std::endl;
 }

1.5.5 null pointer

null pointer is a special address which does not point to any variable.
we can create null pointer by assigning 0 or nllptr.

    int *ptr = 0;
    ptr = nullptr; 

nullptr does not point to anything, so the following code causes segmentation fault.

int main()
 {
     int* ptr = nullptr;

     *ptr = 42; // ヌル参照

     std::cout << "ヌル参照のあと" << std::endl;
 }

1.5.6 cast

#include <iostream>

int main()
{
   char c = 99;
   std::cout << static_cast<int>(c) << std::endl;
}

1.6 配列と文字列

int arr[5];
int arr[5] = {1,2,3,4,5};
int arr[] = {1,2,3,4,5};

If array's length is larger than initialized size, the rest is initialized as 0.

int arr[5] = {1,2,3}
→arr is {1,2,3,0,0}

1.6.2 size of array

#include <iostream>

int main()
{
   int arr[5] = {};
   int size = sizeof(arr);
   std::cout << size << std::endl;
}

size is 20 because sizeof() returns the byte.

#include <iostream>

int main()
{
   int arr[5] = {};
   int size = sizeof(arr);
   std::cout << size/sizeof(int) << std::endl;
}

1.6.3 文字列

\0 is null character.

#include <iostream>

int main()
{
   std::string string = "hello,\0 nullcharacter";

   std::cout << string << std::endl;
}

the output is "hello,".

1.7.2 range based for