c++ - ‘function’ in namespace ‘std’ does not name a type

Question

main.cc:

#include <cstdio>
#include <functional>
#include "segtree.h" // NOLINT

int main() {
  const int n = 8;
  SegmentTree<int> segmentTree(n, 1,
      std::multiplies<int>());
  for (int i = 0; i < n; ++i)
    segmentTree.set(i, i + 1);

  for (int i = 0; i < n; ++i)
    for (int j = i; j < n; ++j) {
      int s = segmentTree.get(i, j);
      std::printf(
          "Function applied from data[%d] to data[%d] is equal to %d \n",
          i, j, s);
    }
  return 0;
}

segtree.h: #ifndef SEGTREE_H_ // NOLINT #define SEGTREE_H_

#include <cassert>
#include <functional>
#include <vector>

template <typename T>
class SegmentTree {
 private:
  std::vector<T> tree;
  int levels;
  int n;
  int el;
  std::function<T(T, T)> f;

  T query(int l, int r, int st, int fi) {
    if (l == r)
      return get(l);

    int mid = (st + fi + 1) / 2;
    if (r < mid) {
      return query(l, r, st,  mid - 1);
    } else if (l >= mid) {
      return query(l, r, mid, fi);
    } else {
      T left  = query(l,   mid - 1, st,  mid - 1);
      T right = query(mid, r,       mid, fi);
      return f(left, right);
    }
  }

 public:
  SegmentTree(const std::vector<T>& data,
      std::function<T(T, T)> f = std::plus <T>()) {
    this->f = f;
    this->n = data.size();

    // build tree
    levels = 1, el = 1;
    while (el < static_cast<int>(data.size())) {
      el <<= 1;
      ++levels;
    }
    tree.assign(2 * el, T());
    for (size_t i = 0; i < data.size(); ++i)
      tree[i + el - 1] = data[i];
    for (int i = el - 2; i >= 0; --i)
      tree[i] = f(tree[2 * i + 1], tree[2 * i + 2]);
  }

  SegmentTree(int n, T val = T(),
      std::function<T(T, T)> f = std::plus <T>()) :
        SegmentTree(std::vector<T>(n, val), f) {
  }


  T get(int l, int r) {
    return query(l, r, 0, n);
  }

  T get(int index) {
    return tree[el - 1 + index];
  }

  void set(int index, T value) {
    int i = el - 1 + index;
    tree[i] = value;
    do {
      i = (i-1) / 2;
      tree[i] = f(tree[i*2 + 1], tree[i*2 + 2]);
    } while (i > 0);
  }
};

Error information after g++ main.cc:

In file included from main.cc:9:0:
segtree.h:22:3: error: ‘function’ in namespace ‘std’ does not name a type
segtree.h:42:12: error: ‘std::function’ has not been declared
segtree.h:42:20: error: expected ‘,’ or ‘...’ before ‘<’ token
segtree.h:60:12: error: ‘std::function’ has not been declared
segtree.h:60:20: error: expected ‘,’ or ‘...’ before ‘<’ token
segtree.h:59:3: error: default argument missing for parameter 3 of ‘SegmentTree<T>::SegmentTree(int, T, int)’
segtree.h: In constructor ‘SegmentTree<T>::SegmentTree(const std::vector<T>&, int)’:
segtree.h:43:15: error: ‘f’ was not declared in this scope
segtree.h: In constructor ‘SegmentTree<T>::SegmentTree(int, T, int)’:
segtree.h:61:45: error: ‘f’ was not declared in this scope
main.cc: In function ‘int main()’:
main.cc:14:29: error: no matching function for call to ‘SegmentTree<int>::SegmentTree(const int&, int, std::multiplies<int>)’
main.cc:14:29: note: candidates are:
segtree.h:59:3: note: SegmentTree<T>::SegmentTree(int, T, int) [with T = int]
segtree.h:59:3: note:   no known conversion for argument 3 from ‘std::multiplies<int>’ to ‘int’
segtree.h:41:3: note: SegmentTree<T>::SegmentTree(const std::vector<T>&, int) [with T = int]
segtree.h:41:3: note:   candidate expects 2 arguments, 3 provided
segtree.h:16:7: note: SegmentTree<int>::SegmentTree(const SegmentTree<int>&)
segtree.h:16:7: note:   candidate expects 1 argument, 3 provided
In file included from main.cc:9:0:
segtree.h: In member function ‘void SegmentTree<T>::set(int, T) [with T = int]’:
main.cc:16:29:   instantiated from here
segtree.h:78:7: error: ‘f’ was not declared in this scope
segtree.h: In member function ‘T SegmentTree<T>::query(int, int, int, int) [with T = int]’:
segtree.h:66:28:   instantiated from ‘T SegmentTree<T>::get(int, int) [with T = int]’
main.cc:20:35:   instantiated from here
segtree.h:36:27: error: ‘f’ was not declared in this scope

I got this piece of code from github, but I am not very familiar with c++. Can anyone help?

Answer 1

Make sure you have quite new version of g++ and try adding -std=c++11 during compilation:

g++ -std=c++11 main.cc

You need to have at least g++-4.7 to make it work.