Using c++ typedef/using type alias

Question

I am reading C++ primer book and completely don\'t understand one line:

 using int_array = int[4]; 
 typedef int int_array[4]; // This line
 for (int_array *         


        

Answer 1

Both type aliases are the same:

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Type alias, alias template (since C++11):
Type alias is a name that refers to a previously defined type (similar to typedef):

using identifier attr(optional) = type-id ; 

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so you may use:

typedef int int_array[4];

or you may just use (it is the same as above):

using int_array = int[4];

When you need to address the memory with 4*sizeof(int) steps, e.g. if the system int size is 4 bytes, then the memory step size is 4*4=16 bytes. even you may use int_array *p; in this case ++p advances p by one memory step e.g. 16 bytes. see:

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1- working sample with using int_array = int[4];:

#include 
using std::cout; using std::endl;

int main()
{
    int ia[3][4] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 };

    // a range for to manage the iteration
    // use type alias
    using int_array = int[4];
    for (int_array& p : ia)
        for (int q : p)
            cout << q << " ";
    cout << endl;

    // ordinary for loop using subscripts
    for (size_t i = 0; i != 3; ++i)
        for (size_t j = 0; j != 4; ++j)
            cout << ia[i][j] << " ";
    cout << endl;

    // using pointers.
    // use type alias
    for (int_array* p = ia; p != ia + 3; ++p)
        for (int *q = *p; q != *p + 4; ++q)
            cout << *q << " ";
    cout << endl;

    return 0;
}

output 1:

0 1 2 3 4 5 6 7 8 9 10 11
0 1 2 3 4 5 6 7 8 9 10 11
0 1 2 3 4 5 6 7 8 9 10 11

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2- working sample using typedef int int_array[4];:

#include 
using std::cout; using std::endl;

int main()
{
    int ia[3][4] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 };

    // a range for to manage the iteration
    // use type alias
    typedef int int_array[4];
    for (int_array& p : ia)
        for (int q : p)
            cout << q << " ";
    cout << endl;

    // ordinary for loop using subscripts
    for (size_t i = 0; i != 3; ++i)
        for (size_t j = 0; j != 4; ++j)
            cout << ia[i][j] << " ";
    cout << endl;

    // using pointers.
    // use type alias
    for (int_array* p = ia; p != ia + 3; ++p)
        for (int *q = *p; q != *p + 4; ++q)
            cout << *q << " ";
    cout << endl;

    return 0;
}

output 2(same):

0 1 2 3 4 5 6 7 8 9 10 11
0 1 2 3 4 5 6 7 8 9 10 11
0 1 2 3 4 5 6 7 8 9 10 11

ref: https://github.com/Mooophy/Cpp-Primer/blob/master/ch03/ex3_44.cpp
note: use -std=c++11 for compile/link.

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3- just for demo(excerpt):
some real world usage in embedded system:

extern const char kpd2ascii[6][6] PROGMEM;
typedef const char cint8a6_t[6];
cint8a6_t *p;
p = kpd2ascii;
kpdBuffer = pgm_read_byte(&p[row][col - 1]);

I hope this helps.