Question: In `C` I have

```
unsigned long arr[256]={0x00000000L,0x01020304L,0x21223212L,...}

```

in C++ in `.h`-file i have

```
private:
    unsigned long arr[256];

```

how-to populate it (fastest way) in `.cpp` file with 256 constant values?

```
arr[256]={...}
// and
arr={...}

```

not work :-\

Answer: 
Use initializer list in constructor:

```
// .h
class C
{
public:
    C();
private:
    unsigned long arr[256];
};

// .cpp
C::C() : arr{0x00000000L, 0x01020304L, 0x21223212L, ...} {}

```

Note that `std::array` may be an alternative which has more intuitive syntax and is assignable.
If you insist on using plain arrays, just `std::memcpy(&this->arr[0], &::initial_values_arr[0], sizeof (::initial_values_arr));`