Member Avatar for #include<>

Why doesn't

typedef [I]ClassName[/I] SomeName[[I]SomeSize[/I]][[I]SomeOtherSize[/I]];

work?

  • 4 Contributors
  • 3 Replies
  • 123 Views
  • 8 Hours Discussion Span
  • Latest Post Latest Post by mrnutty

Dani AI

Generated

Short answer: typedefs follow the same declarator grammar as variable declarations — the identifier goes where a variable name would, and any array brackets attach to that identifier. That means dimensions must be compile-time integral constants (standard C++ has no VLAs). As pointed out, you can name a fixed-size 2D array type, but remember that such a type is an array (not a pointer) and top-level arrays are not directly assignable.

For a reusable, parameterised alias (C++11+), an alias template is convenient:

template<typename T, std::size_t R, std::size_t C>
using Static2D = T[R][C];

struct Item { /* ... */ };
Static2D<Item, 3, 5> grid;   // a 3x5 array of Item

If you want safer/member-friendly types or runtime sizes, prefer standard containers. Nested std::array gives compile-time sizes and value semantics:

constexpr std::size_t R = 3, C = 4;
std::array<std::array<Item, C>, R> fixed_grid;

A flattened std::vector gives contiguous storage with dynamic sizing:

std::vector<Item> data(R * C);
auto at = [&](std::size_t i, std::size_t j) -> Item& { return data[i * C + j]; };

To write functions that preserve sizes, take the array by reference with template parameters:

template<std::size_t R, std::size_t C>
void process(Item (&arr)[R][C]) { /* R and C known here */ }

Tradeoffs: plain C arrays are contiguous but not assignable as top-level objects and they decay to pointers in many contexts; pointer-to-pointer is not the same layout as a true 2D array. ’s wrapper approach is a good next step if methods, bounds checking, or RAII are required. Use std::array/std::vector for safer, idiomatic code unless you need raw arrays for low-level C interop.

Recommended Answers

All 3 Replies

Member Avatar for Unimportant

My mistake, poster below me is correct.

Edited by Unimportant because: n/a
Member Avatar for Ancient Dragon 
const int SomeSize = 5;
const int SomeOtherSize = 5;
class ClassName
{
};

typedef ClassName SomeName[SomeSize][SomeOtherSize];
Edited by Ancient Dragon because: n/a
Member Avatar for mrnutty

Something else to consider :

template<typename T, int ROW, int COLUMN>
class Array2D{
private:
   T _array[ROW][COLUMN];
public:
 Array2D(){/*initialize _array*/}
 //Array2D methods/operations
};

int main(){
 Array2D<int,5,5> array;
}
Edited by mrnutty because: n/a
Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.