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c++voidtrie

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

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.

Solution

  • 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.