error: ‘SIZE’ cannot appear in a constant-expression when using template< class T, int MAX_SIZE >

Question

I am getting a compilation error:

HuffTree.cpp:43:20: error: ‘SIZE’ cannot appear in a constant-expression
  PQueue<HuffNode*, SIZE>;

where the involved lines are:

void HuffTree::buildTree(char * chs, int * freqs, int size )
{
static const int SIZE = size;
PQueue<HuffNode*, SIZE>;

I've tried all different types for "SIZE"

PQueue<HuffNode*, size>; // From the method parameter
static const int SIZE = static_cast<int>(size);

etc. But the only the following compiles:

PQueue<HuffNode*, 10>; // Or any other random int

I also get the related error:

HuffTree.cpp:43:24: error: template argument 2 is invalid
PQueue<HuffNode*, SIZE>;

PQueue is a template class accepting:

template< class T, int MAX_SIZE >
PQueue(T* items, int size);

What type does 'size' need to be for the argument to be accepted?
Using c++11

Answer 1

The problem is that

PQueue<HuffNode*, SIZE>;

is a type and require a SIZE known at compile time.

If you declare SIZE with a value known compile time, as in

PQueue<HuffNode*, 10>;

or also in

static const int SIZE = 10;
PQueue<HuffNode*, SIZE>;

it should works.

But if SIZE depend from a value, size, known only run-time (the input value of a function method is considered to be known runtime), the compiler can't known SIZE compile time.

Addendum: you're using C++11; so try using constexpr instead of const

static constexpr int SIZE = 10;