Self contained test code for partial initializing struct function pointers with lambdas (c++14)

lmbda.cpp

// Copyright: https://mit-license.org/
//
// cpplint lmbda.cpp 2>&1 | grep -v "Is this a non-const reference?"
// g++ -Wall -Wextra -pedantic -std=c++14 -g lmbda.cpp -o lmbda
//
#include <assert.h>

#include <iostream>
#include <vector>

#include <functional>

typedef int num;
typedef num vertex;
typedef num edge;

typedef std::vector<edge> Vertex;
typedef std::vector<std::vector<num>> Edge;

#define _f(g) if (g) g

struct compact5_traversal_visitor {
    std::function<void()> begin_traversal = 0;
    std::function<std::vector<int>()> end_traversal = 0;
    std::function<void(vertex)> begin_vertex = 0;
    std::function<void(vertex)> end_vertex = 0;
    std::function<void(edge)> next_edge = 0;
    std::function<void(vertex, edge)> next_vertex_edge = 0;
};

class graph {
 public:
    std::vector<Vertex> V;
    std::vector<Edge> E;

explicit graph(num n = 0, num m = 0) {
        Vertex v = {};
        Edge e = {};
        V = std::vector<Vertex>(n, v);
        E = std::vector<Edge>(m, e);
    }
};

std::vector<int> compact5_traversal(graph& G, compact5_traversal_visitor c5v) {
    if (!c5v.end_traversal) {
        c5v.end_traversal = []() { return std::vector<int>(0); };
    }

    _f(c5v.begin_traversal)();

    _f(c5v.begin_vertex)(G.V.size()-1);

    return c5v.end_traversal();
}

int main() {
    graph G(3);

    compact5_traversal(G, {
        [G]() { std::cout << "begin_traversal " << G.V.size() << std::endl; },
        [&G]() { std::cout << "end_traversal " << G.V.size() << std::endl;
               return std::vector<int>(0); },
        [&G](vertex v) { std::cout << v << std::endl;
                         G.V.push_back(std::vector<edge>(0)); }
    });

    return 0;
}

Corrected 1st version by advice from older son to use "std::function".
Now compile warning from before is gone, and it just works with locally defined graph G:
https://forums.raspberrypi.com/viewtopic.php?p=1999519#p1999519

pi@pi400-64:~ $ g++ -Wall -Wextra -pedantic -std=c++14 -g lmbda.cpp -o lmbda
pi@pi400-64:~ $ ./lmbda 
begin_traversal 3
2
end_traversal 4
pi@pi400-64:~ $