Please consider the following code:
#include <iostream>
#include <typeinfo>
template< typename Type >
void func( Type var )
{
std::cout << __FUNCTION__ << ": var = " << var << " [" << typeid( var ).name( ) << "]." << std::endl;
std::cout << "-> var is SCALAR. Size = " << sizeof( Type ) << std::endl;
}
#if 1
template< typename Type >
void func( Type * var )
{
std::cout << __FUNCTION__ << ": var = " << var << " [" << typeid( var ).name( ) << "]." << std::endl;
std::cout << "-> var is ARRAY. Size = " << sizeof( Type * ) << std::endl;
}
#endif
int main( )
{
typedef char char16[ 16 ];
char16 c16 = "16 bytes chars.";
std::cout << "Size of char16 = " << sizeof( char16 ) << std::endl;
func( c16 );
return 0;
}
If I compile it and run, I see this:
> g++ -Wall -g3 spec_f_pointer.cpp -o spec_f_pointer
> ./spec_f_pointer
Size of char16 = 16
func: var = 16 bytes chars. [Pc].
-> var is ARRAY. Size = 8
Clearly the sizeof
printed inside func
refers to the size of a pointer, and not the size of the typedef
array, as given in main()
.
Now I wonder how to correctly do the trick for getting my func
to specialize in such a way that it correctly knows about my typedef and its size.
Does anyone here can help me, please?
Really thanks.
EDIT
Implementing a specialization as:
template< typename Type >
void func( Type * const &var )
{
std::cout << __FUNCTION__ << ": var = " << var << " [" << typeid( var ).name( ) << "]." << std::endl;
std::cout << "-> var is ARRAY. Size = " << sizeof( Type * ) << std::endl;
}
The output is:
Size of char16 = 16
func: var = 16 bytes chars. [A16_c].
-> var is SCALAR. Size = 16
I noticed the type change from Pc
to A16_c
.
Does it help?
When used as rvalue expressions, arrays decay to pointers to the first element. The function that you have defined takes a pointer and does what is expected. If you want to maintain the array as an array you need to pass it by reference, and because the number of elements is part of the type you probably want to use that as another template argument:
template <typename T, int N>
void f( T(&arg)[N] ) {
cout << sizeof arg << endl;
}
If you want to specialize your function for arrays, do this:
template<typename T, int N>
void func(T(&var)[N])
{
typedef T Type[N];
std::cout << __FUNCTION__ << " [" << typeid( var ).name( ) << "]." << std::endl;
std::cout << "-> var is ARRAY. Size = " << sizeof( Type ) << std::endl;
std::cout << "Number of elements: " << N << std::endl;
std::cout << "Size of each element: " << sizeof(T) << std::endl;
}
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