使用 C++ 的原则和实践中来自头文件的错误消息

Question

I've just started learning C++ using Programming: Principles and Practice using C++. That book tells me to use a header file which sets things up for me. The header file in question is available at http://www.stroustrup.com/Programming/std_lib_facilities.h

I'm attempting an exercise which asks me to write a prime sieve. I have the following program:

#include "std_lib_facilities.h"

void sieve_erat(int end) {
  vector<bool> primes (end, true);
  int final_element = sqrt(end) + 1;
  for (int i=2; i<final_element; ++i)
    if (primes[i])
      for (int j=i*i; j<end; j += i)
    primes[j] = false;

  for (int p=2; p<end; ++p)
    if (primes[p])
      cout << p << " ";
  cout << '\n';
}

int main() {
  cout << "Enter the number to which I should find the primes: ";
  cin >> max;
  sieve_erat(max);
  return 0;
}

But when I compile on my computer with g++ primes.cpp I get the following output:

~/src/c++ $ g++ primes.cpp 
In file included from /usr/include/c++/4.8.1/ext/hash_map:60:0,
                 from std_lib_facilities.h:34,
                 from primes.cpp:4:
/usr/include/c++/4.8.1/backward/backward_warning.h:32:2: warning: #warning This file includes at least one deprecated or antiquated header which may be removed without further notice at a future date. Please use a non-deprecated interface with equivalent functionality instead. For a listing of replacement headers and interfaces, consult the file backward_warning.h. To disable this warning use -Wno-deprecated. [-Wcpp]
 #warning \
  ^
In file included from primes.cpp:4:0:
std_lib_facilities.h: In instantiation of ‘T& Vector<T>::operator[](unsigned int) [with T = bool]’:
primes.cpp:36:17:   required from here
std_lib_facilities.h:88:38: error: invalid initialization of non-const reference of type ‘bool&’ from an rvalue of type ‘std::vector<bool, std::allocator<bool> >::reference {aka std::_Bit_reference}’
   return std::vector<T>::operator[](i);
                                  ^

I've tried my best to find the answer to this question on the web, but I just can't understand what the message is telling me I've done wrong! Please may somebody be kind enough to point me in the right direction?

Thank you.

Answer 1

(Are you sure that's the code you're compiling? where is max declared?)

std::vector<bool> is a very strange beast which doesn't behave like std::vector<T> for any other T .

Unfortunately even though std::vector<bool> seems like the obvious choice in your case, it forces you to deal with the oddness of std::vector<bool> and it doesn't work with Stroustrup's Vector class template.

The detailed explanation of the error is that Stroustrup's header re-defines vector to refer to his Vector template, which adds some range-checking to the element access operator (ie operator[] , which is the one used when you say primes[i] ). The redefined vector cannot be instantiated with bool because std::vector<bool>::operator[] does not return a normal reference, and the redefined vector expects it to do so.

Answer 2

From a first look, the vector of bool seems to be the problem. Unfortunately, vectors and bool values do not go well together, as iterators are unable to return a bool& reference. See this question for a fairly detailed explanation.