[C++] Passare funzione come parametro di input

[gaten]

Salve ragazzi, ho il seguente codice:

/* sommo le chiavi */
int sumKey(tree *root, int sum){
    if( root == NULL ){
        return sum;
    } else {
        sumKey(root->left);
        sum=sum+root->info;
        sumKey(root->right);
        sum=sum+root->info;
    }
}


/* foo */
int Sum_After_Level_k(tree *root, int levels, int k, int sum){
    if ( levels < k ){
        Sum_After_Level_k(root->left, levels+1, k, sum);
        Sum_After_Level_k(root->right, levels+1, k, sum);
    } else {
        SumKey(root, sum);
        Sum_After_Level_k=SumKey(root, sum);
    }
}

Non riesco a passare la funzione "SumKey" a "Sum_After_Level_k".

Il compilatore mi dà questi errori:

Compiling: C:\Users\Gaten\Desktop\Untitled1.cpp
C:\Users\Gaten\Desktop\Untitled1.cpp: In function 'int sumKey(tree*, int)':
C:\Users\Gaten\Desktop\Untitled1.cpp:92: error: too few arguments to function 'int sumKey(tree*, int)'
C:\Users\Gaten\Desktop\Untitled1.cpp:96: error: at this point in file
C:\Users\Gaten\Desktop\Untitled1.cpp:92: error: too few arguments to function 'int sumKey(tree*, int)'
C:\Users\Gaten\Desktop\Untitled1.cpp:98: error: at this point in file
C:\Users\Gaten\Desktop\Untitled1.cpp: In function 'int Sum_After_Level_k(tree*, int, int, int)':
C:\Users\Gaten\Desktop\Untitled1.cpp:110: error: 'SumKey' was not declared in this scope
Process terminated with status 1 (0 minutes, 0 seconds)
5 errors, 0 warnings

Come faccio a passare la function "SumKey" come parametro di input alla function "Sum_After_Level_k"???

Grazie anticipatamente.