Lazy task

Mircea Baja - 24 May 2025

# Lazy task

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20250524-0-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20250524-0-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20250524-0-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: middle;
      dominant-baseline: middle;
    }
    .svg20250524-0-t2 {
      font-family: sans-serif;
      font-size: 16px;
      dominant-baseline: middle;
    }
    .svg20250524-0-l1 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
    }
    .svg20250524-0-l2 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
      stroke-dasharray: 5;
    }
    .svg20250524-0-task_white {
      stroke: #000000;
      stroke-width: 4;
      fill: #ffffff;
    }
    .svg20250524-0-task_green {
      stroke: #000000;
      stroke-width: 4;
      fill: #8ea604;
    }
    .svg20250524-0-task_grey {
      stroke: #000000;
      stroke-width: 4;
      fill: #cccccc;
    }
    .svg20250524-0-task_neutral {
      stroke: #000000;
      stroke-width: 4;
      fill: #2d93ad;
    }
    .svg20250524-0-task_neutral2 {
      stroke: #000000;
      stroke-width: 4;
      fill: #6ac3d9;
    }
    .svg20250524-0-task_orange {
      stroke: #000000;
      stroke-width: 4;
      fill: #f5bb00;
    }
    .svg20250524-0-task_pink {
      stroke: #000000;
      stroke-width: 4;
      fill: #ff36ab;
    }
    .svg20250524-0-task_purple {
      stroke: #000000;
      stroke-width: 4;
      fill: #9665ac;
    }

</style>
  <defs>
    <marker id="svg20250524-0-arrow" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z"/>
    </marker>
  </defs>
  <rect x="10" y="10" width="80" height="300" class="svg20250524-0-task_white"/>
  <rect x="130" y="70" width="80" height="170" class="svg20250524-0-task_neutral2"/>
  <rect x="250" y="85" width="80" height="140" class="svg20250524-0-task_neutral2"/>
  <rect x="390" y="20" width="80" height="30" class="svg20250524-0-task_grey"/>
  <rect x="390" y="50" width="80" height="220" class="svg20250524-0-task_white"/>
  <rect x="390" y="80" width="80" height="30" class="svg20250524-0-task_purple"/>
  <rect x="390" y="110" width="80" height="30" class="svg20250524-0-task_neutral"/>
  <rect x="390" y="180" width="80" height="30" class="svg20250524-0-task_orange"/>
  <rect x="510" y="70" width="80" height="170" class="svg20250524-0-task_neutral"/>
  <rect x="510" y="90" width="80" height="30" class="svg20250524-0-task_green"/>
  <rect x="510" y="150" width="80" height="30" class="svg20250524-0-task_purple"/>
  <rect x="630" y="70" width="80" height="170" class="svg20250524-0-task_neutral"/>
  <rect x="630" y="90" width="80" height="30" class="svg20250524-0-task_green"/>
  <rect x="630" y="170" width="80" height="30" class="svg20250524-0-task_pink"/>
  <path class="svg20250524-0-l1" d="M 50 30 v 70 l 120 -20 v 40 l 120 -25 v 55"/>
  <path class="svg20250524-0-l2" d="M 290 150 l -120 -20 v 10 l -120 -30 v 100 l 240 -50"/>
  <path class="svg20250524-0-l1" d="M 290 160 v 55 l -120 -65 v 80 l -120 -10 v 70"/>
  <line class="svg20250524-0-l1" x1="470" y1="95" x2="503" y2="73" marker-end="url(#svg20250524-0-arrow)"/>
  <line class="svg20250524-0-l1" x1="510" y1="105" x2="477" y2="122" marker-end="url(#svg20250524-0-arrow)"/>
  <line class="svg20250524-0-l1" x1="590" y1="165" x2="624" y2="73" marker-end="url(#svg20250524-0-arrow)"/>
  <line class="svg20250524-0-l1" x1="630" y1="105" x2="596" y2="73" marker-end="url(#svg20250524-0-arrow)"/>
  <path class="svg20250524-0-l1" d="M 710 185 l 20 -20 v -80 l -14 -13" marker-end="url(#svg20250524-0-arrow)"/>
  <path class="svg20250524-0-l1" d="M 470 195 h 20 v 60 h 240 v -55 l -14 -13" marker-end="url(#svg20250524-0-arrow)"/>
  <text class="svg20250524-0-t1" x="170" y="55">foo</text>
  <text class="svg20250524-0-t1" x="290" y="70">bar</text>
  <text class="svg20250524-0-t2" x="55" y="190">10s</text>
  <text class="svg20250524-0-t2" x="390" y="285">stack</text>
  <text class="svg20250524-0-t2" x="510" y="55">foo frame</text>
  <text class="svg20250524-0-t2" x="630" y="55">bar frame</text>
</svg>
</div>

---

# Motivation

---

# Goal

```cpp
task<void> echo(socket s) {
  buffer buff;
  while (true) {
    std::size_t length = co_await async_read_some(s, buff);
    co_await async_write(s, buff, length);
  }
}
```

- easy to understand behaviour e.g.:
  - scope for the buffer
  - exceptions (but slow, so use sparingly)
  - cancellation

---

# Basic example

```cpp
task<void> bar() {
  co_await async_sleep(10s);
}

task<void> foo() {
  co_await bar();
}

void somewhere() {
  // run foo()
  // also resume sleeping coroutines
}
```

---

# Basic example

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20250524-1-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20250524-1-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20250524-1-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: middle;
      dominant-baseline: middle;
    }
    .svg20250524-1-t2 {
      font-family: sans-serif;
      font-size: 16px;
      dominant-baseline: middle;
    }
    .svg20250524-1-l1 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
    }
    .svg20250524-1-l2 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
      stroke-dasharray: 5;
    }
    .svg20250524-1-task_white {
      stroke: #000000;
      stroke-width: 4;
      fill: #ffffff;
    }
    .svg20250524-1-task_green {
      stroke: #000000;
      stroke-width: 4;
      fill: #8ea604;
    }
    .svg20250524-1-task_grey {
      stroke: #000000;
      stroke-width: 4;
      fill: #cccccc;
    }
    .svg20250524-1-task_neutral {
      stroke: #000000;
      stroke-width: 4;
      fill: #2d93ad;
    }
    .svg20250524-1-task_neutral2 {
      stroke: #000000;
      stroke-width: 4;
      fill: #6ac3d9;
    }
    .svg20250524-1-task_orange {
      stroke: #000000;
      stroke-width: 4;
      fill: #f5bb00;
    }
    .svg20250524-1-task_pink {
      stroke: #000000;
      stroke-width: 4;
      fill: #ff36ab;
    }
    .svg20250524-1-task_purple {
      stroke: #000000;
      stroke-width: 4;
      fill: #9665ac;
    }
  </style>
  <defs>
    <marker id="svg20250524-1-arrow" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z"/>
    </marker>
  </defs>
  <rect x="10" y="10" width="80" height="300" class="svg20250524-1-task_white"/>
  <rect x="130" y="70" width="80" height="170" class="svg20250524-1-task_neutral2"/>
  <rect x="250" y="85" width="80" height="140" class="svg20250524-1-task_neutral2"/>
  <path class="svg20250524-1-l1" d="M 50 30 v 70 l 120 -20 v 40 l 120 -25 v 55"/>
  <path class="svg20250524-1-l2" d="M 290 150 l -120 -20 v 10 l -120 -30 v 100 l 240 -50"/>
  <path class="svg20250524-1-l1" d="M 290 160 v 55 l -120 -65 v 80 l -120 -10 v 70"/>
  <text class="svg20250524-1-t1" x="170" y="55">foo</text>
  <text class="svg20250524-1-t1" x="290" y="70">bar</text>
  <text class="svg20250524-1-t2" x="55" y="190">10s</text>
</svg>
</div>

---

# Basic example - in foo

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20250524-2-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20250524-2-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20250524-2-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: middle;
      dominant-baseline: middle;
    }
    .svg20250524-2-t2 {
      font-family: sans-serif;
      font-size: 16px;
      dominant-baseline: middle;
    }
    .svg20250524-2-l1 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
    }
    .svg20250524-2-l2 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
      stroke-dasharray: 5;
    }
    .svg20250524-2-task_white {
      stroke: #000000;
      stroke-width: 4;
      fill: #ffffff;
    }
    .svg20250524-2-task_green {
      stroke: #000000;
      stroke-width: 4;
      fill: #8ea604;
    }
    .svg20250524-2-task_grey {
      stroke: #000000;
      stroke-width: 4;
      fill: #cccccc;
    }
    .svg20250524-2-task_neutral {
      stroke: #000000;
      stroke-width: 4;
      fill: #2d93ad;
    }
    .svg20250524-2-task_neutral2 {
      stroke: #000000;
      stroke-width: 4;
      fill: #6ac3d9;
    }
    .svg20250524-2-task_orange {
      stroke: #000000;
      stroke-width: 4;
      fill: #f5bb00;
    }
    .svg20250524-2-task_pink {
      stroke: #000000;
      stroke-width: 4;
      fill: #ff36ab;
    }
    .svg20250524-2-task_purple {
      stroke: #000000;
      stroke-width: 4;
      fill: #9665ac;
    }
  </style>
  <defs>
    <marker id="svg20250524-2-arrow" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z"/>
    </marker>
  </defs>
  <rect x="10" y="10" width="80" height="300" class="svg20250524-2-task_white"/>
  <rect x="130" y="70" width="80" height="170" class="svg20250524-2-task_neutral2"/>
  <rect x="390" y="20" width="80" height="30" class="svg20250524-2-task_grey"/>
  <rect x="390" y="50" width="80" height="220" class="svg20250524-2-task_white"/>
  <rect x="390" y="80" width="80" height="30" class="svg20250524-2-task_purple"/>
  <rect x="510" y="70" width="80" height="170" class="svg20250524-2-task_neutral"/>
  <path class="svg20250524-2-l1" d="M 50 30 v 70 l 120 -20 v 40"/>
  <line class="svg20250524-2-l1" x1="470" y1="95" x2="503" y2="73" marker-end="url(#svg20250524-2-arrow)"/>
  <text class="svg20250524-2-t1" x="170" y="55">foo</text>
  <text class="svg20250524-2-t2" x="390" y="285">stack</text>
  <text class="svg20250524-2-t2" x="510" y="55">foo frame</text>
</svg>
</div>

---

# Basic example - in bar *

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20250524-3-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20250524-3-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20250524-3-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: middle;
      dominant-baseline: middle;
    }
    .svg20250524-3-t2 {
      font-family: sans-serif;
      font-size: 16px;
      dominant-baseline: middle;
    }
    .svg20250524-3-l1 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
    }
    .svg20250524-3-l2 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
      stroke-dasharray: 5;
    }
    .svg20250524-3-task_white {
      stroke: #000000;
      stroke-width: 4;
      fill: #ffffff;
    }
    .svg20250524-3-task_green {
      stroke: #000000;
      stroke-width: 4;
      fill: #8ea604;
    }
    .svg20250524-3-task_grey {
      stroke: #000000;
      stroke-width: 4;
      fill: #cccccc;
    }
    .svg20250524-3-task_neutral {
      stroke: #000000;
      stroke-width: 4;
      fill: #2d93ad;
    }
    .svg20250524-3-task_neutral2 {
      stroke: #000000;
      stroke-width: 4;
      fill: #6ac3d9;
    }
    .svg20250524-3-task_orange {
      stroke: #000000;
      stroke-width: 4;
      fill: #f5bb00;
    }
    .svg20250524-3-task_pink {
      stroke: #000000;
      stroke-width: 4;
      fill: #ff36ab;
    }
    .svg20250524-3-task_purple {
      stroke: #000000;
      stroke-width: 4;
      fill: #9665ac;
    }
  </style>
  <defs>
    <marker id="svg20250524-3-arrow" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z"/>
    </marker>
  </defs>
  <rect x="10" y="10" width="80" height="300" class="svg20250524-3-task_white"/>
  <rect x="130" y="70" width="80" height="170" class="svg20250524-3-task_neutral2"/>
  <rect x="250" y="85" width="80" height="140" class="svg20250524-3-task_neutral2"/>
  <rect x="390" y="20" width="80" height="30" class="svg20250524-3-task_grey"/>
  <rect x="390" y="50" width="80" height="220" class="svg20250524-3-task_white"/>
  <rect x="390" y="80" width="80" height="30" class="svg20250524-3-task_purple"/>
  <rect x="510" y="70" width="80" height="170" class="svg20250524-3-task_neutral"/>
  <rect x="510" y="150" width="80" height="30" class="svg20250524-3-task_purple"/>
  <rect x="630" y="70" width="80" height="170" class="svg20250524-3-task_neutral"/>
  <rect x="630" y="90" width="80" height="30" class="svg20250524-3-task_green"/>
  <path class="svg20250524-3-l1" d="M 50 30 v 70 l 120 -20 v 40 l 120 -25"/>
  <line class="svg20250524-3-l1" x1="470" y1="95" x2="503" y2="73" marker-end="url(#svg20250524-3-arrow)"/>
  <line class="svg20250524-3-l1" x1="590" y1="165" x2="624" y2="73" marker-end="url(#svg20250524-3-arrow)"/>
  <line class="svg20250524-3-l1" x1="630" y1="105" x2="596" y2="73" marker-end="url(#svg20250524-3-arrow)"/>
  <text class="svg20250524-3-t1" x="170" y="55">foo</text>
  <text class="svg20250524-3-t1" x="290" y="70">bar</text>
  <text class="svg20250524-3-t2" x="390" y="285">stack</text>
  <text class="svg20250524-3-t2" x="510" y="55">foo frame</text>
  <text class="svg20250524-3-t2" x="630" y="55">bar frame</text>
</svg>
</div>

---

# Basic example - sleep - wakeup

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20250524-4-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20250524-4-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20250524-4-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: middle;
      dominant-baseline: middle;
    }
    .svg20250524-4-t2 {
      font-family: sans-serif;
      font-size: 16px;
      dominant-baseline: middle;
    }
    .svg20250524-4-l1 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
    }
    .svg20250524-4-l2 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
      stroke-dasharray: 5;
    }
    .svg20250524-4-task_white {
      stroke: #000000;
      stroke-width: 4;
      fill: #ffffff;
    }
    .svg20250524-4-task_green {
      stroke: #000000;
      stroke-width: 4;
      fill: #8ea604;
    }
    .svg20250524-4-task_grey {
      stroke: #000000;
      stroke-width: 4;
      fill: #cccccc;
    }
    .svg20250524-4-task_neutral {
      stroke: #000000;
      stroke-width: 4;
      fill: #2d93ad;
    }
    .svg20250524-4-task_neutral2 {
      stroke: #000000;
      stroke-width: 4;
      fill: #6ac3d9;
    }
    .svg20250524-4-task_orange {
      stroke: #000000;
      stroke-width: 4;
      fill: #f5bb00;
    }
    .svg20250524-4-task_pink {
      stroke: #000000;
      stroke-width: 4;
      fill: #ff36ab;
    }
    .svg20250524-4-task_purple {
      stroke: #000000;
      stroke-width: 4;
      fill: #9665ac;
    }
  </style>
  <defs>
    <marker id="svg20250524-4-arrow" viewBox="0, 0, 10, 10" refX="5" refY="5" markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z"/>
    </marker>
  </defs>
  <rect x="10" y="10" width="80" height="300" class="svg20250524-4-task_white"/>
  <rect x="130" y="70" width="80" height="170" class="svg20250524-4-task_neutral2"/>
  <rect x="250" y="85" width="80" height="140" class="svg20250524-4-task_neutral2"/>
  <rect x="390" y="20" width="80" height="30" class="svg20250524-4-task_grey"/>
  <rect x="390" y="50" width="80" height="220" class="svg20250524-4-task_white"/>
  <rect x="390" y="80" width="80" height="30" class="svg20250524-4-task_purple"/>
  <rect x="510" y="70" width="80" height="170" class="svg20250524-4-task_neutral"/>
  <rect x="510" y="150" width="80" height="30" class="svg20250524-4-task_purple"/>
  <rect x="630" y="70" width="80" height="170" class="svg20250524-4-task_neutral"/>
  <rect x="630" y="90" width="80" height="30" class="svg20250524-4-task_green"/>
  <path class="svg20250524-4-l1" d="M 50 30 v 70 l 120 -20 v 40 l 120 -25 v 55"/>
  <path class="svg20250524-4-l2" d="M 290 150 l -120 -20 v 10 l -120 -30 v 100 l 240 -50"/>
  <line class="svg20250524-4-l1" x1="470" y1="95" x2="503" y2="73" marker-end="url(#svg20250524-4-arrow)"/>
  <line class="svg20250524-4-l1" x1="590" y1="165" x2="624" y2="73" marker-end="url(#svg20250524-4-arrow)"/>
  <line class="svg20250524-4-l1" x1="630" y1="105" x2="596" y2="73" marker-end="url(#svg20250524-4-arrow)"/>
  <path class="svg20250524-4-l1" d="M 470 195 h 20 v 60 h 240 v -169 l -14 -13" marker-end="url(#svg20250524-4-arrow)"/>
  <text class="svg20250524-4-t1" x="170" y="55">foo</text>
  <text class="svg20250524-4-t1" x="290" y="70">bar</text>
  <text class="svg20250524-4-t2" x="55" y="190">10s</text>
  <text class="svg20250524-4-t2" x="390" y="285">stack</text>
  <text class="svg20250524-4-t2" x="510" y="55">foo frame</text>
  <text class="svg20250524-4-t2" x="630" y="55">bar frame</text>
</svg>
</div>

---

# Basic example - back in foo

<div align="center">
<svg xmlns="http://www.w3.org/2000/svg" id="svg20250524-5-box" width="100%" viewBox="0 0 800 320">
  <style>
    #svg20250524-5-box {
      border: 1px solid #e8e8e8;
      background-color: #f5f5f5;
    }
    .svg20250524-5-t1 {
      font-family: sans-serif;
      font-size: 16px;
      text-anchor: middle;
      dominant-baseline: middle;
    }
    .svg20250524-5-t2 {
      font-family: sans-serif;
      font-size: 16px;
      dominant-baseline: middle;
    }
    .svg20250524-5-l1 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
    }
    .svg20250524-5-l2 {
      stroke: #000000;
      stroke-width: 2;
      fill: none;
      stroke-dasharray: 5;
    }
    .svg20250524-5-task_white {
      stroke: #000000;
      stroke-width: 4;
      fill: #ffffff;
    }
    .svg20250524-5-task_green {
      stroke: #000000;
      stroke-width: 4;
      fill: #8ea604;
    }
    .svg20250524-5-task_grey {
      stroke: #000000;
      stroke-width: 4;
      fill: #cccccc;
    }
    .svg20250524-5-task_neutral {
      stroke: #000000;
      stroke-width: 4;
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      stroke-width: 4;
      fill: #9665ac;
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  </style>
  <defs>
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      <path d="M 0 0 L 10 5 L 0 10 L 2.5 5 Z"/>
    </marker>
  </defs>
  <rect x="10" y="10" width="80" height="300" class="svg20250524-5-task_white"/>
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  <path class="svg20250524-5-l1" d="M 50 30 v 70 l 120 -20 v 40 l 120 -25 v 55"/>
  <path class="svg20250524-5-l2" d="M 290 150 l -120 -20 v 10 l -120 -30 v 100 l 240 -50"/>
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  <text class="svg20250524-5-t1" x="170" y="55">foo</text>
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  <text class="svg20250524-5-t2" x="510" y="55">foo frame</text>
</svg>
</div>

---

# Code

---

# Promise base

```cpp
template<typename T>
class promise_base {
  std::variant<std::monostate, T, std::exception_ptr> result_;

public:
  template<typename U>
    requires std::convertible_to<U, T>
  void return_value(U && x) noexcept(std::is_nothrow_constructible_v<T, U>)
  {
    assert(0 == result_.index());
    result_.template emplace<1>(std::forward<U>(x));
  }

void unhandled_exception() noexcept {
    assert(0 == result_.index());
    result_.template emplace<2>(std::current_exception());
  }
```

---

# Promise base

```cpp
  T get_result() const {
    switch(result_.index()) {
      case 1:
        return std::move(std::get<1>(result_));
      case 2:
        std::rethrow_exception(std::get<2>(result_));
      default:
        std::terminate();
    }
  }
}; // promise_base
```

---

# Promise base - void

```cpp
template<>
class promise_base<void> {
  std::exception_ptr exception_{};

public:
  void return_void() noexcept {
  }

void unhandled_exception() noexcept {
    assert(nullptr == exception_);
    exception_ = std::current_exception();
  }

void get_result() const {
    if (exception_) {
      std::rethrow_exception(exception_);
    }
  }

std::exception_ptr get_result_exception() const noexcept {
    return exception_;
  }
};
```

---

# Promise

```cpp
template<typename T>
class [[nodiscard]] task {
public:
  class promise_type : public promise_base<T> {
    friend task;

std::coroutine_handle<> parent_coro_;

public:
    promise_type() noexcept = default;

promise_type(const promise_type&) = delete;
    promise_type& operator=(const promise_type&) = delete;

task get_return_object() noexcept {
      return { std::coroutine_handle<promise_type>::from_promise(*this) };
    }
```

---

# Promise - lazy

```cpp
    std::suspend_always initial_suspend() noexcept {
      return {};
    }

struct final_awaiter {
      [[nodiscard]] constexpr bool await_ready() const noexcept {
        return false;
      }

std::coroutine_handle<> await_suspend(
          std::coroutine_handle<promise_type> child_coro) noexcept {
        return child_coro.promise().parent_coro_;
      }

[[noreturn]] constexpr void await_resume() const noexcept {
        std::unreachable(); //std::terminate();
      }
    }; // final_awaiter

final_awaiter final_suspend() noexcept {
      return {};
    }
  }; // promise_type
```

---

# Task

```cpp
private:
  unique_coroutine_handle<promise_type> unique_child_coro_;

task(std::coroutine_handle<promise_type> child_coro) noexcept :
    unique_child_coro_{ child_coro }
  {
  }

public:
  task(const co&) = delete;
  task& operator=(const co&) = delete;
```

---

# Task awaiter

```cpp
  [[nodiscard]] constexpr bool await_ready() const noexcept {
    return false;
  }

std::coroutine_handle<> await_suspend(
      std::coroutine_handle<> parent_coro) noexcept {
    std::coroutine_handle<promise_type> child_coro = unique_child_coro_.get();
    child_coro.promise().parent_coro_ = parent_coro;
    return child_coro;
  }

T await_resume() const {
    return unique_child_coro_.get().promise().get_result();
  }
}; // task
```

---

# Discussion

---

# History

This is close to the task/lazy [P1056r1
proposal](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1056r1.html)
which did not make it.

---

# Q: How do we start one?

- e.g. in our simple exemple how do we start `foo()`?
- the problem is that we need to `co_await` in order to have a coroutine
  handle, and we can only `co_await` in a coroutine
- A: the answer is long, will explain another time

---

# Bad: no cancellation

- needs major changes to support cancellation

---

# Good: lazy

Bad example: `future.then` is concurrent with work being done
```cpp
future<int> f = get_future_for_async_activity();
future<string> f2 = f.then([](int i) {
  return std::to_string(i);
});
std::cout << f2.get() << '\n';
```
- setting "continuation" (the lambda specified in `then`) requires
  synchronization

- also Q: What to do in final_suspend for eager tasks if the parent can't be
  resumed becaus it did not suspend? A: more synchronization

---

# Good: lazy

Another reason is (correct) error handling becomes impractical for anything but
the most trivial examples of eager work see [Structured Concurrency: Writing
Safer Concurrent Code with Coroutines... - Lewis Baker - CppCon
2019](https://www.youtube.com/watch?v=1Wy5sq3s2rg)

```cpp
future f1 = eager_async_work1();
bool joined_f1 = false;
std::exception_ptr ex;
try {
  co_await eager_async_work2();
  joined_f1 = true;
  co_await std::move(f1);
  co_return;
} catch(...) {
  ex = current_exception();
}
if (!joined_f1) co_await std::move(f1);
std::rethrow_exception(ex);
```
---

# Bad: Scope for bugs

- same as for the std::generator

```cpp
task<void> foo() {
  task<void> bar("alpha");
  co_await bar;
}
```

```cpp
task<void> bar(const std::string& letter) {
  std::cout << letter << '\n';
  co_return;
}
```

---

# The move trick

Used by Lewis Baker's coro library, also see [Raymond Chen: Making it
impossible to co_await a task
twice](https://devblogs.microsoft.com/oldnewthing/20210414-00/?p=105095)

```cpp
template<typename T>
class [[nodiscard]] task {
  // ...
  task_awaiter operator co_await() && {
    return {std::move(unique_child_coro_};
  }
};
```

Meant to prevent
```cpp
task<void> foo() {
  task<void> bar("alpha");
  co_await bar;
  co_await bar;
}
```

**Clever, but not a good solution for things that are meant to be done only
once**. In particular if std::move is used in both places it compiles, but is
incorrect code. Also it still has the reference argument bug.

---

# Questions?