error: invalid use of void expression

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error: invalid use of void expression

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#1 by (3 votes)

2

In this code I skip the following error: invalid use of void expression. More precisely in return ciclo_Euleriano(g, g.getVertices().pop_front(), g.getVertices().pop_front()); and I do not know why it is.

bool ciclo_Euleriano(const Grafo<T> & g, T u, T first) { //u es el vertice
    bool found = false;
    if (!aristas.empty()) {
        list<typename Grafo<T>::Arco> adyacentes = g.getAdyacentes(u);
        for (typename list<typename Grafo<T>::Arco>::iterator it = adyacentes.begin(); it != adyacentes.end() && !found; it++) {
            if (Camino_no_Usado(u, it->getAdyacente(), aristas)) {
                addArista(camino, u, it->getAdyacente(), g.getCosto(u, *it));
                eliminar(u, *it, aristas);
                found = ciclo_Euleriano(g, *it, first);
                if (!found) {
                    addArista(aristas, u, *it, g.getCosto(u, *it));
                    camino.pop_back();
                }
            }
        }
    } else
        if (u == first)
            found = true;
    return found;
}

bool ciclo_Euleriano(const Grafo<T> & g) {
    list<typename Grafo<T>::Arco> aristas = g.getAristas();
    camino.clear();
    return ciclo_Euleriano(g, g.getVertices().pop_front(), g.getVertices().pop_front());
}

Here I leave the getVertices() :

list<T> getVertices() const {
    list<T> vertices;
    for (typename map<T, map<T, int> >::const_iterator itV = grafo.begin(); itV != grafo.end(); itV++)
        vertices.push_front(itV->first);
    return vertices;
}

c++

asked by Guido Modarelli 09.07.2018 в 21:05

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1 answer

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score 3 · Accepted Answer

The type void by definition is an incomplete type, which can not be completed. It can not be part of expressions or be instantiated.

You are calling the ciclo_Euleriano function by passing the result of calling two functions that return void .

Given that Grafo<T>::getVertices returns a std::list , we can see that the function pop_front returns void :

std::list::pop_front .

Surely what you wanted was to get the heads ( front ) call ciclo_Euleriano and then delete the head element:

bool ciclo_Euleriano(const Grafo<T> & g) {
    const auto &aristas = g.getAristas();
    auto v = g.getVertices().front();

    camino.clear();
    g.getVertices().pop_front();

    return ciclo_Euleriano(g, v, v);
}

As an additional note, your code would improve a lot in readability by using auto and some C ++ utilities:

list<T> getVertices() const {
    list<T> vertices;

    std::copy(grafo.begin(), grafo.end(), std::front_inserter(vertices));
    return vertices;
}