In a function declaration, what does passing a fixed size array signify? [duplicate]

Answer 1

I always though that passing fixed size C++ arrays was a "half baked" feature of C++. For example, ignored size matching or only being able to specify the first index size, etc... Until recently I learn this idiom:

template<size_t N1, size_t N2> // enable_if magic can be added as well
function(double(&m)[N1][N2]){
  ... do something with array m...knowing its size!
}

Reference: Can someone explain this template code that gives me the size of an array?

Answer 2

The 25 in the parameter declaration is ignored by the compiler. It's the same as if you'd written string ar_dictionary[]. This is because a parameter declaration of array type is implicitly adjusted to a pointer to the element's type.

So the following three function declarations are equivalent:

void read_dictionary(string ar_dictionary[25], int& dictionary_size)
void read_dictionary(string ar_dictionary[],   int& dictionary_size)
void read_dictionary(string *ar_dictionary,    int& dictionary_size)

Even in the case of the first function, with the size of the array explicitly declared, sizeof(ar_dictionary) will return the same value as sizeof(void*).

See this sample on Codepad:

#include <string>
#include <iostream>

using namespace std;

void read_dictionary(string ar_dictionary[25], int& dictionary_size)
{
    cout << sizeof(ar_dictionary) << endl;  
    cout << sizeof(void*) << endl;  
}

int main()
{
    string test[25];
    int dictionary_size = 25;
    read_dictionary(test, dictionary_size);

    return 0;
}

Output (the exact value is, of course, implementation-dependent; this is purely for example purposes):

4
4

Answer 3

In a one dimensional array It has no significance and is ignored by the compiler. In a two or more dimensional array It can be useful and is used by the function as a way to determine the row length of the matrix(or multi dimensional array). for example :

int 2dArr(int arr[][10]){
   return arr[1][2];
}

this function would know the address of arr[1][2] according to the specified length, and also the compiler should not accept different sizes of arrays for this function -

int arr[30][30];
2dArr(arr);

is not allowed and would be a compiler error(g++) :

error: cannot convert int (*)[30] to int (*)[10]