How can you deduce the size of an array in a function template?

Question

I have a bunch of structs like:

struct A { ... } struct B { ... } struct C { ... }

I want to devise a function that can accept arrays of these structs and iterate through each element of the array and call another function like:

template <typename T> ostream& process(ostream& os, const T* array) { // output each element of array to os (but how do we know the length?) } A a_array[10]; // in practice, this is actually operator<<, so I cannot pass in the // size explicitly process(..., a_array);

Update: I cannot use any of the std containers here. It has to be an array unfortunately!

Why are you not using std::vector or std::array?

Paul Manta
– Paul Manta

2012-01-04 17:08:49 +00:00
Commented Jan 4, 2012 at 17:08 — Paul Manta
– Paul Manta, Commented Jan 4, 2012 at 17:08

vallentin · Accepted Answer · 2016-10-14 04:52:34Z

10

Array-to-pointer decay is really, really bad.

Fortunately, C++ has array references, which know their size.

template<typename T, size_t N> ostream& process(ostream& os, const T (&arr)[N]) { // use N }

edited Oct 14, 2016 at 4:52

vallentin

26.5k6 gold badges70 silver badges93 bronze badges

answered Jan 4, 2012 at 17:05

Puppy

147k40 gold badges271 silver badges481 bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

zneak Over a year ago

I've always seen const T (&arr)[N], but I suppose it works the same.

Puppy Over a year ago

I've no idea, it's been a really long time since I used that syntax.

CashCow Over a year ago

I think it is advantageous to drop the const. The template will automatically make the type const if it needs to be, but if you pass in a non-const array I seem to recall it won't know how to handle it.

Dietmar Kühl Over a year ago

The notation const T(&arr)[Size] definitely works to deduce the size of a statically sized array. Leaving off the const has the advantage that the constness is correctly deduced, e.g. when getting an end iterator for the array.

Puppy Over a year ago

@Dietmar: Oh noes! Code bloat! My binary, it might consume a couple more KB, compared to my hundreds of MB of art and such.

|

Fox32 · Accepted Answer · 2012-01-04 17:18:54Z

You could use a std::vector<T> instead of an simple array.

template <typename T> ostream& process(ostream& os, const std::vector<T> &array) { for(std::vector<T>::const_iterator iterator = array.begin(); iterator != array.end(); ++iterator) { //... } }

Or you can go the std::array way (If your compiler support it and N is constant).

template <typename T, int N> ostream& process(ostream& os, const std::array<T, N> &array) { for(std::array<T, N>::const_iterator iterator = array.begin(); iterator != array.end(); ++iterator) { //... } } // Usage: array<int, 10> test; process(..., test);

If the compiler doesn't have std::array, boost::array is always an option

moshbear · Accepted Answer · 2012-01-06 09:02:33Z

You need to use the following format for arrays:

template <typename T, size_t N> void foo(const T (&arr)[N]) { ... }

Otherwise, size information will be lost.

DYoung · Accepted Answer · 2012-01-04 17:39:29Z

Or, a simple template bounds checked array.

template< typename T, unsigned int Size > class Array { public: T& operator[]( unsigned int index ) { assert( index < Size ); return mElements[ index ]; } const T& operator[]( unsigned int index ) const { assert( index < Size ); return mElements[ index ]; } unsigned int Capacity() const { return Size; } private: T mElements[ Size ]; };

And then

template< typename T, unsigned int Size > void Process( Array< T, Size >& array ) { for( unsigned int i = 0; i < Size; ++i ) { //use array[i] } }

And to tie it together

Array< int, 10 > array; Process( array );

It's a bit of a roll your own solution, but it's probably roughly equivalent (although a less functional array class) to std::Array or boost

Collectives™ on Stack Overflow

How can you deduce the size of an array in a function template?

4 Answers 4

9 Comments

2 Comments

Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

9 Comments

2 Comments

Comments

Comments

Related