Class's static struct pointer declaration in C++ - c++

Question

Given the following snippet:
/* trie.h file */
using namespace std;
#include <list>
typedef struct tn {
char ch;
list<struct tn*> ptrs;
} TrieNode;
class Trie {
public:
static const TrieNode* EMPTY;
//... other member functions
};
/* trie.cpp file */
#include "trie.h"
// declare, define static variables of the Trie class
TrieNode* Trie::EMPTY = (TrieNode*) malloc( sizeof(TrieNode) ); // <-- seems to work fine
// the statements below seem to yield errors
Trie::EMPTY->ch = '.';
Trie::EMPTY->ptrs = nullptr;
I get the error: "This declaration has no storage type or type specifier" if I try to instantiate the struct member variables of the static constant variable EMPTY. I know storing EMPTYas a struct object rather than a pointer to the struct object would be easier but was curious how this would work. Thanks.

Answer 1

You can't put statements like Trie::EMPTY->ch = '.'; and Trie::EMPTY->ptrs = nullptr; in global scope, they can only be executed inside of functions, constructors, etc.
Try something more like this instead:
/* trie.h file */
#include <list>
struct TrieNode {
char ch;
std::list<TrieNode*> ptrs;
};
class Trie {
public:
static const TrieNode* EMPTY;
//... other member functions
};
/* trie.cpp file */
#include "trie.h"
// declare, define static variables of the Trie class
static const TrieNode emptyNode{'.'};
const TrieNode* Trie::EMPTY = &emptyNode;
Live Demo

Answer 2

In a namespace (outside any function) you may place only declarations.
Statements like these:
Trie::EMPTY->ch = '.';
Trie::EMPTY->ptrs = nullptr;
are not allowed to be placed in a namespace, because they are not declarations.
Moreover, this statement:
Trie::EMPTY->ptrs = nullptr;
does not make sense, because the object ptrs is not a pointer, and a std::list cannot be initialized from nullptr.
Pay attention to that instead of the C function malloc(), you should use the C++ operator new.
This definition:
TrieNode* Trie::EMPTY = (TrieNode*) malloc( sizeof(TrieNode) );
is also incorrect, because you forgot to specify the qualifier const.
It should be rewritten like this:
const TrieNode* Trie::EMPTY = new TrieNode { '.' };
Here is a demonstrative program
#include <iostream>
#include <list>
typedef struct tn {
char ch;
std::list<struct tn*> ptrs;
} TrieNode;
class Trie {
public:
static const TrieNode* EMPTY;
//... other member functions
};
// the definition below must be placed in a cpp module
// it presents here only for demonstration.
const TrieNode* Trie::EMPTY = new TrieNode { '.' };
int main()
{
return 0;
}
Before exiting the program you should free the allocated memory.
Instead of the raw pointer you could use the smart pointer std::unique_ptr.