what does !temp->left mean in C?

Question

I am studying binary search trees right now, and I came across this code

void free_tree(Node* root) {
    // Deallocates memory corresponding
    // to every node in the tree.
    Node* temp = root;
    if (!temp)
        return;
    free_tree(temp->left);
    free_tree(temp->right);
    if (!temp->left && !temp->right) {
        free(temp);
        return;
    }
}

what does !temp mean in this context? Does it mean if temp == NULL? and does that mean !temp->left or !temp->right mean "if right of temp == NULL"? I'm kind of confused with this code in general.

I tried searching for related questions, but so far got no answers.

Answer 1

From the C Standard (6.5.3.3 Unary arithmetic operators)

5 The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is equivalent to (0==E).

and (6.3.2.3 Pointers)

3 An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. 66) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

So these if statements

if (!temp)

and

if (!temp->left && !temp->right) {

is equivalent to

if ( temp == NULL )

and

if ( temp->left == NULL && temp->right == NULL ) {

Pay attention to that your function is wrong. It frees only memory for nodes data members left and right of which are null pointers.

A correct function can look for example the following wat

void free_tree( Node *root ) 
{
    // Deallocates memory corresponding
    // to every node in the tree.
    if ( root != NULL )
    {
        free_tree( root->left );
        free_tree( root->right );

        free( root );
    }
}

Though it will be better to declare and define it the following way

void free_tree( Node **root ) 
{
    // Deallocates memory corresponding
    // to every node in the tree.
    if ( *root != NULL )
    {
        free_tree( &( *root )->left );
        free_tree( &( *root )->right );

        free( *root );
        *root = NULL;
    }
}

After calling the function the original pointer to the root node passed to the function by reference through pointer to it will be equal to NULL.