about templates

[lixx]

Im quite baffled with the code I'm dealing with:



template <typename T>
int getSize(T *arrayname) {
return sizeof(arrayname)/sizeof(T);
}
....
int test[16];
cout << sizeof(test) << endl;


On my computer, it displays 1. Upon further tracing
of variables, sizeof(arrayname) is equal to 4 and also
sizeof(T). So 4/4=1. So I can infer that the int size
on my pc is about 4 bytes. Does templates really
doesn't work with sizeof???

[StOoZ]

what u are doing is basically asking for the size of the type of the pointer.... it wont give u 64 in this case (if int = 4 byte).
it wont work even without templates... this is because its a pointer.

[JupiterV2]

The size of a pointer is static so (example in C):

#include <stdio.h>
#include <stdlib.h>

int main()
{
int* x;

printf("sizeof(x)=%d\n", sizeof(x));

x = calloc(10, sizeof(int));

printf("sizeof(x)=%d\n", sizeof(x));

return 0;
}

...will always return the same value no matter the contents of x nor the amount of memory allocated to x (in my case, 4 bytes). The only way I know to retain the number of elements in an array is to store it in some fashion (example in C):

#include <stdio.h>
#include <stdlib.h>

typedef struct
{
int length;
int* array;
} array_t;

int main()
{
array_t my_array;

my_array.length = 5;
my_array.array = (int*) calloc(my_array.length, sizeof(int));

printf("The size of my_array is %d.\n", my_array.length);

return 0;
}

In C++, though, it's typically better practice to use one of the STL types like a vector rather than an array (example in C++):

#include <iostream>
#include <vector>

int main()
{
std::vector<int> my_int_array;

for(int x = 0; x < 5; x++) my_int_array.push_back(x);

std::cout << "Size of my array: " << my_int_array.size() << std::endl;

return 0;
}