C++20 协程和 iostream

Question

I am playing around with C++ 20's Coroutines. The sample is compiled with clang++.

The compiler error I am facing is

error: invalid operands to binary expression ('std::ostream' (aka 'basic_ostream') and 'cppcoro::generator')

which is about the following line

std::cout << numbers << " ";  

the full code snipped looks like this:

#include <thread>
#include <iostream>
#include <vector>
#include <cppcoro/generator.hpp>

using namespace std;

// coroutine
cppcoro::generator<int> generatorForNumbers(int begin, int inc = 1) 
{
  // for Schleife ohne Abbruchbedingung
  for (int i = begin;; i += inc) 
  {
    co_yield i;
  }  
}

int main() 
{

    auto numbers = generatorForNumbers(-10);                 
  
    for (int i= 1; i <= 20; ++i)
    {
        std::cout << numbers << " ";       
    }
  
    std::cout << "\n\n";
        
    // inline works
    for (auto n: generatorForNumbers(0, 5))
    {
        std::cout << n << " ";  
    }

    std::cout << std::endl;
}

Executable code snipped can be found here: https://godbolt.org/z/4cxhqxPP7

Answer 1

From the documentation of cppcoro::generator , the only "meaningful" operations you can do to a generator is get begin and end iterators via begin() and end() . This is precisely why your second use for (auto n: generatorForNumbers(0, 5)) works. It iterates over the generator. Though you technically hit undefined behavior since there's no stopping condition for the generator so it overflows an int .

You can't print the generator directly. You have to iterate over it. So you could do this instead:

auto numbers = generatorForNumbers(-10);

auto it = numbers.begin();
for (int i= 1; i <= 20; ++i)
{
    std::cout << *it << " ";
    ++it;
}

or better (in my opinion):

int i = 0;
for (auto n : generatorForNumbers(-10))
{
    if (++i > 20)
        break;
    std::cout << n << " ";
}

or even better using ranges (thanks to @cigien in the comments):

for (auto n : generatorForNumbers(-10) | std::views::take(20))
{
    std::cout << n << " ";
}