Value not usable in constant expression C++

Question

The following code is in C++ I am encountering the error of value is not usable in a constant expression error

int sumNumbers(TreeNode* root) {

    stack<pair<TreeNode*, int>> st;
    st.push(make_pair(root, root->val));
    int sum = 0;

    while(!st.empty()){
        pair<TreeNode*, int> temp = st.top();
        st.pop();
        TreeNode* node = temp.first;
        int value = temp.second;

        if(node->left==NULL && node->right==NULL){
            sum += value;
        }

        if(node->left){
            st.push(pair< node->left, value*10 + node->left->val >);
        }

        if(node->right){
            st.push(pair< node->right, value*10 + node->right->val >);
        }
    }

    return sum;
}

The error is in the line:

if(node->left){
            st.push(pair< node->left, value*10 + node->left->val >);
        }

The error is:

Line 29: Char 37: error: the value of 'node' is not usable in a constant expression st.push(pair< node->left, value*10 + node->left->val >);

I am not able to figure out why this error is encountered here?

Readability suggestion : if(auto nl = node->left) { st.push(make_pair< nl, value*10 + nl->val >); . An if statement may define a local variable for the value being compared. — MSalters
– MSalters, Commented Feb 2, 2019 at 0:42

Matthieu Brucher · Accepted Answer · 2019-02-02 00:22:27Z

6

You want to use make_pair instead to create an object, setting the template types automatically:

st.push(make_pair(node->left, value*10 + node->left->val));

And same for the right side.

answered Feb 2, 2019 at 0:22

Matthieu Brucher

22.1k7 gold badges43 silver badges66 bronze badges

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

1 Comment

betrunken Over a year ago

Alternatively, just use list initialization: st.push({node->left, value*10 + node->left->val})

Collectives™ on Stack Overflow

Value not usable in constant expression C++

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related