c++ "error: invalid use of void expression"

Question

I'm trying to implement a trie and I get the following error:

error: invalid use of void expression
  5   |   collect(node->next[c], pre.push_back(c), q);
      |   ^~~~~~~

This is my function:

void string_map<T>::collect(Node* node, string pre, std::queue<string> q) const {
    if (node == nullptr) {return;}
    if (node->definition != nullptr) {q.push(pre); }
    for (int c = 0; c < 256; c++) { 
        collect(node->next[c], pre.push_back(c), q); 
    }
}

This is the only error I get. Everything else work fine.

Any help? Please.

score 3 · Accepted Answer · answered Jul 04 '20 at 00:46

3

http://www.cplusplus.com/reference/string/string/push_back/

string::push_back returns a Void type (ie, doesn't return) and you are using the return value of this call as an argument here:

collect(node->next[c], pre.push_back(c), q);

I am not sure exactly what you are trying to do, but this is the cause for the error.

answered Jul 04 '20 at 00:46

Jeremy

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Thanks, I couldn't stop looking at my own function, and not the type of push_back () – Martín Funes Jul 04 '20 at 01:00

c++ "error: invalid use of void expression"

1 Answers1